Holographic storage medium, holographic tape cartridge, system, and holographic tape library

Abstract

A holographic data storage medium shaped as tape is provided. The holographic data storage medium has a thickness of 60μm or less. A holographic tape cartridge for use with a holographic data storage system is also provided. A system for receiving a holographic tape cartridge comprising a holographic data storage medium is also provided. A holographic tape library is also provided.

Description

[0001] Holographic data storage medium, holographic tape cartridge, system, and holographic tape library

[0002] Field of the disclosure

[0003] The present disclosure relates to a holographic data storage medium, particularly a holographic data storage medium shaped as tape. The present disclosure also relates to a holographic tape cartridge for use with a holographic data storage system, a system for receiving a holographic tape cartridge comprising a holographic data storage medium, and a holographic tape library.

[0004] Background to the Disclosure

[0005] Holographic data storage is a method of storing data on a holographic medium as a hologram. Data is stored in the holographic medium as an interference pattern between a signal beam, which carries data, and a reference beam. To retrieve the stored data, the medium may then be illuminated with a reconstruction beam, which has the same or very similar properties to the reference beam.

[0006] Holographic data storage allows the use of the volume of a storage medium to increase data capacity. Compared to conventional optical storage systems, holographic storage has the potential to provide relatively high data density and short access times. Multiplexing techniques, such as angular and wavelength multiplexing, have been developed to increase the amount of data that may be stored in a holographic medium. However, present holographic data storage methods are still limited in their data capacity, as well as in the speed of writing, reading and copying.

[0007] Accordingly, there is a constant requirement to increase the data storage density and speed of writing and retrieval of holographic media.

[0008] Summary of the Disclosure

[0009] According to a first aspect of the present disclosure, there is described a method of storing data in a holographic medium, comprising: recording a plurality of holograms in one or more at least partially overlapping regions in a holographic medium, data being stored in a combination of the plurality of holograms.

[0010] Data may be stored and / or encoded in the holographic medium based on a combination of the plurality of holograms. The data may be stored and / or encoded across the plurality of holograms. The data may be stored and / or encoded in dependence on the (content of the) plurality of holograms. The data may be computer-readable data stored in a format. Preferably, stored data can only be accessed and understood based on the plurality of holograms, taken together as a combination.

[0011] The combination of holograms may be referred to as a ‘combined hologram’. The combined hologram may be formed of the plurality of holograms. By ‘formed’, it is meant that the combined hologram consists of the superposition of the plurality of the holograms.

[0012] The holograms may be recorded separately or simultaneously. The plurality of holograms may overlap; the overlap may be partial or complete (such that the plurality of holograms are recorded at the same location in the holographic medium).

[0013] The holograms may be recorded in a region of the volume of the medium. The holograms may be volume holograms (also referred to as ‘thick’ holograms), meaning they occupy a three-dimensional region within the volume of the medium, typically having a depth or thickness much larger than the wavelength of radiation used for recording. The holograms may instead be thin holograms, meaning they have a depth or thickness typically similar to the wavelength of radiation used for recording.

[0014] The holograms may be multiplexed holograms. As used herein, the term ‘multiplexing’ preferably connotes recording a plurality of holograms in a single region of the holographic medium by varying at least one recording parameter. Recording parameters that may be varied include, for example, angle, wavelength, phase, shift, and include combinations of parameters, e.g. both angle and wavelength. For example, angle multiplexing may involve varying the angle of the reference beam incident on the medium during recording to store a plurality of holograms in the same volume. The multiplexed holograms may be retrieved by using the same recording parameters used to record the respective holograms. The holograms are preferably wavelength multiplexed.

[0015] Preferably, each of the plurality of holograms stores one of a respective plurality of arrays, each of the arrays preferably comprising a plurality of array elements, data being stored in dependence on the plurality of array elements in each of the plurality of arrays. That is, the data may only be understood by reference to the plurality of arrays, more preferably the plurality of array elements in each of the plurality of arrays.

[0016] Preferably, data is stored and / or encoded in dependence on multiple elements in multiple arrays, which are stored across multiple holograms. The plurality of elements in each array preferably correspond, such that the plurality of elements in one of the plurality of arrays correspond to the plurality of elements in a different one of the plurality of arrays.

[0017] Holograms may be recorded at the same location, preferably such that when the holograms are illuminated by a reconstruction beam a first element in a first array reconstructed from a first hologram appears at the same location as a corresponding first element in a second array reconstructed from a second hologram.

[0018] Each of the elements in a given array preferably corresponds (in terms of location in the array) to an element in a different array.

[0019] Preferably, data is stored in dependence on a combination of the plurality of array elements in each of the plurality of arrays.

[0020] Preferably, each of the plurality of array elements of the array stored in one of the holograms has a value. The stored data may be defined by the combination of values across corresponding elements in each array (being stored by a respective hologram), more specifically by a plurality of said combinations of values.

[0021] Preferably, the value is defined by at least one data beam used to record the relevant hologram. That is, the content of the array stored by each hologram may be defined by the at least one data beam used to record the relevant hologram.

[0022] Preferably, the at least one data beam comprises a plurality of beam portions corresponding to the plurality of array elements. That is, the beam may be separated into portions across its beam width / diameter. The beam portions may be defined by a three-dimensional arrangement, such as a grid (thereby leading to the relevant array having a similar grid structure).

[0023] Preferably, the value of each of the plurality of array elements of the array stored in one of the holograms is defined by a corresponding beam portion, each of the beam portions comprising one of: radiation; or no radiation.

[0024] Preferably, the combination is formed of multiple elements across multiple arrays.

[0025] Preferably, data is stored in the form of a further array comprising a combination of the plurality of arrays, the further array comprising a plurality of further array elements. That is, the further array may store the ‘final’ data, i.e. the data including the content that a user intends to store. Each of the plurality of further array elements may relate to a combination of (preferably corresponding) array elements in different arrays.

[0026] The further array may also be referred to as a combined array or a multi -wavelength array. Each of the plurality of further array elements may each correspond to a data point having a data value. Each of the plurality of arrays may be referred to as a component array.

[0027] Preferably, the plurality of further array elements corresponds to the plurality of array elements of each of the arrays.

[0028] A further array element may be considered as effectively corresponding to an array element of each of the arrays. Preferably, when the holograms are illuminated by a reconstruction beam the plurality of array elements appear in the same location and the corresponding further array element is formed of their combination.

[0029] Preferably, the content of each of the plurality of further array elements is defined by the corresponding array elements in each of the arrays.

[0030] The content of the further array elements is preferably the value of each of the data points which each of the plurality of further array elements represent.

[0031] Preferably, wherein each of the plurality of array elements in each of the arrays represents one or more bits.

[0032] A bit is a binary value corresponding to either 0 or 1. An array element may correspond to 0 if when the hologram is replayed the array element is ‘dark’ and there is no radiation of the appropriate wavelength; an array element may correspond to 1 if when the hologram is replayed the array element is Tight’ and there is radiation of the appropriate wavelength. That is, if the relevant beam portion for the relevant array element includes radiation, the array element may correspond to 1, and if the relevant beam portion for the relevant array element does not include radiation, the array element may correspond to 0.

[0033] Further data values may be provided by modulating the intensity of radiation at each array element. For example, 4 intensity values may give 2 bits, 8 data values 3 bits, and so on.

[0034] Preferably, each of the further array elements are bit arrays being formed of the bits represented by the corresponding array elements in each of the arrays.

[0035] Preferably, the number of bits in the bit array is defined by the number of holograms and the number of bits per array element in each hologram. The bit arrays are preferably used as integers in data storage.

[0036] Preferably, at least one property of each of the plurality of holograms differs thereby to allow each of the plurality of holograms to be distinguished.

[0037] The holograms may be multiplexed, with potential types of multiplexing given above. This may allow holograms to be distinguished in a reading process.

[0038] Preferably, the method involves recording the plurality of holograms at a respective plurality of wavelengths.

[0039] Data may be stored / encoded in the holograms on the basis of the wavelengths of radiation which were recorded at each element. Each array stored in each hologram may be recorded at one of the plurality of wavelengths. The further array may comprise the combination of the wavelengths of the arrays. The data value stored in each further array element may be determined by a combination of wavelengths present in the further array element.

[0040] Preferably, the plurality of wavelengths comprises at least three wavelengths in the visible spectrum, preferably wherein the wavelengths are evenly spaced in the visible spectrum or wherein the at least three wavelengths are between 350nm and 500nm.

[0041] The wavelengths may include a red, a green and a blue wavelength (an RGB system). The wavelengths may include three wavelengths in the blue-violet region of the visible spectrum. Further wavelengths may be provided in the visible spectrum, or outside the visible spectrum.

[0042] Preferably, the plurality of wavelengths comprises at least one of: at least one wavelength in the visible spectrum, at least one wavelength in the infrared spectrum, at least one wavelength in the ultraviolet spectrum, at least one wavelength in the X-ray spectrum.

[0043] Preferably, the method involves recording the plurality of holograms at a plurality of radiation intensities.

[0044] Preferably, each of the array elements of the plurality of arrays has an associated one of the plurality of radiation intensities.

[0045] The plurality of radiation intensities may be variations in intensity of a signal beam(s) used to record the holograms. A plurality of intensities in each hologram may allow each array element of each array to store further data values, including additional bits. The plurality of holograms may be at a respective plurality of wavelengths and a plurality of radiation intensities, preferably where the radiation intensities vary across portions of the data beam, more preferably so as to provide a plurality of possible values for each of the plurality of array elements of the array stored in one of the holograms.

[0046] Preferably, the method further comprises: using one or more radiation sources, generating a plurality of radiation beams having a plurality of wavelengths; from the plurality of radiation beams, generating one or more signal beams and one or more reference beams; using one or more spatial light modulators, SLM, to produce from the one or more signal beams one or more data beams carrying data; directing the one or more data beams and the one or more reference beam at the holographic medium, thereby recording the plurality of holograms in the holographic medium.

[0047] Preferably, the one or more data beams correspond to (or are) the at least one data beams that define the value in each of the plurality of array elements of the array stored in one of the holograms.

[0048] The data encoded (or modulated) into the signal beams to produce the data beams may be the arrays. Each array may be carried by the one or more data beams at a different wavelength.

[0049] Preferably, each of the one or more SLMs receives a signal beam comprising a plurality of wavelengths simultaneously or consecutively. A single SLM may operate on radiation of a plurality of wavelengths. A single SLM may therefore modulate multiple arrays into a data beam or a plurality of data beams. The single SLM may be illuminated by the plurality of wavelengths simultaneously, producing a single data beam comprising a plurality of wavelengths (and a plurality of arrays). To allow this, the SLM preferably has a wavelength-selective filter. Alternatively, the single SLM may be illuminated by the plurality of wavelengths consecutively, producing a plurality of data beams consecutively, each data beam comprising a single array.

[0050] Preferably, each of a plurality of SLMs receives a signal beam comprising a single wavelength of radiation to produce one of a plurality of data beams.

[0051] Each SLM preferably receives only a single wavelength. A single array may be modulated into a single data beam by each SLM.

[0052] Preferably, the method further comprises combining the plurality of data beams into a combined data beam, and directing the combined data beam onto the holographic medium; more preferably wherein the data beams are combined using a dichroic element, yet more preferably a dichroic prism.

[0053] The combined data beam may carry the further array comprising a plurality of arrays at a plurality of wavelengths. The combined data beam may be used to record a plurality of holograms (and thus a plurality of arrays) onto a region of the holographic medium simultaneously.

[0054] Preferably, the method further comprises directing each of the data beams onto separate regions of the holographic medium.

[0055] Each hologram may be recorded separately. Each hologram may belong to a different plurality of holograms (to a different combined hologram). The holograms may be recorded onto non-overlapping regions of the holographic medium. Further pluralities of data beams may be further directed onto the holographic medium, such that each region receives a plurality of data beams and has the respective plurality of holograms (combined hologram) recorded thereon. The plurality of holograms may be recorded one by one on each region. Each data beam may comprise a different array at a different wavelength.

[0056] Preferably, the one or more radiation sources comprise one or more tunable radiation sources. As used herein, the term “tunable” preferably connotes that the wavelength of radiation produced by the radiation source may be changed.

[0057] Preferably, the one or more radiation sources are at least one of coherent, monochromatic, and collimated, preferably wherein the one or more radiation sources are lasers.

[0058] Preferably, the one or more radiation sources are continuous-wave radiation sources with beams controlled by shutters, or pulsed radiation sources.

[0059] Preferably, the one or more SLMs are reflective SLMs, preferably digital micromirror devices or liquid crystal on silicon devices, or transmissive SLMs, preferably liquid crystal devices.

[0060] Preferably, the method further comprises receiving the one or more data beams at an aperture, more preferably wherein the aperture is configmed to filter three or fewer orders of diffracted light, two or fewer orders of diffracted light, or one order of diffracted light.

[0061] Preferably, a plurality of signal beams and a corresponding plurality of reference beams are incident on the holographic medium, wherein each of the plurality of reference beams is incident on the holographic medium at a different angle to record angularly multiplexed data in the holographic medium.

[0062] Preferably, the plurality of holograms are recorded as reflection holograms, transmission holograms, co-axial reflection holograms, or co-axial transmission holograms.

[0063] Preferably, the method further comprises recording the plurality of holograms in one overlapping region. The holograms may be partially overlapping or entirely overlapping in the one overlapping region.

[0064] Preferably, method further comprises recording the plurality of holograms at the same location in the holographic medium.

[0065] The holograms may be coincident and overlap substantially or entirely.

[0066] Preferably, the method further comprises recording at least one further plurality of holograms in the holographic medium, further data being stored in a combination of the further plurality of holograms. The further plurality of holograms (or further combined hologram) may store further data. More preferably, the stored data is another plurality of arrays forming another further array.

[0067] Preferably, the method comprises recording the at least one further plurality of holograms at a different location in the holographic medium to the location of the plurality of holograms.

[0068] Preferably, the method further comprises recording a plurality of plurality of holograms in the holographic medium, the plurality of plurality of holograms including the plurality of holograms (and optionally the at least one further plurality of holograms).

[0069] The plurality of plurality of holograms may also be referred to as the plurality of combined holograms. The term ‘plurality of plurality of holograms’ may refer to a plurality of multiplexed or combined holograms being recorded at different locations. That is, in the term ‘plurality of plurality of holograms’, the second term ‘plurality’ may refer to multiple (at least partially) overlapping or multiplexed holograms, while the first term ‘plurality’ may refer to multiple instances of the second ‘plurality ’, preferably being at different locations in the holographic medium.

[0070] Preferably, the plurality of plurality of holograms are arranged in the holographic medium in a regular arrangement, preferably a grid arrangement, more preferably consisting of rows and columns.

[0071] Preferably, the method further comprises moving the holographic medium between a plurality of recording locations, thereby recording the plurality of plurality of holograms on the holographic medium.

[0072] Each of the plurality of plurality of holograms may be recorded simultaneously on each region, or may be recorded non-simultaneously, at different times. Holograms of different pluralities of holograms may be recorded simultaneously.

[0073] Preferably, the method further comprises moving the holographic medium continuously during recording and / or between recordings.

[0074] Preferably, the holographic medium is mounted on a first reel and a second reel, more preferably wherein at least one of the reels is removable, yet more preferably wherein at least one of the reels is comprised in a cartridge.

[0075] Preferably, the method further comprises moving the holographic medium by rotation of the first reel and second reel.

[0076] Preferably, the method further comprises encoding data into a data array capable of being represented by a combination of plurality of holograms.

[0077] Preferably, the plurality of holograms comprises at least two holograms, preferably at least three holograms, more preferably at least four holograms, most preferably at least five holograms.

[0078] According to a third aspect of the present disclosure, there is described a method of reading data stored in a holographic medium by the method of the first or second aspect, comprising: using one or more radiation sources, generating a plurality of radiation beams having the plurality of wavelengths; from the plurality of radiation beams, generating one or more reconstruction beams; directing the one or more reconstruction beams at the holographic medium to produce one or more data beams; reading the one or more data beams using a detector. Preferably, the detector is a CMOS sensor, a CCD sensor, or a camera.

[0079] According to a fourth aspect of the present disclosure, there is described a system implementing the method of the first, second, or third aspect. Preferably, the system further comprises at least one of: a lens, preferably wherein the lens is at least one of: collimating, converging, diverging, achromatic, convex, concave; a spatial filter; a beamsplitter; a dichroic filter; and a mirror, preferably a collimating or curved mirror.

[0080] According to a fifth aspect of the present disclosure, there is described a holographic medium comprising a plurality of holograms stored in one or more at least partially overlapping regions of the holographic medium, data being stored in a combination of the plurality of holograms, the holograms preferably being written thereon by the method of the first or second aspect. Preferably, the holographic medium is a photopolymer or comprises a photopolymer layer.

[0081] According to a sixth aspect of the present disclosure, there is described a method for storing data on a holographic medium comprising storing data in a hologram recorded at a plurality of intensity values. Preferably, an array comprising a plurality of array elements is stored in the hologram. Preferably, each of the array elements represents a plurality of data values, wherein the data value is determined by the intensity value at the corresponding array element.

[0082] The method may further comprise: using one or more radiation sources, generating a plurality of radiation beams having a plurality of wavelengths; from the plurality of radiation beams, generating one or more signal beams and one or more reference beams; using one or more spatial light modulators, SLM, to produce from the one or more signal beams one or more data beams carrying data; directing the one or more data beams and the one or more reference beam at the holographic medium to record a plurality of holograms in one or more at least partially overlapping regions in the holographic medium.

[0083] According to a seventh aspect of the present disclosure, there is described a method of writing data to a holographic medium, comprising: using one or more radiation sources, generating a plurality of radiation beams having a plurality of wavelengths; from the plurality of radiation beams, generating one or more signal beams and one or more reference beams; using one or more spatial light modulators, SLM, to produce from the one or more signal beams one or more data beams carrying data; directing the one or more data beams and the one or more reference beam at the holographic medium to record a plurality of holograms in one or more at least partially overlapping regions in the holographic medium.

[0084] Preferably, each of the one or more SLMs receives a signal beam comprising a plurality of wavelengths simultaneously or consecutively.

[0085] Preferably, each of a plurality of SLMs receives a signal beam comprising a single wavelength of radiation to produce one of a plurality of data beams.

[0086] Preferably, the method further comprises combining the plurality of data beams into a combined data beam, and the combined data beam is directed onto the holographic medium; more preferably wherein the data beams are combined using a dichroic element, yet more preferably a dichroic prism.

[0087] Preferably, the method further comprises directing each of the data beams onto separate regions of the holographic medium.

[0088] According to an eighth aspect of the present disclosure, there is described a method of storing data in a holographic medium, comprising: recording a hologram in the holographic medium using a data beam and a reference beam, each region of the data beam having one of at least three intensity values.

[0089] Preferably, the hologram is recorded at a single wavelength of radiation. Holograms recorded at a single wavelength of radiation in the above manner may be referred to as grayscale holograms.

[0090] Preferably, the hologram stores an array storing data. The array may be referred to as a grayscale array.

[0091] Preferably, the array comprises a plurality of array elements.

[0092] Preferably, each of the plurality of array elements corresponds to a region of the data beam.

[0093] Preferably, each of the plurality of array elements has a value.

[0094] Preferably, the value is defined by the data beam.

[0095] Preferably, the value is defined by the intensity value of the corresponding region of the data beam.

[0096] Preferably, each region of the data beam has four or more intensity values, five or more intensity values, six or more intensity values, or eight or more intensity values. Preferably, one of the intensity values corresponds to a zero or minimum intensity and one of the data values corresponds to a maximum intensity.

[0097] Preferably, a plurality of holograms storing a plurality of arrays are recorded on the holographic medium.

[0098] Preferably, data is stored in a combination of the plurality of holograms.

[0099] Preferably, the plurality of holograms are recorded on the holographic medium according to the seventh aspect.

[0100] According to a ninth aspect of the present disclosure, there is described a holographic storage medium shaped as tape. Optionally, the holographic data storage medium is formed as tape. The term “tape” as used herein preferably connotes a narrow strip of material and / or a piece of material which has a relatively large length and a relatively small width (and preferably a relatively small depth), preferably being suitable for winding onto a reel and preferably being relatively flexible. The term “data storage” as used herein preferably connotes storage of electronic, digital, and / or machine (computer)-readable data. As used herein, the term “holographic tape” is used synonymously with “a holographic data storage medium shaped (or formed) as tape”. According to an aspect of the present disclosure, there is described use of holographic tape in data storage.

[0101] Preferably, the holographic storage medium shaped as a tape has a thickness of 60pm or less.

[0102] Preferably, the holographic data storage medium further comprises a photosensitive layer and at least one substrate layer; preferably wherein the photosensitive layer comprises a photopolymer.

[0103] The photosensitive layer may have a thickness of 15pm or less, preferably 12pm or less, more preferably 10pm or less, more preferably 7pm or less, more preferably 5pm or less, more preferably 3pm or less.

[0104] Preferably, the at least one substrate layer comprises a polymer, preferably an optically clear polymer.

[0105] Preferably, the holographic data storage medium comprises a first substrate layer attached to a first side of the photosensitive layer and a second substrate layer attached to a second side of the photosensitive layer.

[0106] Preferably, the second substrate layer is removable from the holographic data storage medium.

[0107] Preferably, a width of the photosensitive layer is less than a width of the first substrate layer and the second substrate layer.

[0108] Preferably, the at least one substrate layer has a thickness of 35pm or less, preferably 30pm or less, more preferably 20pm or less, more preferably 10pm or less, more preferably 5pm or less, more preferably 3pm or less.

[0109] Preferably, the holographic data storage medium has the same width as Linear Tape-Open (LTO) tape; more preferably wherein the holographic data storage medium has a width of 12.65mm.

[0110] The holographic data storage medium may have a thickness of 100pm or less, preferably 80pm or less, more preferably 60pm or less, more preferably 40pm or less, more preferably 20pm or less, more preferably 15pm or less, more preferably 10pm or less, more preferably 5.2pm.

[0111] According to a tenth aspect of the present disclosure, there is provided a holographic tape cartridge for use with a holographic data storage system, comprising: a holographic data storage medium, preferably according to the ninth aspect; a reel, wherein a first end of the holographic data storage medium is attached to the reel; and a casing to hold the reel and holographic data storage medium.

[0112] Preferably, the holographic tape cartridge has a single reel. The holographic tape cartridge may have only one reel.

[0113] Preferably, a second end of the holographic storage medium comprises an attachment means, more preferably wherein the attachment means is a leader pin.

[0114] Preferably, the holographic tape cartridge further comprises at least one of: an opening for accessing the holographic data storage medium; and a mechanism allowing the reel to be rotated from the exterior of the casing.

[0115] Preferably, the cartridge has the same outer dimensions as a Linear Tape-Open (LTO) cartridge. The casing may have a width of 102.0mm by 105.4mm and a depth of 21.5mm.

[0116] According to an eleventh aspect of the present disclosure, there is provided a system for receiving a holographic tape cartridge comprising a holographic data storage medium, the holographic tape cartridge preferably being according the tenth aspect, the system comprising: a further reel to receive a second end of the holographic data storage medium; and means for recording holograms and / or reading holograms on the holographic storage medium within the holographic tape cartridge.

[0117] Preferably, the system further comprises an attachment arm to receive the second end of the holographic data storage medium, more preferably by means of a leader pin, and means for directing the second end of the holographic data storage medium onto the second reel, more preferably wherein the second end of the holographic data storage medium attaches to the second reel by means of the leader pin.

[0118] Preferably, the system further comprises at least one rolling element in contact with the holographic data storage medium.

[0119] Preferably, the means for recording holograms and / or reading holograms is configured to record and / or read holograms on a part of the holographic data storage medium extending between the reel and the further reel; more preferably a part of the holographic data storage medium extending between a rolling element and a further rolling element. Preferably, the means for recording holograms and / or reading holograms is configured to record and / or read holograms on the holographic data storage medium while holographic data storage medium is in motion, more preferably wherein the holographic data storage medium is in motion at a continuous speed. More preferably, a distance the holographic data storage medium moves during recording and / or reading of a hologram is equal to or less than one tenth (10%) of a wavelength of radiation used for recording and / or reading. The distance the holographic medium moves during recording and / or reading of a hologram may also be equal or less than 8%, 6%, 5%, 4%, 3%, 2%, or 1% of the wavelength of radiation used for the recording and / or reading operation.

[0120] Preferably, the system is adapted to write data on the holographic data storage medium, further comprising: a transport mechanism for transporting the holographic tape; a motion detector configured to detect motion of the tape caused by the transport mechanism; a controller; a laser; and a spatial light modulator, SLM; wherein the controller is configured to receive a signal from the motion detector indicating that the tape has moved a set distance and further configmed to cause the activation of the laser and SLM as a result.

[0121] Preferably, the system is adapted to read data on the holographic data storage medium, further comprising: a transport mechanism for transporting the holographic tape; a motion detector configmed to detect motion of the tape caused by the transport mechanism; a controller; a laser and a sensor; wherein the controller is configured to receive a signal from the motion detector indicating that the tape has moved a set distance, and further configmed to cause the activation of the laser and sensor as a result.

[0122] Preferably, the SLM is configmed to be deactivated during reading of data and wherein the sensor is configured to be deactivated during writing of data.

[0123] Preferably, wherein the motion detector is a rotational motion detector, or an optical motion detector configured to read markings on the tape.

[0124] According to a twelfth aspect of the present disclosure, there is described a holographic tape library comprising: a plurality of slots each for holding a respective one of a plurality of holographic tape cartridges, each holographic tape cartridge comprising holographic tape; and a transport mechanism for transporting each such cartridge between its respective slot and a system for reading and / or writing data to the holographic tape on each such holographic tape cartridge.

[0125] Preferably, the holographic tape library further comprises the system for reading and / or writing data to the holographic tape on each such holographic tape cartridge, preferably wherein the system for reading and / or writing data to the holographic tape on each such holographic tape cartridge is a system according to the eleventh aspect.

[0126] Preferably, the system of the eleventh aspect or the tape library of the twelfth aspect further comprises at least one holographic tape cartridge according to the tenth aspect and / or at least one holographic data storage medium according to the ninth aspect.

[0127] According to a thirteenth aspect of the present disclosure, there is described a holographic data storage device comprising a reel, and a holographic data storage medium according to the ninth aspect.

[0128] Preferably, the holographic data storage medium comprises a first end attached to the reel.

[0129] The holographic data storage medium may comprise a second end, preferably the second end comprising an attachment means, more preferably wherein the attachment means is a leader pin.

[0130] Preferably, the holographic data storage device further comprises a casing to house the reel and holographic data storage medium.

[0131] Preferably, the casing forms a cartridge for use with a holographic data storage system, more preferably wherein the cartridge comprises at least one of: an opening for accessing the holographic data storage medium; and a mechanism allowing the reel to be rotated from external to the casing.

[0132] Preferably, the cartridge has the same outer dimensions as a Linear Tape-Open (LTO) cartridge, more preferably wherein the casing has a width of 102.0mm by 105.4mm and a depth of 21.5mm.

[0133] According to an fourteenth aspect of the present disclosure, there is described a system comprising a holographic data storage device according to the thirteenth aspect, further comprising a further reel to receive a second end of the holographic data storage medium. Preferably, the system further comprises an attachment arm to receive the second end of the holographic data storage medium, more preferably by means of a leader pin, and means for directing the second end of the holographic data storage medium onto the second reel, more preferably wherein the second end of the holographic data storage medium attaches to the second reel by means of the leader pin.

[0134] Preferably, the system further comprises at least one rolling element in contact with the holographic data storage medium.

[0135] Preferably, the system further comprises means for recording holograms on the holographic data storage medium. Holograms may be recorded in a regular arrangement, preferably a grid arrangement, more preferably consisting of rows and columns. That is, holograms may be recorded across both the length and the width of the holographic data storage medium. The system may comprise means for transporting the holographic data storage medium relative to the means for recording holograms. Preferably, the means for transporting the holographic data storage medium is configmed to transport the holographic data storage medium along its length, more preferably wherein the means for transporting the holographic data storage medium is a reel-to-reel transport arrangement. A further means for transporting the holographic data storage medium relative to the means for recording holograms may be provided, preferably wherein said further means is configmed to transport the holographic data storage medium relative to the means for recording holograms along the width of the holographic data storage medium; more preferably wherein the further means is configmed to move the means for recording holograms along the width of the holographic data storage medium. Alternatively, the further means is configmed to move the holographic data storage medium, for example wherein the further means comprises a movable stage. The means for recording holograms may be a means for producing a data beam.

[0136] Preferably, the means for recording holograms is configured to record holograms on a part of the holographic data storage medium extending between the reel and the further reel; more preferably a part of the holographic data storage medium extending between a rolling element and a further rolling element.

[0137] Preferably, the means for recording holograms is configured to record holograms on the holographic data storage medium while the holographic data storage medium is in motion between the first reel and the second reel.

[0138] According to a fifteenth aspect of the present disclosure, there is provided a system comprising: a plurality of holographic data storage media, the plurality of holographic data storage media being the same form of media; means for copying data from at least one of the plurality of holographic data storage media to a different at least one of the plurality of holographic data storage media; and means for transporting the plurality of holographic data storage media relative to the means for copying.

[0139] Preferably, the plurality of holographic data storage media are each shaped as tape.

[0140] According to a sixteenth aspect of the present disclosure, there is provided a system comprising: a plurality of holographic data storage media according to the nineth and means for copying data from at least one of the plurality of holographic data storage media to a different at least one of the plurality of holographic data storage media.

[0141] Preferably, the system comprises means for transporting the plurality of holographic data storage media relative to the means for copying.

[0142] Preferably, the means for copying data comprises at least one copy beam; wherein the at least one copy beam is configmed to cause data to be copied from at least one of the plurality of holographic data storage media to a different at least one of the plurality of holographic data storage media.

[0143] Preferably, the system comprises means operable to produce at least one copy beam.

[0144] Preferably, the means for transporting the holographic data storage medium is configured to transport the holographic data storage medium along its length, more preferably wherein the means for transporting the holographic data storage medium is a reel-to-reel transport arrangement. A further means for transporting the holographic data storage medium relative to the means for copying may be provided, preferably wherein said further means is configured to transport the holographic data storage medium relative to the means for copying along the width of the holographic data storage medium; more preferably wherein the further means is configured to move the means for copying along the width of the holographic data storage medium. Alternatively, the further means is configured to move the holographic data storage medium, for example wherein the further means comprises a movable stage. Preferably, the means for transporting is operable to transport the plurality of holographic data storage media simultaneously; more preferably synchronously.

[0145] Preferably, the means for copying is configured to operate on the plurality of holographic data storage media while the plurality of holographic data storage media is being transported by the means for transporting.

[0146] Preferably, the means for transporting comprises a plurality of transport mechanisms each being operable to transport a respective one of the plurality of holographic data storage media.

[0147] Preferably, the plurality of transport mechanisms comprise a respective plurality of reels; wherein each of the reels is attached to an end of one of the plurality of holographic data storage media.

[0148] Preferably, at least one of the reels is housed in a cartridge and is attached to an end of a respective one of the plurality of holographic data storage media, such that the attached holographic data storage medium is capable of being stored in the cartridge.

[0149] Preferably, the cartridge is removable from the system.

[0150] Preferably, at least one of the reels and a respective at least one holographic data storage medium form a holographic data storage device according to the thirteenth aspect.

[0151] Preferably, each of the plurality of transport mechanisms comprises a further reel, wherein each of the plurality of holographic data storage media extends between a respective reel and a respective further reel.

[0152] Preferably, the means for copying is configured to operate on parts of the plurality of holographic data storage media extending between the reel and the further reel; more preferably parts of the plurality of holographic data storage media extending between a rolling element and a further rolling element.

[0153] Preferably, the means for transporting is configured to hold the plurality of holographic data storage media in suitable proximity for copying data; more preferably wherein the means for transporting is configmed to hold the plurality of holographic data storage media in contact with each other.

[0154] Preferably, the means for transporting comprises at least one guide component.

[0155] Preferably, the at least one guide component is movable thereby to move at least one of the plurality of holographic data storage media into a position in which the plurality of holographic data storage media me in suitable proximity for copying data; more preferably in contact.

[0156] Preferably, the at least one of the plurality of holographic data storage media has stored thereon multiplexed data that is recorded by at least one of: wavelength multiplexing, angular multiplexing, peristrophic multiplexing, shift multiplexing, and spatial multiplexing.

[0157] Preferably, the means for copying data is configmed to copy the multiplexed data from at least one of the plurality of holographic data storage media to a different at least one of the plurality of holographic data storage media.

[0158] Preferably, the at least one of the plurality of holographic data storage media has stored thereon peristrophically multiplexed data, and wherein the peristrophically multiplexed data is copied from the at least one of the plurality of holographic data storage media to a different at least one of the plurality of holographic data storage media by a beam having the same size, angle and aspect as a data beam used to record the peristrophically multiplexed data.

[0159] Preferably, the peristrophically multiplexed data is recorded as reflection holograms or as transmission holograms.

[0160] Preferably, data is both shift multiplexed and peristrophically multiplexed.

[0161] The plurality of holographic data storage media may comprise a master holographic data storage medium; and at least one copy holographic data storage medium.

[0162] Preferably, the plurality of holographic data storage media comprise a master holographic data storage medium; and a holographic data storage medium.

[0163] Preferably, the plurality of holographic data storage media comprise a master holographic data storage medium; and a plurality of copy holographic data storage media.

[0164] Preferably, the at least one copy beam is partially diffracted by the holographic data stored in the master holographic medium to produce at least one data beam; wherein the at least one data beam interferes with the at least one copy beam at the at least one copy holographic medium to reproduce the holographic data into the copy holographic medium.

[0165] Preferably, the master holographic data storage medium stores at least one hologram for storing data.

[0166] Preferably, the at least hologram is each formed of a combination of a plurality of component holograms, data being stored in a combination of the plurality of holograms; preferably wherein the plurality of holograms are recorded in one or more overlapping regions in the master holographic data storage medium.

[0167] Preferably, the plurality of component holograms are recorded at a respective plurality of different wavelengths.

[0168] Preferably, the master holographic data storage medium stores a plurality of holograms for storing data, wherein the plurality of holograms are stored in a regular arrangement, more preferably a grid arrangement, yet more preferably consisting of rows and columns.

[0169] Preferably, a width of the at least one copy beam is configured to be one of: equal to; equal to an integer multiple of; and greater than one of: a diameter of each of the holograms; and a pitch of the plurality of holograms.

[0170] Preferably, a size of the copy beam may be larger, preferably only slightly larger, than a data and / or reference beam used to record the original data, or the same size as the data and / or reference beam.

[0171] Preferably, the at least one copy beam is continuously incident on the master holographic medium and copy holographic medium.

[0172] Preferably, the at least one copy beam is incident on the master holographic medium and copy holographic medium in pulses, more preferably when the pulses are produced when the at least one copy beam is aligned with one or more holographic recordings stored on the master holographic medium.

[0173] Preferably, each of the plurality of copy beams comprises radiation of different wavelengths.

[0174] Preferably, the plurality of copy beams is incident onto the master holographic medium and copy holographic medium at different angles, thereby to copy angularly multiplexed holographic data stored in the master holographic medium.

[0175] Preferably, the plurality of copy beams are non-overlapping, more preferably wherein a pitch of the plurality of copy beams is equal to a pitch of a plurality of holographic recordings on the master holographic medium.

[0176] Preferably, a breadth of the copy beam is equal to or greater than a width of the plurality of holographic data storage media.

[0177] Preferably, the copy beam has a different profile to a reference beam used to record holographic data on the master holographic medium, more preferably wherein the copy beam may be wider than the reference beam.

[0178] Optionally, the copy beam is incident onto the master holographic medium before the copy holographic medium.

[0179] Optionally, the copy beam is incident onto the copy holographic medium before the master holographic medium.

[0180] Preferably, the copy beam is produced by a radiation source that may be one or more of: coherent, monochromatic, collimated, more preferably wherein the copy beam may be produced by a laser, yet more preferably a continuous diode or a pulsed laser.

[0181] Preferably, the system further comprises at least one radiation source to produce at least one radiation beam, more preferably wherein the at least one radiation beam is at least one of: monochromatic, coherent, and collimated.

[0182] Preferably, the system further comprises: means for producing at least one signal beam and at least one reference beam from the at least one radiation beam; at least one spatial light modulator to encode data in the at least one signal beam to produce at least one modulated signal beam; and means for directing the at least one reference beam and the at least one modulated signal beam onto the holographic data storage medium thereby to record data onto the holographic data storage medium.

[0183] Preferably, the system further comprises: means for producing a plurality of signal beams; a plurality of spatial light modulators to produce a plurality of modulated signal beam elements; and one or more dichroic elements to combine the plurality of modulated signal beams into a single combined signal beam. Preferably, the system further comprises: means for producing a plurality of signal beams and a plurality of reference beams; a plurality of spatial light modulators to produce a plurality of modulated signal beams; means for directing each modulated signal beam onto a different region of the holographic data storage medium, and a reference beam onto each same region.

[0184] Preferably, the system further comprises means for reading holograms from the holographic data storage medium.

[0185] Preferably, the means for reading holograms comprises: means for producing at least one reconstraction beam; means for directing the at least one reconstruction beam at a holographic data storage medium storing data thereby to reconstruct the stored data; a detector to read the reconstructed stored data.

[0186] Preferably, the holographic data storage medium is in motion when illuminated by the at least one reference beam, the at least one signal beam, and / or the at least one reconstruction beam, preferably wherein the motion of the holographic data storage medium is provided by rotation of the first reel and second reel.

[0187] Preferably, the reference beam is incident on the holographic data storage medium from a plurality of angles to read and / or write a plurality of angle multiplexed holograms.

[0188] Preferably, at least one of the radiation beams has a wavelength in the visible spectrum, the infrared spectrum, the ultraviolet spectrum, or the X-ray spectrum.

[0189] Preferably, a plurality of radiation beams having a plurality of wavelengths are produced by the at least one radiation source to read and / or write a plurality of wavelength multiplexed holograms.

[0190] According to a seventeenth aspect of the disclosure, there is provided a system and method for recording data onto a holographic medium whilst the holographic medium is in motion and / or for reading data from a holographic medium whilst the holographic medium is in motion. Preferably, the holographic medium is moved at a constant speed during recording and / or reading a plurality of consecutive holograms on the holographic medium. Preferably, the holographic medium is not stopped or slowed down between recording and / or reading consecutive holograms. Preferably, recording and / or reading each of the consecutive holograms corresponds to a pulse of a laser. Preferably, the distance the holographic medium moves during recording and / or reading each of the consecutive holograms is equal to or less than one tenth (10%) of the wavelength of radiation used the recording and / or reading. The distance the holographic medium moves during recording and / or reading each of the consecutive holograms may also be equal or less than 8%, 6%, 5%, 4%, 3%, 2%, or 1% of the wavelength of radiation used for the recording and / or reading operation.

[0191] Any feature in one aspect of the disclosure may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

[0192] Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory.

[0193] It should also be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and / or supplied and / or used independently.

[0194] The disclosure extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings.

[0195] As used herein, the terms "signal beam" and “data beam” preferably connotes to a beam used to transmit a data signal. This may include a beam that has been modulated by a modulator such as a spatial light modulator (SLM) and which consequently carries data, or a beam that is incident on a spatial light modulator.

[0196] As used herein, the term “radiation detector,” “light sensor" or “detector” preferably connotes any type of device capable of detecting the presence or intensity of electromagnetic radiation. For example, such a device may be a camera or quad cell, complementary metal-oxide-semiconductor (CMOS) imaging sensors or arrays, charge -coupled device (CCD) arrays, or any other suitable device.

[0197] As used herein, the terms "holographic grating", "holograph" or "hologram" preferably connote an interference pattern formed by interference of a data / signal beam and a reference beam. The data / signal beam may carry data encoded into the beam with a data modulator such as a spatial light modulator. As used herein, the term “holographic medium” or "holographic storage medium" preferably connotes a medium that is capable of storing one or more holograms as patterns of varying refractive index in the medium in at least one layer, material, or other constituent element (such as a photopolymer layer or element) of the medium. Examples of a holographic medium which may be used include but are not limited to photopolymers such as Bayfol HX (produced by Covestro GmbH, Leverkusen Germany) and silver halide film coatings (such as those produced by Harman Technology Ltd. Mobberley UK).

[0198] As used herein, the term "radiation source" or “light source” preferably connotes a source of electromagnetic radiation having a single wavelength or multiple wavelengths. The radiation source may be a laser, one or more light emitting diodes (LEDs), or any other suitable device.

[0199] As used herein, the term "spatial light modulator" (SLM) preferably connotes a device that modulates abeam to produce a beam carrying data or information. The SLM may modulate the beam by, for example, modulating the amplitude, spatial intensity and / or phase profile of the beam.

[0200] The disclosure will now be described, by way of example, with reference to the accompanying drawings. Description of the Drawings

[0201] Figure la shows apparatus for holographically recording data as a multi-wavelength array on a holographic medium using a transmissive spatial light modulator (SLM);

[0202] Figure lb shows apparatus for holographically recording data as a multi-wavelength array on a holographic medium using a reflective SLM;

[0203] Figure 2a and 2b show details of data stored in a holographic medium by a multi-wavelength array;

[0204] Figure 3 shows holographic data storage apparatus for recording multi-wavelength arrays on a holographic medium using a plurality of SLMs;

[0205] Figure 4 shows an alternative embodiment of holographic data storage apparatus for recording multiwavelength arrays on a holographic medium using a plurality of SLMs;

[0206] Figure 5a shows holographic data storage apparatus for recording a plurality of component arrays on a plurality of regions of the holographic medium simultaneously;

[0207] Figure 5b shows the process of shifting the holographic medium of Figure 5a to record a plurality of arrays;

[0208] Figure 6 shows apparatus suitable for reading multi-wavelength arrays from a holographic medium;

[0209] Figure 7 shows alternative apparatus suitable for reading multi-wavelength arrays from a holographic medium;

[0210] Figure 8 shows apparatus suitable for recording intensity -modulated arrays in a holographic medium;

[0211] Figure 9 shows the holographic medium comprising holographically stored intensity-modulated arrays;

[0212] Figure 10 shows apparatus suitable for recording intensity-modulated multi-wavelength arrays in a holographic medium;

[0213] Figure Ila and 11b show details of data stored in a holographic medium by an intensity-modulated multiwavelength array;

[0214] Figures 12a and 12b show an alternative embodiment of a holographic data storage apparatus for recording multi-wavelength arrays on a holographic medium;

[0215] Figure 13 shows a digital micro mirror device used as an SLM;

[0216] Figure 14 shows the operation of a Fourier filter on the signal beam;

[0217] Figure 15 shows holographic data storage apparatus for recording a multi- wavelength array on a holographic medium as a transmission hologram;

[0218] Figure 16 shows holographic data storage apparatus for recording a multi-wavelength array on a holographic medium as a reflection hologram and retrieving the array using a detector;

[0219] Figure 17 shows holographic data storage apparatus for recording a multi-wavelength array on a holographic medium as a transmission co-axial hologram;

[0220] Figure 18 shows holographic data storage apparatus for recording a multi- wavelength array on a holographic medium as a reflection co-axial hologram;

[0221] Figure 19 shows arrangements for storing a plurality of multi-wavelength arrays on a holographic medium; Figure 20 shows a holographic medium to store a plurality of angularly multiplexed arrays; and

[0222] Figures 21a and 21b show a method of retrieving holographically stored data from the holographic medium;

[0223] Figure 22 shows a holographic storage device comprising a holographic storage medium on a reel;

[0224] Figure 23 shows a cartridge for holographic storage comprising a holographic storage medium on a reel;

[0225] Figure 24 shows apparatus suitable for writing and / or reading holographic data on a cartridge;

[0226] Figures 25a-e show the structure and operation of engagement apparatus for take-up of a holographic storage medium from a cartridge;

[0227] Figures 26, 27 and 28 show examples of encoders that can be used with a holographic storage medium;

[0228] Figure 29 and 30 show an encoding layer in a holographic storage medium;

[0229] Figures 3 la and 3 lb show the structure of a holographic storage medium;

[0230] Figure 32 shows a reel-to-reel system for contact copying holograms from a master medium to a copy medium;

[0231] Figure 33 shows the process of writing original holograms on a master medium;

[0232] Figures 34a and 34b show arrangements for contact copying holograms by transmission and reflection through the master medium;

[0233] Figures 35a, 35b and 35c shows the relation between possible angles of a copy beam and to the angle of an original reference beam;

[0234] Figures 36a, 36b, 36c, 36d and 36e shows possible copy beam configmations;

[0235] Figure 37 shows a plurality of copy beams used to copy angularly multiplexed holographic data;

[0236] Figures 38a and 38b show a reel-to-reel system for copying data stored on holographic tape cartridges;

[0237] Figure 39 shows an arrangement for copying to a plurality of copy media;

[0238] Figures 40a, 40b, 40c, 40d, 40e and 40f show methods of recording, copying and retrieving transmission shift-peristrophically multiplexed recordings;

[0239] Figures 41a, 41b, 41c and 41d show methods of recording, copying and retrieving reflection shift- peristrophically multiplexed recordings;

[0240] Figure 42 shows a system for reading and writing holograms on a holographic tape;

[0241] Figure 43 shows details of the tape system of Figure 42;

[0242] Figures 44a and 44b show a system for reading and writing holograms on a holographic tape within a server rack;

[0243] Figure 45 shows an optical arrangement that may be used with a holographic tape system;

[0244] Figure 46 shows a flow diagram for initialising a holographic system for reading and / or writing data on holographic tape;

[0245] Figure 47 shows a flow diagram for writing data on holographic tape using a holographic system; and Figure 48 shows a flow diagram for reading data from holographic tape using a holographic system.

[0246] Detailed description

[0247] Data storage

[0248] Referring to Figure la, there is shown apparatus 100 for recording data in a holographic medium (which may also be referred to as a holographic storage medium). The apparatus comprises three monochromatic lasers 102a, 102b, 102c, wherein each laser produces light of a different wavelength. The three lasers produce three coherent, monochromatic laser beams 104a, 104b, 104c. Light from the lasers is controlled by shutters 124. The lasers may produce light from the human visual spectrum - for example, one laser may produce light in the red portion of the visual spectrum, another in the green portion of the visual spectrum, and another in the blue portion of the visual spectrum (also referred to as an RGB system). Alternatively, other (e.g. narrower) bands of the visual spectrum, or radiation outside of the visual spectrum, may be used.

[0249] The three laser beams 104a, 104b, 104c pass through dichroic filters 106 to form a single combined beam comprising all three component wavelengths produced by the lasers. For example, if an RGB system is used, the combined beam is a white beam. The combined beam passes through beamsplitter 108, which separates the beam into a signal beam 110 and a reference beam 112. The beamspliter 108 may be a partially silvered mirror, a dielectric beam splitting mirror, or a cube beamspliter. The beamspliter 108 may also be a variable beamspliter. This may be achieved, for example, by providing a movable waveplate in front of a polarisation dependent beamspliter, which splits light at a ratio according to the polarisation of incident light; the waveplate may be moved to alter the polarisation of light incident on the beamspliter, which changes the ratio of intensities of the beams produced by the beamspliter.

[0250] The signal beam 110 and reference beam 112 are directed onto the holographic medium 130, to write holograms onto the holographic medium. The holographic medium 130 is preferably a thin sheet or tape and is preferably a photopolymer or comprises a photopolymer layer. The storage medium 130 may comprise a plurality of layers, such as a protective polymer layer above and below the photosensitive layer.

[0251] To allow writing of holograms onto the storage medium 130, the reference beam 112, which does not contain any information, is directed by mirrors 114 onto the holographic medium 130, whilst the signal beam 110 is directed along an optical path where data is encoded into the signal beam 110 (the signal beam with encoded data may be referred to as a data beam). The signal beam may be controlled by a shuter 126, which allows the signal beam 110 to be blocked and only the reference beam 112 to be incident on the holographic medium. This allows the apparatus to be switched between a reading more and a writing mode, where both beams are provided for the writing mode and only the reference beam is provided for use as a reconstruction beam in the reading mode. Following the shuter 126, the signal beam 110 then passes through a spatial filter 116 to improve its homogeneity and remove any aberrations that may be present in its wavefront. The spatial filter 116 produces a diverging beam, for example such that the signal beam has a wavefront of similar dimensions to the spatial light modulator 120. After the spatial filter 116, the signal beam 100 passes through a collimator 118, such as a collimating lens or collimating lens assembly, which produces a collimated beam.

[0252] The collimated signal beam is incident on the spatial light modulator (SLM) 120. The SLM 120 shown in Figure la is a transmissive SLM, which modulates a light beam that is transmited through the SLM. The SLM may be a liquid crystal display (LCD) or any other suitable device. The SLM 120 comprises a grid of pixels which selectively transmit light from the signal beam into the optical path. In possible other embodiments, such as that shown in Figure lb, the SLM may be a reflective SLM, which reflects incident light rather than transmiting it.

[0253] The SLM 120 is used to modulate the signal beam with data. The SLM comprises a grid of pixels, and at each pixel the SLM controls whether light is transmited through. Each pixel comprises a component (also referred to as a sub-pixel) corresponding to light of each wavelength, and a wavelength-selective filter (which may also be referred to as a colour selective filter or a colour filter) which allows through only light of that wavelength at that subpixel. Therefore, the SLM can control which wavelengths of light are transmited through at each pixel. The wavelength-selective filter may be a Bayer filter for an RGB system, for example.

[0254] The SLM modulates the signal beam to encode data into the signal beam, producing a data beam 111. The SLM produces a plurality of arrays (which may also be referred to as component arrays), each of the plurality of arrays having a different wavelength. The plurality of arrays combined form a further array (also referred to as a multiwavelength array). The data beam 111 carries the multi- wavelength array produced by the effect of the SLM on the signal beam. The multi-wavelength array comprises a grid of pixels. Each pixel has a plurality of components corresponding to the plurality of wavelengths in the signal beam. The multi-wavelength array in this way is formed of a plurality of component arrays, each component array having a single wavelength, overlaid on one another. Each component array comprises a grid of pixels corresponding to the pixels of the other component arrays and the pixels of the multi -wavelength array. Each pixel stores data in a binary format. Each pixel of a component array has a value corresponding to 0 or 1 depending on whether light of that wavelength is present in the pixel or not. This is modulated into the signal beam by the SLM by the SLM blocking light at pixels which have a value of 0, and allowing transmission of light at pixels which have a value of 1. Data is stored in the multi-wavelength array across all three wavelengths. For example, if each pixel of each component array stores one bit of data, each pixel of the multiwavelength array stores 3 bits of data. Further values may be provided by intermediate intensities in which only a portion of light is allowed through, as further described below with reference to Figures 8-11. Following passage through the SLM 120, the data beam 111 passes through a focussing lens or lens array 122, which focusses the data beam 111 onto the storage medium 130, such that the data beam 111 has a small crosssection when incident onto the storage medium. This may advantageously improve interference with the reference beam and reduce the size of the holographic recording. The storage medium is preferably placed in the focal plane of the lens 122.

[0255] The focussing lens 122 is preferably achromatic, such that the lens has the same or a substantially similar focal length for each of the component wavelengths used. If the lens has a high variation in focal length for each wavelength, this may cause chromatic aberration and result in the multiple component arrays of the multi-wavelength array being misaligned when the multi-wavelength array is retrieved, potentially causing difficulties in reading data. It may be easier to achieve a similar focal length for multiple wavelengths when the wavelengths in a narrower band of the electromagnetic spectrum (for example, three laser sources in the blue region of the visual spectrum). The focussing lens 122 also advantageously has a high numerical aperture (preferably between 0.8 and 1.0, or 1.0 or higher, particularly if a medium other than air is used) to focus light onto the holographic medium 130. The lens 122 may be a lens assembly comprising a plurality of lenses.

[0256] The reference beam 112, which is divided from the signal beam 110 at the beamsplitter 108, is reflected through mirrors 114 onto the holographic medium 130, such that it is incident on the same region of the storage medium 130 as the signal beam 110. The signal beam 110 and reference beam 112 form an interference pattern on the storage medium 130, and this interference pattern is recorded in the storage medium 130 as variations in the refractive index of the medium. In this way, a plurality of holograms, each having a different wavelength and each storing data in the form of an array is recorded onto the holographic medium. The holograms are stored as interference fringes in the medium, and the interference fringes are formed by variations in the refractive index of the medium. The interference fringes forming the hologram may also be referred to as a grating, or for a volume hologram, a Bragg grating.

[0257] The plurality of holograms is stored in one or more at least partially overlapping regions of the holographic medium, such that there is overlap between the holograms. Preferably, the plurality of holograms are recorded onto one overlapping region of the medium, and even more preferably the holograms are recorded at the same location in the holographic medium, such that they entirely or substantially overlap. Data is stored in a combination of the plurality of holograms (which may also be referred to as component holograms); the combination of the plurality of holograms may also be referred to as a set of holograms or a multi-wavelength hologram. Each of the component holograms stores a component array, and the plurality of hologram together stores the multi-wavelength array having data stored therein by a combination of wavelengths.

[0258] For each hologram to be recorded onto the holographic medium 130, the medium 130 is illuminated by the signal and reference beams for a set time period corresponding to an exposure time. For continuous wave lasers, this process comprises opening the shutters 124 to provide the signal and reference beams incident on storage medium 130 and closing the shutters 124 after the exposure time. For such a continuous wave laser controlled by shutters 124, the exposure time is preferably between 0.05ps and 50ps. Alternatively, a pulsed laser may be used, which is directly controlled to provide an exposure pulse without requiring the use of shutters. For a pulsed laser, the exposure time may be lower, for example anywhere between 5fs and several hundred nanoseconds.

[0259] Following a first exposure, the holographic medium 130 may be moved to align a second region of the holographic medium 130 with the signal beam 110 and reference beam 112. The shutters may then be opened (or pulsed lasers activated) to provide a second exposure to record a second, different, plurality of holograms storing further data in the second region of the holographic medium 130.

[0260] The holographic medium 130 is mounted on a supply (or feed) reel 132 and a take-up reel 134. The holographic medium is initially wound onto the supply reel 132. The holographic medium is preferably in the form of a tape on the reels. The supply reel 132 may be the reel of a cartridge, which is inserted into the holographic storage apparatus, as is further described with reference to Figures 22-24. In this case the holographic medium has a first end attached to the supply reel and a second end that is free, and the holographic storage apparatus 100 may comprise a feeding mechanism of the storage apparatus to take up the free end of the holographic medium and feeding it through the apparatus to attach to the take-up reel 134. Further details of the structure and format of a reel-to-reel system and a holographic tape, as well as a suitable take-up mechanism, are described below with reference to Figures 22-25.

[0261] The reels 132 and 134 rotate, driven by a motor, to allow different regions of the medium to be illuminated by the signal beam 110 and reference beam 112. The passage of the holographic medium 130 may be assisted by rollers 136, which may help to hold the holographic medium 130 in a suitable position for reading or writing of holograms, and rotate with the motion of the holographic medium (where they may be driven by a motor or may rotate passively).

[0262] The movement of the holographic medium is achieved through rotation of reels 132 and 134 (where the rotation of reels 132 and 134 may be driven by a motor or any other suitable means). This causes the holographic medium 130 to move relative to the data and reference beams 110 and 112 (which may be collectively referred to as the recording beams), and aligns different areas of the holographic medium to the recording beams. This allows holograms to be written onto different regions of the holographic medium, through a process called shift multiplexing.

[0263] Preferably, the holographic medium 130 is kept in continuous motion which continues both between exposures and during each exposure period, without the medium 130 stopping during each exposure. To achieve this, the speed of the holographic medium is adjusted compared to the exposure time. The distance moved by the reel during an exposure is suitably a value equal to less than one tenth of the wavelength of the radiation incident on the holographic medium (based on experimental testing). If beams of multiple wavelengths are incident on the holographic medium, the relevant wavelength is the shortest of the wavelengths incident; for example, for an RGB system, the distance is less than one tenth of the wavelength of red light (for example, for a red beam of 700nm, the storage medium 130 moves less than 70nm during each exposure).

[0264] To record holograms onto the holographic medium 130 when the reel is in continuous motion, pulses of the recording beams are incident onto the holographic medium at even intervals, suitably timed such that the holographic medium has moved a sufficient distance for the resulting holograms not to overlap. For example, for a hologram pitch of 1.25mm (which may be used for a hologram diameter of 1.00mm) between the pluralities of holograms, each exposure is provided when the holographic medium has moved 1.25 mm from the position of the previous exposure.

[0265] Alternatively, the holographic medium may not be kept in continuous motion, but may be moved in a stepwise manner, with the medium 130 being stopped during each recording and moved inbetween recordings. This may be suitable for longer exposure periods, which may cause distortion of holograms if the holographic medium is kept in continuous motion. However, such a stepwise method may increase the time taken to record onto the medium, as the reels have to be continually stopped and restarted. In a further alternative, since the distance moved by the storage medium 130 between each exposure is generally much less than the distance moved by the storage medium during each exposure, the speed of the holographic medium may be varied by speeding up the motion of the medium between exposures and slowing the medium down during exposures.

[0266] Once the desired number of recordings is made, or the end of the holographic medium is reached, the direction of rotation of the reels 132 and 134 may be reversed, causing accumulated holographic medium 130 to unwind from the take-up reel 134 and back onto the supply reel 132. The medium may be disengaged from the takeup reel 134, allowing the supply reel 132 with the wound written holographic medium 130 to be removed from the holographic storage apparatus 100. For example, if the supply reel 132 is on a cartridge, the cartridge may be removed from a mounting system.

[0267] Whilst a reel-to-reel medium system is shown here, in alternative embodiments the holographic medium may be provided in a different format. For example, the holographic medium may be provided as a disk (where holograms are recorded on different areas of the disk by rotation of the disk), as a flat sheet on a linear translation stage, or any other suitable method that allows movement of the holographic medium past the signal and reference beams 110 and 112.

[0268] The above process produces a row of holograms recorded along the length of the holographic medium 130. To utilize the width of the holographic medium, multiple rows may be recorded on the medium, producing a grid with a plurality of rows and columns of holograms (such as that seen in Figures 2a and 2b). To achieve this, the recording beams of the holographic storage apparatus may be moved parallel to the width of the holographic medium (for example using the movable stage shown in Figure 12b), allowing holograms to be recorded along multiple regions along the width of the medium.

[0269] To read holograms written onto a holographic medium 130, the medium is illuminated with a reconstruction beam, which has the same or generally similar properties (such as incidence angle, beam shape, wavelength and coherence) to the reference beam used to record the holograms. The reconstraction beam interferes with the fringes formed on the holographic medium, producing abeam that contains the multi -wavelength array stored in the hologram.

[0270] The apparatus 100 shown in Figure 1 may be used in this way to read data previously written onto the holographic medium 130. To retrieve stored data, the signal beam 110 is deactivated by shutter 126 (or by other means, such as the SLM 120) and the holographic medium 130 is illuminated by the reference beam 112 only, which acts as the reconstruction beam. The signal beam 110 is not required as it may interfere with reading data. If the data holographically stored on the storage medium 130 has been written with the apparatus 100 (or with other similar apparatus that produces the same recording beams), then it is straightforward to replicate the reconstruction beam to have the same properties as the original reference beam, allowing high read quality. Since the reconstruction beam comprises the same plurality of wavelengths as used for recording, each of the plurality of holograms recorded at the plurality of wavelengths in the region of the holographic medium are retrieved simultaneously. This retrieves all component arrays from each of the holograms and reproduces the multi-wavelength array.

[0271] The resulting beam comprising the reproduced multi-wavelength array is then incident on a detector 140, which may be any suitable device to detect electromagnetic radiation, including visible light, ultraviolet (UV) radiation, infrared (IR) radiation, X-rays, etc., such as a camera, CCD or CMOS sensor, or any other suitable device. Optics such as a lens 138 (or a plurality of lenses) may be provided between the holographic medium 130 and the detector 140 to focus the image of the array stored in the holograms to be read by the detector. The retrieved array is sent to a processor, computer or other retrieval device from the detector 140, where the data encoded in the array is decoded.

[0272] In an embodiment, to provide improved array retrieval quality, a three-CCD detector may be used, where incoming red, green and blue light (or any other wavelengths if a different set of component wavelengths is used) is split by a beam-splitter and light of each wavelength is incident onto a separate CCD sensor.

[0273] A separate reading device, such as that shown in Figure 7, which does not contain components for writing data (such as an SLM 130 and optical components 108, 116, 118, 122) may also be used to retrieve data stored on holographic medium. This may be advantageous for archival use, where a large amount of stored data is not required to be modified but may frequently need to be retrieved.

[0274] Figure lb shows hologram reading and writing apparatus 101, which uses a reflective SLM 121. Unlike the transmissive SLM 120, reflective SLM 121 produces a modulated light beam that is reflected from the surface of the SLM. Writing components such as holographic medium 130 and focussing lens 122 are placed in the optical path of the signal beam 110 reflected from the SLM 121 and the reels 132, 134 and rollers 136 are also positioned accordingly. The mirror (or plurality of mirrors) 114 is positioned to direct the reference beam 112 onto the same position of the holographic medium 130 as the signal beam 110. The reflective SLM 121 may be a digital micromirror device (DMD), further shown in Figure 13, a liquid crystal on silicon (LCoS) device, or any other suitable device.

[0275] The embodiments shown in Figures la and lb comprise three sources to produce radiation at three different wavelengths for recording holograms. This allows three wavelength multiplexed holograms to be recorded in a single region of the holographic medium. However, any number of radiation sources may be used, such as two, four, five or more radiation sources to produce two, four, five or more wavelengths respectively. As a result, two, four, five or more wavelength multiplexed holograms can be recorded in a single region of the holographic medium, storing the same number of component arrays, and allowing data to be stored in multi-wavelength arrays with a greater number of wavelengths. This allows improved data density with each additional wavelength used.

[0276] Figure 2a shows how data is stored on a holographic medium 230 in the form of a multi-wavelength array formed of a plurality of component arrays. Each region of the holographic medium 230 has a plurality of holograms 246 recorded in each region, each of the plurality of holograms recorded at a different wavelength. Multiple pluralities of holograms are stored on different regions of the holographic medium. The pluralities of holograms may be recorded in rows and columns, as shown. Whilst 3 sets of holograms are shown along the width of the medium, the actual number may be greater depending on the hologram pitch and medium dimensions. For example, for a hologram diameter of 1.00mm and a hologram pitch of 1.25mm, 10 sets of holograms may be recorded along the width of the medium for a medium with a width of 12.65mm. Shift multiplexing together with another form of multiplexing (for example angular multiplexing) may be further used to increase the number of holograms stored in the width of the medium. For example, if angular multiplexing with 10 different angles of the reference beam is used, the overlap between subsequent holograms can be greatly increased and the pitch can be reduced by a factor of 10 to 0.125 mm, allowing for 100 sets of holograms to be recorded along the width of the medium.

[0277] The pluralities of holograms 246 are recorded in different regions of the holographic medium. Preferably, the holographic medium is a single homogeneous sheet, and data may be stored at any point of the photopolymer, so the regions are not physically defined on the medium. However, in an embodiment, the regions for storage may have particular properties to separate them from the rest of the sheet - for example, they may have improved photosensitivity, whilst the rest of the holographic medium is not photosensitive.

[0278] Each plurality of holograms 246 stores a multi-wavelength array 280, which comprises three monochromatic component arrays 270a, 270b, 270c, each recorded at a corresponding component wavelength. For the example of the RGB system, component array 170a may be recorded at a wavelength of 600-700nm (red), component array 170b may be recorded at a wavelength of 500-600nm (green) and component array 170c may be recorded at a wavelength of 400-500nm (blue). The plurality of holograms storing the multi-wavelength array is written onto a single region of the holographic medium such that when the reference beam illuminates the region, all component arrays of the multiwavelength array are reproduced overlapping one another. The three overlapping arrays form a single multiwavelength array 280.

[0279] The multi-wavelength array comprises a grid of elements, which may be pixels or groups of pixels (say 4 pixels per group). Each element corresponds to a data point, and the value of each data point is represented by the combination of wavelengths in the element. For the example of an RGB system, possible data values corresponding to combinations of wavelengths present in a region of the multi-wavelength array are shown in the table below. A first data value is associated with no wavelengths present (black); second, third and fourth data values are associated with the presence of only red, only green and only blue wavelengths respectively; fifth, sixth and seventh data values are associated with the presence of combinations of two of the component wavelengths (corresponding to cyan, magenta or yellow) ; and an eighth data value is associated with the presence of all three component wavelengths (corresponding to white). This allows 8 data values (corresponding to 3 bits) to be stored in a single pixel of a three-wavelength system. In this way, the multi-wavelength array stores data across the plurality of wavelengths (the data stored in each multi -wavelength array may also be referred to as a data page).

[0280] The below wavelength combinations may also be considered in terms of an additive colour mixing model, which corresponds to how the human eye perceives colour. Each of the wavelengths used to record the holograms may be considered to correspond to a primary colour (red, green and blue); further secondary colours (cyan, magenta, yellow) are formed by combination of the primary colours. Each primary and secondary colour corresponds to a different data value stored. The additive colour mixing model may be a useful explanation of how combinations of wavelengths are used in holograms, but since the data writing / retrieval systems used to record data in the multiwavelength arrays is not concerned with human vision, this system uses the underlying wavelengths and the colour model is only intended for illustration.

[0281] Whilst an RGB system is described above, the same density of 3 bits per pixel can be achieved with a system using any three primary wavelengths (for example, wavelengths in a smaller band of the visual spectrum, or wavelengths outside the visible spectrum). Whilst a combination of red, green and blue may be advantageous for systems designed for viewing by humans due to the RGB colour model corresponding well to the colour sensitivity of the human eye, since the multi-wavelength arrays are read by a detector connected to a processor or computer, sets of wavelengths may be used that are more suitable to other considerations, such as the sensitivity of photopolymers used in the holographic medium or the sensitivity of the detector. For example, a blue-violet system comprising multiple wavelengths in the blue-violet region of the optical spectrum (between 375nm-510nm) may be used. For example, any selection of lasers with the following wavelengths may be used: 375nm, 395nm, 405nm, 420nm, 445nm, 460nm, 473nm, 488nm, 505nm. Lasers with wavelengths close to the blue-violet region may also be used, for example with wavelengths of 355nm or 532nm. This may allow improved retrieval by the detector.

[0282] Figure 2b shows how the data stored in pixels of the multi-wavelength array is determined from pixels of the component arrays. The first columns 271a, 271b, 271c of component arrays 270a, 270b, 270c are shown. Each element of each column, corresponding to a pixel of the arrays, has a value of 0 or 1 depending on whether light of a certain wavelength is present or not. The first column 281 of multi-wavelength array 280 is also shown. Each element of this column, corresponding to a pixel of the multi-wavelength array, is a combination of the wavelengths present in each corresponding pixel of the component arrays. For example, in the first row, columns 27 la, 270b, 270c have values of 0, 0, 1 respectively; these values are taken together to produce the data value stored in the first element of column 281 - which has a value of 001 (corresponding to blue, as only blue light is present). Similarly, the second row has values of 0, 1, 1 in columns 271a, 271b, 271c respectively; as a result the second element of column 281 has a value of 011, corresponding to cyan, from the combination of green and blue light (and no red light). The wavelengths in each pixel of the multi-wavelength array are recorded by a detector when reading the array, and a processor decodes the data stored in the array by assigning a data value to each combination of wavelengths as shown.

[0283] Storing data across a plurality of wavelengths can allow improved writing and retrieval of the data as the components can be read and decoded simultaneously. For example, a retrieval beam having the plurality of wavelengths can read each plurality of wavelength multiplexed holograms at once, and a computer system can process the plurality of component arrays together as a multi-wavelength array, rather than having to process the data stored in each hologram separately. Storing data across a plurality of wavelengths can also allow improved writing; for example, in the systems shown in Figures 1-4, each plurality of wavelength multiplexed holograms can be written onto the holographic medium simultaneously, and the corresponding processing required to write these holograms is simpler as a single data page is encoded into a single multi- wavelength array, rather than the data being needed to be processed into a plurality of arrays.

[0284] When writing data onto a holographic medium, a portion of the data written is not used for writing user information, but for metadata, formatting, indexing, redundancy, alignment, etc. If data is stored in an array, this requires using a portion of the array, reducing the amount of useable area for storing information. Since storing data across a plurality of wavelengths in a multi-wavelength array allows each array to store much more data, and the amount of data needed for overheads does not significantly increase with array size, then the proportion of usable storage in the array is increased. This can allow increased data density.

[0285] A number of factors are relevant to the selection of appropriate wavelengths. In a volume hologram, a Bragg grating is formed, and the Bragg condition must be satisfied by a reconstruction beam to retrieve the stored hologram. At different wavelengths, the fringes of the Bragg grating have different spacing, and light of each wavelength only interacts with a grating having a particular spacing, with the relation of the wavelength to the spacing given by Bragg’ s law. The wavelengths are suitable separated sufficiently (preferably by around lOnm or more, 20nm or more, 30nm or more, 50nm or more, 75nm or more) to ensure each wavelength only interacts with the appropriate grating having the appropriate wavelength and no other gratings. Within these constraints, providing a plurality of wavelengths close together can allow a high data density. It is also important to ensure that the wavelengths used are readily distinguished by the detector used in the reading process; if a lower quality detector is used this may limit how close the wavelengths can be to one another. To further reduce interference between different wavelengths, beams at different wavelengths may have different polarisation states to each other, or reference beams of different wavelengths may be incident at different angles (in which case different angles are also used for the different wavelength of the reconstruction beam). The power level of each wavelength is also suitably adjusted to account for the sensitivity curve of the storage medium (for example, the relative wavelength sensitivity of Bayfol HX200 film for red, green and blue light is 1 : 1.6:2, so the wavelengths and intensities are preferably adjusted accordingly).

[0286] The desired minimum wavelength separation to minimise interference between two recorded holographic gratings may be provided by AA = A2 / 2nd

[0287] Wherein AX is the minimum wavelength separation, X is the centre wavelength of each laser, n is the refractive index of the holographic medium, and d is the length of the hologram in the holographic medium.

[0288] Wavelengths of the electromagnetic spectrum outside of the human visual spectrum (around 375nm to 750nm) may also be used, such as ultraviolet, infrared, or X-ray radiation. To allow such wavelengths to be used, radiation sources with an appropriate wavelength may be provided. These may include infrared lasers, ultraviolet lasers, masers (to produce microwaves) and X-ray lasers (to produce X-rays). The SLM 120 may be appropriately adapted for use with such wavelengths. For example, a wavelength-selective filter may be used that is not an RGB Bayer filter, but that filters other wavelengths of radiation. Such a filter may also be used with the detector 140.

[0289] The lasers 102 are preferably continuous wave single mode diode lasers, which produce a continuous beam that is controlled by the shutters 124. They are preferably small SLM diode lasers with TEMoo output. Single mode diode lasers may advantageously provide a low -cost, easy to produce system, whilst still enabling sufficient resolution for recording and retrieving holographic multi-wavelength arrays. The shutters 124 (and 126) may be opto-acoustic modulators (AOM) or other suitable mechanical or opto-mechanical devices.

[0290] The lasers suitably have a sufficient coherence length to provide a stable standing wave by interference at the holographic medium. Since diode laser coherence may be highly sensitive to temperature change and driver current, precautions to avoid overheating may be appropriate. This may include the provision of heat sinks or coolers such as Peltier thermoelectric coolers.

[0291] Instead of continuous-wave lasers controlled by a shutter, single-pulsed lasers may be used, such as pulsed diode lasers or other pulsed lasers. Pulsed lasers may advantageously provide a shorter exposure time and an increased power of the laser beam and may not require the use of shutters. Pulsed lasers may help to improve the speed of operation of the reading / writing apparatus, reduce the requirement for stability control in the system, improve data reading consistency and reduce the risk of shutter issues. Pulsed laser use may entail additional requirements for high energy mirror surfaces; for example, standard DMD mirror coatings may not be suitable for operation with the higher intensity light produced by pulsed lasers.

[0292] Instead of recording all of the plurality of holograms in a single region of the medium simultaneously, the holographic medium may be illuminated by each wavelength individually, thus recording the component holograms one hologram at a time. For example, the holographic medium 130 shown in Figures la and lb may initially be illuminated by light from only the first laser 102a (by controlling shutters 124 or otherwise) and the SLM 120 set in accordance with the component array corresponding to the wavelength of laser 102a. This array is recorded onto the holographic medium as a first hologram, laser 102a is deactivated, and the process is repeated for the other lasers 102b and 102c. In this way, the plurality of component holograms is written onto a single region of the storage medium one component at a time. Note that this method differs from the writing method shown in Figure 5a, which provides a system that is able to write on multiple regions of the holographic medium simultaneously.

[0293] A system in which each component hologram is written sequentially may also utilise a variable-wavelength laser able to produce a plurality of wavelengths. To record each component hologram, instead of sequentially using light from each of a plurality of lasers, light from a single laser, which cycles between a plurality of wavelengths, is used. For example, for each region of the holographic medium, the laser may be pulsed multiple times, each time at a different wavelength, and the SLM configmed appropriately to record a different component array at each wavelength. Similarly , a plurality of pulses at a plurality of different appropriate wavelengths may be used to reconstruct the multiwavelength array one component array at a time.

[0294] Due to factors such as the manipulation of the signal beam 110 by the SLM 120 and diffraction losses, a portion of energy may be lost from the signal beam 110. In comparison, the reference beam 112 undergoes much less modification and modulation, and less energy is typically lost in the reference beam path. This may lead to the reference beam 112 being brighter than the signal beam 110 at the holographic medium 130 if the initial intensity of both beams is equal. Therefore, the beamsplitter 108 may be tuned or tunable to provide a greater proportion of energy of the laser beam to the signal beam than the reference beam.

[0295] As described above, the SLM shown may be a multi-wavelength (colour) liquid crystal display (LCD). The LCD comprises a liquid crystal layer comprising a plurality of pixels, wherein the liquid crystal layer is able to adjust the polarisation of light passing through each pixel (from 0° to 90°) in response to an electrical signal. The light subsequently passes through a wavelength-selective filter following the liquid crystal display in order to produce each component array of the multi-wavelength array. If an RGB system is used, the wavelength-selective filter is preferably a Bayer filter, though other filters may be used, particularly a different set of component wavelengths than red, green and blue is used. Following the wavelength-selective filter, the light passes through a polarising filter which allows through light polarised at 90° and blocks light polarised at 0°. Light polarised between 0° and 90° is able to pass through at a reduced intensity, the intensity reduction depending on the value of the polarisation. The LCD may also comprise a polarising filter polarised at 0° in front of the liquid crystal layer, which ensures that all light incident on the liquid crystal layer has a set polarisation. This may be used if, for example, an unpolarised light source is used. In this way, a single LCD with a wavelength-selective filter is able to produce a multi-wavelength array comprising a plurality of monochromatic component array multi- wavelength arrays and is able to modulate the intensity of the light at each pixel. However, in embodiments in which a plurality of LCDs is used (such as that shown in Figure 3) and each LCD acts on a separate beam comprising only one wavelength, the wavelength-selective filter is not required and may be omitted. The LCD may be a TFT-LCD, which comprises a thin-film transistor (TFT) layer before the liquid crystal layer to provide control over the pixels of the LCD.

[0296] Whilst only binary values (pixel entirely on or pixel entirely off) have been discussed so far, in other embodiments each pixel may have an intermediate intensity value, allowing each pixel to store further data values, as will be described later.

[0297] Figure 3 shows an alternative embodiment of the system in which the holographic data storage apparatus is modified from the apparatus in Figure 1 by a plurality of SLMs 320a, 320b, 320c, one for each wavelength of light used. As in Figure 1, a plurality of laser light sources 302a, 302b, 302c is used to produce three light beams 304a, 304b, 304c, each controlled by a shutter 305, but before passing through a dichroic filter, eachofthe light beams 304a, 304b, 304c first passes through a beamsplitter 308, which separates each beam into a signal beam component and a reference beam component. The reference beam components pass through dichroic filters 306, where they are combined into a single reference beam 312 and are reflected by mirrors 314 towards the holographic medium 330.

[0298] The signal beam components 310a, 310b, 310c are directed towards the plurality of SLMs 320a, 320b, 320c. The signal beam components first pass through spatial filters 316 and collimating lenses 318. After passing through the collimating lenses 318, each signal beam component passes through an SLM. The SLMs 320 shown in Figure 3 are transmissive SLMs, such as LCDs, but reflective SLMs, such as digital micromirror devices (DMDs) or a liquid crystal on silicon (LCoS) devices, may be used, with appropriate modification of the apparatus to correspond to the different signal beam component path. Each SLM acts on each signal beam component independently, such that each signal beam component is modulated with different data to produce three data beam components. Each SLM modulates each signal beam component with the appropriate component array (of the appropriate wavelength) of the multi-wavelength array. For example, if laser light source 302a is red, 302b is green, and 302c is blue (in an RGB system), SLM 320a produces a red component array of the multi -wave length array, SLM 320b produces a green component array, and SLM 320c produces a blue component array. If LCDs are used, then unlike the embodiments shown in Figure la or lb a wavelength-selective filter is not required to be used with the LCDs as each LCD only receives light of a single wavelength.

[0299] The three data beam components carrying the three component arrays pass through dichroic prism 350, which combines the three data beam components into a single data beam 311. This data beam thus carries a single multiwavelength array having all three wavelengths produced by the laser sources 302a, 302b, 302c. Data is encoded in the multi-wavelength array in the manner previously described. The optical properties of the prism are suitably selected to ensure the three monochromatic component arrays transmitted by each signal beam component align to correctly form a single multi-wavelength array in the signal beam.

[0300] The data beam 311 passes through focussing lens 322, which focusses it onto a point on the holographic medium 330. As before, a series of fringes is formed from interference of the reference beam 312 and data beam 311 for each wavelength, and the fringes are recorded onto the holographic medium 330, thereby storing the multiwavelength array and the data encoded therein as a plurality of holograms on the medium.

[0301] Compared to the embodiment shown in Figures 1 which uses a single SLM, the arrangement shown in Figure 3 may be simpler to produce and may provide improved resolution. For example, to produce a multi-wavelength array by a single SLM as in Figures 1, each pixel may be split into a plurality of sub-pixels, each sub-pixel having one of the component wavelengths. This may reduce the resolution available for the multi-wavelength array as each pixel is made larger than the grating size of the SLM. In contrast, the use of an individual SLM for each wavelength allows each pixel of the SLM to represent a single pixel of the multi-wavelength array, as the grating of the SLM is not grouped into sub-pixels of different wavelengths. In addition, a multi-wavelength SLM may be more expensive or difficult to manufacture. However, a multi-SLM set-up such as in Figure 3 may require accurate alignment of the multiple signal beam component arrays by the dichroic prism 350.

[0302] Figure 4 shows an alternative embodiment of the system in Figure 3 in which the dichroic prism 350 is replaced by a dichroic cube 450. Light beams from three lasers 402a, 402b, 402c are split by beamsplitters 408 to produce signal beam components 410a, 410b, 410c which are incident on three SLMs 420a, 420b, 420c to produce three data beam components. The data beam component are combined by the dichroic cube 450 and the resultant data beam 411 is incident on the holographic medium 430, where it forms interference fringes with the reference beam 412. A polarising cube may be used instead of a dichroic cube 450.

[0303] In the arrangements shown in Figures 3 and 4, it is possible to reduce chromatic aberrations caused by components having different focal lengths for different wavelengths by adjusting the properties of components along each channel. This can be achieved by tuning spatial filters 416 and collimating lenses 418 for each channel to account for differences in focal length, or by inserting an additional component between collimating lenses 418 and SLMs 420 (for example, glass of varying thickness, as sometimes used in 3LCD projectors).

[0304] Figures 5a and 5b show holographic data storage apparatus 500 which uses three sets of recording beams to write onto three regions of the holographic medium 530 simultaneously. The holographic medium 530, which is provided as a medium on supply reel 532 and is wound onto take-up reel 534, is moved relative to the position of the three sets of recording beams and each region of the holographic medium first receives a first pair of beams (data beam 511a and reference beam 512a), then a second pair of beams (data beam 511b and reference beam 512b), and then a third pair of beams (data beam 511c and reference beam 512c). Each data beam has a different wavelength, whilst each reference beam may have a single wavelength corresponding to the wavelength of the respective data beam or a combination of all three wavelengths. As the holographic medium moves through the apparatus, the three component arrays are written onto each region of the holographic medium sequentially.

[0305] Three lasers 502a, 502b, 502c produce three monochromatic light beams 504a, 504b, 504c of three different wavelengths, where the beams are controlled by shutters 524. These beams are split by beamsplitters 508 into three signal beams 510a, 510b, 510c, and three reference beam components, where the reference beam components are combined into reference beam 512 by dichroic filters or dichroic mirrors 506.

[0306] The three signal beams 510a, 510b, 510c are directed towards SLMs 520a, 520b, 520c to produce three data beams 511a, 511b, 511c. Each signal beam passes through a separate spatial filter 516 and a collimating lens 518 before it is incident on one of the SLMs. Each beam is incident on a separate SLM: signal beam 520a is incident on SLM 530a, signal beam 520b is incident on SLM 530b, and signal beam 520c is incident on SLM 530c. Each of the SLMs modulates each beam with a component array, where the component array is a single-wavelength component of a multi -wavelength array. The component arrays are not components of the same multi- wavelength array but are components of three adjacent multi-wavelength arrays, so each array is recorded in three exposures: a first component in a first exposure by data beam 51 la, a second component in a second exposure by data beam 511b, and a second component in a third exposure by data beam 511c. Each data beam is focussed by a focussing lens 522 and is incident onto the holographic medium 530. The data beams are not combined, but each beam is incident onto a different region 528a, 528b, 528c of the holographic medium 530. Data beam 510a is incident on region 528a of the holographic medium, data beam 510b onto region 528b, and data beam 510c onto region 528c. The reference beam 512 is split into beams 512a, 512b and 512c, which are directed onto corresponding regions 528a, 528b, 528c of the holographic medium 530. This may be achieved through the use of beamsplitters to produce multiple reference beams, each of which is directed towards one of the regions.

[0307] Alternatively, instead of combining the three reference beam components into a single reference beam 512 comprising all three wavelengths and splitting the single reference beam 512 up into the three reference beams 512a, 512b, 512c, it is possible to keep the reference beam components separate. In such a system, a first reference beam is produced from radiation beam 504a and is directed to region 528a, and similarly for regions 528b and 528c. Therefore, each region receives a reference beam comprising only one wavelength, corresponding to the wavelength of the data beam incident on that region.

[0308] Interference fringes are formed at each of the regions between the data beam and reference beam, and a hologram storing a component array is recorded in this way simultaneously at each region. After the holograms are recorded, shutters 524 or 526 may be used to deactivate the recording beams, ending the exposure (where the exposure time may be the same or different for each of the recordings, and different exposure times may be provided by shutters 526 or by a system comprising three separate monochromatic reference beams).

[0309] After this recording process happens, the holographic storage material 530 moves such that each data beam is incident on a region neighbouring the previous region. The holographic medium 530 is preferably continuously moved throughout the recording process, including during each exposure, where the distance that the holographic medium 530 moves during each exposure is no more than one tenth of the wavelength of the (shortest) wavelength of light used for recording. The recording process is repeated, and a different hologram storing a different component array at a different wavelength is recorded onto each of the regions by each of the data beams. For example, if data beam 510a is red, 510b is green, and 510c is blue, region 528a may first have a red component array recorded thereon (by data beam 510a), then a green component array recorded thereon (by data beam 510b), then a blue component array recorded thereon (by data beam 510c). In this way, the components of a multi- wavelength array are written onto each region of the holographic medium sequentially.

[0310] Figure 5b shows further details of the writing process. The holographic medium 530 is shown with a plurality of regions 528a, 528b, 528c, 528d, etc. arranged in a row. Note that these regions are shown simply for illustration and may not in any way be different to the rest of the storage medium; preferably, any arbitrary section of the storage medium may be used for recording. Note also that the reference beams are not shown; they are incident on the same regions of the storage medium as the data beams.

[0311] The reels 532 and 534 rotate in direction 542 to keep the holographic medium in continuous motion relative to the object beams 510 and writing heads (comprising optics 522, SLMs 520, and other appropriate components). When the desired regions of the holographic medium are aligned with the appropriate writing head, the data beams 51 la, 51 lb, 511c and the appropriate reference beams are directed onto the holographic medium 530 (either through opening shutters 524 or directly operating pulsed lasers) to provide an exposure to record a hologram at each region of the holographic medium. For example, when region 528a is aligned with data beam 511a, region 528b is aligned with beam 51 lb, and region 528c with beam 511c, the lasers may be operated and a an component array (for an RGB system, a red component on region 510a, a green component on region 510b, and a blue component on region 510c) is recorded on the medium in the form of interference fringes between each data beam and reference beam. The lasers are then switched off or the shutters 524 are closed, and the holographic medium continues to move.

[0312] A second exposure occurs after the holographic medium has moved an appropriate distance, equivalent to the hologram pitch, to align each writing head with a subsequent region. This occurs when data beam 51 la is aligned with region 528d (previously unexposed), data beam 51 lb is aligned with region 528a and data beam 511c is aligned with region 528b. The lasers are activated to provide a further exposure to record the appropriate component arrays on these regions. In this way, each plurality of holograms is recorded onto a single region of the holographic medium sequentially, one hologram at a time, and multiple holograms not forming part of the same plurality of holograms but forming part of different pluralities of holograms are recorded onto multiple regions of the holographic medium simultaneously. If the process is repeated and the storage medium is once again shifted, region 528a will have an additional red component array written thereon, resulting in a full multi-wavelength array being stored in the region. Each hologram storing each component arrays recorded in each region is overlaid with any previous one such that they are exactly aligned. This allows each plurality of holograms to be read by illuminating the region with an appropriate reconstruction beam, the reference beam having the same angle and comprising the same component wavelength as the reference beam used for writing the multi-wavelength arrays. This reproduces the multi-wavelength array, allowing it to be read using a detector or other equipment.

[0313] As the exposure process is repeated, further regions will have a plurality of holograms storing a multiwavelength array written thereon. As each multi-wavelength array may be used to store a data page, having a plurality of regions storing a plurality of multi-wavelength arrays allows a plurality of data pages to be stored on the storage medium.

[0314] Advantageously, the apparatus shown in Figures 5a and 5b may allow simpler optics for forming a multiwavelength array than the embodiments shown in the previous figures. In addition, since each signal / data beam uses a separate spatial filter 516, separate lenses 518 and 522, and a separate SLM 520, it may be easier to align the multiple data beams at the same depth within the storage medium. In embodiments where a single spatial filter, collimating lens and / or focussing lens is used on multiple beams having a plurality of wavelengths, the lenses may have different focal lengths for each wavelength, resulting in the multi-wavelength array being formed at different positions within the holographic medium.

[0315] In some cases, the first two and last two regions of a row on the holographic medium may not be illuminated by all three beams of all three wavelengths, and so may not have a complete multi-wavelength array. For example, if regions 528b and 528c are the first and second regions on the storage medium respectively, region 528a is only illuminated by red beam 511a and region 528b is only illuminated by red beam 511a and green beam 51 lb. A similar situation may occur on the final two regions of a row.

[0316] This may be remedied by starting the write process with the first region 528c aligned with data beam 511a, and illuminating the holographic medium 530 with only data beam 51 la. The other data beams remain deactivated, for example with shutters 524. Subsequently, as the storage medium moves across the object beams, when regions 528b and 528c are aligned with the positions of beams 511a and 51 lb, those two beams are activated, and a further monochromatic component array is written onto the two regions 528b and 528c. Following this, as the storage medium 530 continues moving, the data beams are activated normally when the holographic medium 530 has moved an appropriate distance equal to the desired hologram pitch, allowing the recording process to proceed normally as described above. At the end of the row, when the final two regions are reached, the recording may finish in a similar way, with the penultimate stage of recording involving only two beams (51 lb and 511c) and the final stage involving only one beam (511c).

[0317] Alternatively, the recording may simply start with the recording proceeding without the first two regions receiving all three data beams. In this case, the first two regions may be used to store a smaller amount of data, for example with one or two bits per pixel corresponding to one or two wavelengths.

[0318] Whilst one row of pluralities of holograms is shown in Figure 5b, multiple rows may be used to record data on the holographic medium 530. Following the recording of data on one row of the holographic medium 530, the storage medium may be shifted in a direction perpendicular to the direction of the rows (such as by the movable stage shown in Figure 12b). In this way, data may be stored in a grid with rows and columns of pluralities of holograms on the holographic medium 530, each plurality of holograms storing a multi-wavelength array.

[0319] Figures 6 and 7 show apparatus 600 and 700 which may be used to read the multi-wavelength holographic recordings produced in a holographic medium.

[0320] The apparatus 600 shown in Figure 6 is similar to that shown in Figure 1 for writing the holographic recording, but shows additional details of its operation for reconstructing data stored on holographic medium 630. The shutter 626 may be used to block the signal beam from reaching the optics 616, 618, 622, SLM 620 and storage medium 630. Light combined by the dichroic elements 606 produces reconstruction beam 644 that is incident onto the holographic medium 630.

[0321] The reconstruction beam 644 has the same or broadly similar properties to the original reference beam used when writing onto the storage medium 630, as variations from the original reference beam may affect reconstructed image quality. Since apparatus 600 shown in Figure 6 may also be used for writing onto the holographic medium, it is straightforward to produce a reconstruction beam that has the same properties as the reference beam used for writing the hologram by the same apparatus.

[0322] The reconstruction beam 616 is directed onto the storage medium by mirrors 614. The interaction of the reconstraction beam 644 with the fringes of the holograms stored on the storage medium 630 produces a reconstructed data beam, which carries the original multi-wavelength array that was written onto the holographic medium. The image of the multi-wavelength array may then be captured by detector 640, and may be processed by software, on a processor, computer, etc., to retrieve the data encoded in the multi-wavelength array.

[0323] Light from the holographic medium 630 comprising the stored multi-wavelength array may pass through optical elements before being incident on the detector 640. For example, the light may pass through a focussing lens 638a and a collimating lens 638b. The lenses are achromatic to avoid distortion and aberration of the plurality of wavelengths of the pluralily of element arrays forming the multi-wavelength array.

[0324] The detector 640 may be a CCD sensor or a CMOS sensor, or any other suitable sensor. The detector may have a wavelength-selective filter, such as a Bayer filter, to split light from the multi-wavelength array back into its component array wavelengths and allow the multi-wavelength array to be reproduced. If a different filter than a Bayer filter is used with the SLM for writing the multi-wavelength array, then this filter may also be used with the detector 640 instead of a Bayer filter. It is also possible to pulse the different wavelength lasers sequentially and capture independent images with a monochrome sensor for each wavelength.

[0325] Figure 7 shows an alternative embodiment of a reading apparatus 700 which comprises only optics for reading holograms and does not comprise optics that may be used for writing. The apparatus comprises three lasers 702a, 702b, 702c with the same wavelengths as used for writing the multi-wavelength arrays, which are combined by dichroic elements 706 into a single reconstruction beam 744. Since this apparatus is not used for writing, the laser sources and optics to produce the reconstruction beam are configured and aligned to reproduce the arrangement used to produce the reference beam of the writing apparatus that was used.

[0326] The reconstruction beam 744 is reflected onto the storage medium 730 at the same angle as when writing, and the multi-wavelength array is reproduced from the fringes stored on the storage medium 730 and is captured by detector 740. The apparatus may comprise optics between the holographic medium 730 and the detector 740, such as a focussing lens 738a and a collimating lens 738b. The apparatus 700 does not comprise an SLM or any optics to produce and modify a signal beam. Advantageously, this may provide a more compact and simple apparatus that can be used in situations where only multi-wavelength array retrieval is required (for example, for archival purposes).

[0327] The reconstruction beam suitably has similar properties to the original reference beam used in the apparatus used to produce the hologram. In particular, having a very similar angle of incidence and beam shape is advantageous for clearly reproducing the hologram. Small differences in reconstruction beam angle from the original reference beam may reduce the brightness of the reconstructed hologram, whilst a significantly different angle may result in the hologram not being reconstructed at all. Variations in the shape of the beam, which may be affected by factors such as the placement, focal length and numerical aperture of lenses and filters, may similarly reduce the quality of the reconstructed image.

[0328] The wavelength (and similarly the coherence) of the reconstruction beam may have larger variations from the original reference beam than the angle of incidence and beam shape, and small variations in wavelength generally affect the quality of the reconstructed image to a lesser extent. Therefore, whilst for recording, higher quality lasers with strong coherence and a strong monochromaticity (i.e. a well-defined laser peak) may be desirable, for reconstruction lower quality lasers may be used which have a lower coherence or a wavelength that has some variation from the original reference beam. Figure 8 shows holographic recording apparatus 800 that is able to record intensity-modulated arrays onto a holographic medium 830 wherein each pixel of the array may take on more than two values. The apparatus comprises a single coherent and monochromatic light source 802, such as a laser, that produces a single light beam 804. This light may be of any suitable wavelength, inside or outside the visible spectrum, and is preferably of a wavelength that is suited for the properties of the holographic medium 830 and the detector 840. The laser is preferably a continuous- wave diode laser which is controlled by shutter 824 to expose a multi-wavelength array on the holographic medium. Alternatively, the laser may be a pulsed laser, in which case a shutter 824 may not be required.

[0329] The light beam 804 is split by a beamsplitter 808, which may be of any of the types described previously. This produces a signal beam 810 and a reference beam 812. The signal beam passes through optics such as spatial filter 816 and collimating lens 818 before being incident on the SLM 820.

[0330] The SLM 820 comprises a plurality of elements (such as pixels) formed into a regular grid which modulates the signal beam with an array storing data to produce a data beam 811. In previous embodiments, it was described that each pixel of the SLM could take on an “off’ state corresponding to light being blocked (corresponding to a 0 binary value) and an “on” state corresponding to light being transmitted (corresponding to a 1 binary value). However, in this implementation, each pixel of the SLM is also able to take at least one intermediate value (which may also be referred to as a grayscale value), in which a portion of light is transmitted at each pixel. For example, each pixel of the SLM may be set to an intensity of 0%, 33%, 67%, and 100%. This corresponds to four data values - for example, 00, 01, 10, 11 respectively - allowing each pixel of the array to store two bits of data.

[0331] The above values are only an illustration - any number of intermediate values may be used, and any suitable intensities may be used (though they are preferably evenly spaced between 0% and 100% to allow them to be easily distinguished by a detector).

[0332] In the embodiment shown, the SLM 820 is a transmissive SLM, such as a liquid crystal display (LCD). An LCD comprises a liquid crystal layer which forms a grid of pixels, and may be electrically controlled at each pixel to adjust the polarisation of light passing through each pixel. For example, it may be able to change the polarisation of light by between 0° (no change) to 90°. Arranged behind the liquid crystal layer is a polarising filter arranged to allow light polarised at 90° through and block light polarised at 0°. Whilst in previous embodiments only polarisations of 0° and 90° were described, here a plurality of intermediate values may also be used. For example, pixels of the LCD may be set to change the polarisation of light by 15°, 30°, 45°, 60°, 75°, and other values. When this light passes through the polarising filter, a proportion of light passes through the filter correlating to the polarisation of the light by the liquid crystal layer. In this way, additional data values corresponding to intermediate intensities may be provided at each pixel in the array.

[0333] In another embodiment, the SLM 820 may be a different type of transmissive SLM, or a reflective SLM (requiring the appropriate changes to the layout of the apparatus as described with reference to Figure lb) such as a digital micromirror device (DMD), a liquid crystal on silicon (LCoS) device, or any other suitable device. If a DMD is used, the micromirrors of the DMD are set to oscillate between an on and off configuration, reducing the intensity of light reflected according to the proportion between the time the micro mirrors are in an active and inactive state.

[0334] Following the SLM 820, the data beam carrying the array passes through focussing lens 822, which focusses the signal beam onto the holographic medium 830, which is preferably a photopolymer. The data beam 812 is reflected by mirrors 814 such that it is incident onto the same region of the holographic medium 830 as the signal beam 810. A plurality of interference fringes is formed on the holographic medium and recorded on the medium, thereby storing the array on the holographic medium 830 as a hologram.

[0335] Apparatus 800 also comprises detector 840, which may be used to read recorded holograms from storage medium 830. This may be achieved by using shutter 826 to block the signal beam and providing only the reference beam 812 (as a reconstruction beam) onto the storage medium. Alternatively, the apparatus may not comprise any reading optics and may be writing apparatus only. Apparatus for writing may be provided separately.

[0336] Figure 9 shows further details of arrays 970 stored on a holographic medium 930. The holographic medium may comprise a grid of rows and columns of stored holograms 946. Each array 970 comprises a grid of pixels and each pixel can take on one of a plurality of intensity values. In the array 970 shown, each pixel takes on one of 4 values of 0% (minimum intensity), 33%, 67%, 100% (maximum intensity). For a 256-level colour system, this corresponds to 0, 85, 170, 255. Each intensity value corresponds to a binary data value: 00, 01, 10, 11 respectively, as described above. Other numbers and values of intermediate intensities may be used, preferably with the intensities evenly spaced. For example, 8 intensity values may be used for 4 bits per pixel, 16 intensity values for 5 bits per pixel, and so on for 6, 7, 8 and more bits per pixel (wherein the number of intensity values is given by 2An, wherein n is the number of bits). A number of intensity values not corresponding to a power of 2 (and thus not to an integer number of bits) may also be used, but this is less desirable as it may be more difficult to encode and decode data stored in this manner.

[0337] Figures 10 and 11 show an embodiment that combines the principles of the apparatus shown in Figures 1 to 5 (the recording of multi-wavelength arrays) and Figures 8 and 9 (the recording of intensity -modulated arrays with more than 2 possible data values per pixel) to record multi-wavelength arrays wherein each pixel of each component array stores one of more than 2 possible data values through intermediate intensities. The reading and writing apparatus 1000, shown in Figure 10, is similar to apparatus 100, comprising three monochromatic coherent light sources 1002a, 1002b, 1002c (each having a different wavelength), which produce a signal beam 1010 and a reference beam 1012 by means of dichroic elements 1006 and beamsplitters 1008. The signal beam 1010 passes through a spatial filter 1016 and a collimating lens 1018 to form a wide collimated beam that is incident on spatial light modulator (SLM) 1020.

[0338] The SLM 1020 produces a multi-wavelength array in the signal beam 1010, wherein each pixel of each component array represents one of 3 or more possible values. The SLM is preferably a multi-wavelength (colour) LCD, similar to the LCD described in Figure 1, with the capacity to produce intermediate intensity values at each pixel. Each pixel of the SLM comprises a plurality of sub-pixels corresponding to each wavelength of light used. As described with reference to Figure 8 each sub-pixel of the SLM can take on an “off’ state corresponding to light being blocked, an “on” state corresponding to light being transmitted, and one or more intermediate values, in which a portion of light is transmitted at each pixel. This produces an intensity-modulated component array for each wavelength in the signal beam 1010, and the overlaid component arrays form a multi-wavelength array as before.

[0339] Following the SLM 1020, the signal beam passes through a focussing lens 1022, which focuses the light onto holographic medium 1030. The storage medium 1030 is preferably a photosensitive polymer. The reference beam 1014 is reflected onto the same region of the holographic medium 1030, and an interference pattern comprising a plurality of fringes is formed between the signal beam and reference beam. The interference pattern is recorded onto the holographic medium.

[0340] The apparatus 1000 also comprises a detector 1040, which may be used to retrieve the stored multiwavelength array from the holographic medium. The shutter 1026 may be used to block the signal beam 1010, such that only the reference beam 1012 is incident on the holographic medium 830. Optics (not shown) may be present between the holographic medium and the detector 840, such as a focussing lens and a collimating lens.

[0341] Whilst the apparatus 1000 shown in Figure 10 follows the arrangement 100 of the apparatus shown in Figure 1 (using a single SLM to form a single signal beam incident on the holographic medium), multi-wavelength arrays comprising intensity information may also be recorded on a holographic medium by the arrangements shown in Figures 3 and 4, where a separate SLM is provided for each wavelength of light used and the component arrays produced by each SLM are combined into a single signal beam incident on the holographic medium. Alternatively, the arrangement shown in Figures 5a and 5b may also be used, wherein a plurality of SLMs are used to produce a plurality of data beams which are incident on the holographic medium separately at different regions, and the holographic medium is moved between each exposure to record each of the plurality of holograms storing a multiwavelength array at each region sequentially (with each pixel of each SLM and therefore of each component array being able to take on one of at least 3 intensity values.

[0342] Figure Ila shows how data is stored on holographic medium 1130 in the form of variable intensity multiwavelength arrays 1180. The holographic medium comprises a plurality of pluralities of holograms 1046 in a grid, as in previous embodiments. Each hologram of each plurality of holograms stores a component array at a specific intensity. As shown, component arrays 1170a, 1170b, 1170c, which overlap to form combined multi-wavelength array 1180. As shown in Figure Ila, each pixel of each of the component arrays 1170 takes one of four intensity values - 0% (0 in a 256-level colour system), 33% (85 in a 256-level colour system), 67% (170 in a 256-level colour system), and 100% (255 in a 256-level colour system). These correspond to binary values of 00, 01, 10, 11 respectively, so each pixel can store 2 bits of data.

[0343] Each pixel of the multi- wavelength array is a combination of the three corresponding pixels of the component arrays. Therefore, each pixel of the multi-wavelength array can take on one of 64 values, each corresponding to a unique combination of intensities of each of the component wavelengths. This means each pixel is able to store 6 bits of data. Each combination (i.e. each colour of the pixel) has an associated binary value, which is retrieved when the multi-wavelength array is read and processed. For a plurality of multi-wavelength arrays stored on the holographic medium (for example as rows and columns), a plurality of data pages can be stored at high density.

[0344] Figure 1 lb shows further details of how the data stored in pixels of the multi-wavelength array is determined from pixels of the intensity -modulated component arrays. The first columns 1171a, 1171b, 1171c of component arrays 1170a, 1170b, 1170c and the first column 1181 of multi-wavelength array 1180 are shown. Each element of each array has a value corresponding to the intensity of light of the respective wavelength. Each element of the multiwavelength array then has a value corresponding to the combination of values of each corresponding pixel in each component array. For example, in the first row, columns 1171a, 1170b, 1170c have values of 01, 01, 11 respectively; these values are taken together to produce the data value stored in the first element of column 181 - which has a value of 010111. Each other element of the multi-wavelength array is determined in the same way. For example, in row 6 of the first column, each of the red, green and blue components have a value of 11 (maximum intensity); as a result row 6 of the first column of the multi -wavelength array has a value of 111111, corresponding to white light. The combination of intensities of wavelengths in each pixel of the multi-wavelength array is recorded by a detector when reading the array, and a processor decodes the data stored in the array by assigning a data value to each combination of intensities of wavelengths as shown.

[0345] Such arrays may be produced using the apparatus shown in Figure 10, or other apparatus shown previously, wherein the SLM or plurality of SLMs are configured to allow through a variable portion of light incident on the SLM. For example, this may be an LCD in which the polarisation of the liquid crystal layer is set to an intermediate value between 0° and 90°, as described with reference to Figures 8 and 9 above, or a DMD in which the micromirrors are set to oscillate between the on and off configuration, reducing the intensity of light reflected according to the proportion between the time the micro mirrors are in an active and in active state . A single SLM may be used, in which case a wavelength-selective filter may be provided (such as a Bayer filter for an LCD), or each wavelength may be incident on the SLM separately and sequentially. Alternatively, a plurality of SLMs may be used, and the modulated beams from the SLMs either combined (Figures 3 and 4) or incident on the storage medium separately (Figure 5).

[0346] To read the variable intensity multi-wavelength arrays, the same reading apparatus may be used as for the 3- bit arrays shown in Figure 2a. Suitable reading apparatus has a plurality of light sources corresponding to the wavelengths of the component arrays, and a detector of suitable sensitivity to distinguish a plurality of intensity values for each channel.

[0347] The number of intensity values per pixel may be selected to optimise for improved data storage capacity and for detector readability, wherein the intensity values are preferably evenly spaced. For example, in other embodiments, each pixel of each component array may store 3 or more bits, 4 or more bits, 6 or more bits, or 8 or more bits. For example, a system with 3 bits for each channel would have 8 intensity values corresponding to 100%, 85%, 71%, 57%, 43%, 28%, 14% and 0% (or 255, 218, 181, 145, 109, 73, 36 and 0 respectively for a 256-level colour system).

[0348] A significant example is 8 bits stored per pixel per wavelength, corresponding to 256 intensity values per wavelength, allows 24 bits to be stored per pixel of a multi-wavelength array and allowing 24-bit colour (16,777,216 colours, each colour corresponding to a unique combination of intensities of each wavelength) to be reproduced in the array. As 24-bit colour is commonly used in modern electronic devices, this may be advantageous in allowing straightforward conversion of data into multi-wavelength arrays, as well as allowing high data storage density. Other numbers of intensity values may be used that do not correspond to powers of two (and thus to an integer number of bits), such as 5 intensity values per pixel, 7 intensity values per pixel, and so on, which may be desirable from the optical properties of the system and the material properties of the holographic medium. However, as they do not correspond to an integer number of bits, they may be less suitable for conversion to a binary data file.

[0349] The use of 24-bit colour may also advantageously allow more efficient decoding by a computer. Having 8 bits in each channel (8 bits for each component of a pixel) may allow improved processing by processors that have 8- bit channels (for example, a 64-bit processor, which has 8 8-bit channels), where three 8-bit channels may be used to process each pixel of the 24-bit multi-wavelength array. This allows 8 full RGB pixels to be processed in three processes. Similarly, having multi-wavelength arrays with dimensions that are divisible by 8 may be advantageous for decoding efficiency, allowing full use of all channels. Example dimensions of multi-wavelength arrays that may be suitable include 384 by 384 pixels (442,368 individual red, green and blue pixels, or 442,368 bytes if each channel is 8 bytes), 448 by 448 pixels, 512 by 512 pixels, etc., although any multi-wavelength array size may be used. Though the multi-wavelength arrays are preferably square and have an equal number of vertical and horizontal pixels, they may also have a different number of vertical pixels to horizontal pixels.

[0350] However, the more bits per pixel are used, the more difficult it may be to form a suitably clear multiwavelength array as a hologram, as optical errors (for example from chromatic aberration caused by optical elements having different focal lengths for different wavelengths of light, from small differences in optical path taken by light of each wavelength to the storage medium, and from other sources) may build up, preventing each pixel of the multiwavelength array from being clearly resolved. Errors may also be caused or compounded by the detector, which may not have a high enough sensitivity to distinguish between all colours used.

[0351] To avoid data loss from such issues, data pages may be stored redundantly, wherein portions of the data page are stored in a plurality of regions of the multi-wavelength array, allowing the data page to still be retrieved if the multi-wavelength array is damaged.

[0352] Advantageously, since holograms are stored as an interference pattern between a data beam and a reference beam, information on each pixel is not stored at a single point on the hologram but is instead recorded over the entirety of the hologram. As a result, if a portion of the hologram is damaged, this does not mean that a portion of the multiwavelength array will be missing; instead, the hologram degrade by losing resolution and quality. Advantageously, this means data may not be lost if a hologram is damaged; however, it may make reading a damaged hologram more difficult for the entirety of the data (i.e. an entire array) stored on the hologram.

[0353] In addition to redundancy, each array may comprise regions that store format information, encoding and decoding information, and alignment information (regions of the multi-wavelength array may comprise certain patterns which signal to decoding software the alignment and bounds of the multi -wavelength array).

[0354] Figure 12a shows another embodiment of holographic recording apparatus 1200, comprising a Fourier filter 1250, a movable prism 1254 for angular multiplexing, and a rotating cylinder 1252 for receiving the holographic medium 1230. The apparatus 1200 shown comprises a reflective SLM 1220, though a transmissive SLM may also be used.

[0355] As shown in previous embodiments, the apparatus comprises three laser light sources 1202a, 1202b, 1202c, which may be controlled by switches 1224. The beams produced by the lasers are combined using dichroic filters 1206 and a beamsplitter 1208 is used to produce a signal beam 1210 and a reference beam 1212. The signal beam passes through a spatial filter 1216 and is reflected by a plane mirror 1214 onto a collimating mirror 1248, which reflects and collimates the signal beam. The collimating mirror may be a spherical mirror, parabolic mirror, or any other suitable element. A spherical mirror may be provided in the case that small angles of incidence and reflection are desirable, whilst a parabolic mirror may be advantageous if off-axis incidence is necessitated by the physical or dimensional constraints of the optical set-up.

[0356] The collimated signal beam is incident onto SLM 1220. The SLM 1220 reflects a portion of light containing encoded data (a data beam) towards an achromatic lens 1222a, or system of lenses, and subsequent writing optics. The remaining light is reflected away from the subsequent optics. Preferably, the SLM is a digital micromirror device (DMD), such as that shown in Figure 13. The DMD comprises a grid of micromirrors which canbe tilted on their axes to reflect light onto the writing optics or otherwise divert the light away from the optical path. Further details of a DMD 1300 are shown in Figure 13. The DMD 1300 comprises a plurality of tiltable micromirrors 1366, wherein each micromirror may represent a pixel. The angle of each micromirror 1366 may be adjusted (in an embodiment, this angle may be in the range -12° to 12° from the original position), which determines whether light is able to pass along the optical path towards the storage medium or is diverted away from the optical path. For example, parallel portions 1310a, 1310b, 1310c, 13 lOd of the signal beam are incident on different portions of the DMD. Rays 1310a and 1310b are incident on non-activated micromirrors. The reflected rays 131 la and 131 lb are therefore reflected away from the optical path. In contrast, rays 1311c and 13 lid are incident on activated micromirrors, which are tilted from their inactivated position, and they rays reflected back as rays 1311c and 131 Id towards the focussing lenses and holographic medium.

[0357] To produce a multi-wavelength array comprising three component arrays of different wavelengths, the lasers 1202a, 1202b, 1202c shown in Figure 12a may be turned on sequentially (for example using the switches 1224) and the micromirrors 1366 of the DMD 1300 adjusted accordingly. For example, the multi-wavelength array may comprise a constituent red component array, green component array, and blue component array, with 1202a, 1202b and 1202c being red, green and blue lasers respectively. To record this on the holographic medium 1230, at the first stage only light from the red laser 1202a is used (which may be controlled using the shutters 1224, or otherwise, for example through directly controlling pulsed lasers), and the micromirrors of the DMD are set such that they produce a red data beam carrying the red component array, and the component array is recorded on the holographic medium as a hologram formed from an interference pattern between the data beam 1211 and reference beam 1212. The red laser 1202a is deactivated and the green laser 1202b is activated, and the DMD is adjusted to produce a data beam carrying the green component. This is recorded on the holographic medium as a further hologram overlapping with the first, and the green laser 1202b is deactivated and the blue laser 1202c is activated. The final blue component array is produced by the DMD and recorded in the storage medium 1230 as a third hologram. This stores all three holograms on an overlapping region of the holographic medium. Following this, the holographic medium 1230 may be moved to expose a different region of the medium to the object and reference beam, and the process may be repeated to record a different plurality of holograms storing a different multi-wavelength array storing further data onto the holographic medium. This preferably occurs whilst the holographic medium is in motion, and the holographic medium suitably moves no more than a distance equal to one tenth of the wavelength of the incident light during the exposure time for each respective wavelength.

[0358] The multi-wavelength array may be used to store data, as described previously, where each colour represented at each pixel (or group of pixels) in the multi-wavelength array corresponds to a different data value. Each pixel of the multi-wavelength array may store 3 bits (8 data values), or more data values (and more bits) if each pixel of each component array can take on more than 2 intensity values. To record an intermediate intensity using a DMD 1300, a micromirror 1366 of the DMD 1300 may be quickly toggled between its on and off configuration when it is illuminated by the signal beam. This changes the intensity of the light through pulse width modulation, and the micromirror reflects a smaller amount of the light onto the holographic medium, where the reflected intensity is determined by the ratio at which the mirror is in each configmation.

[0359] Typical DMDs that may be used may have 5.4 pm, 7.6pm, 10.8pm and 13.6pm pitch and may support XGA, 1080p, WUXGA, and WQXGA resolutions.

[0360] Returning to Figure 12a, after being reflected from the SLM 1220, the data beam 1211 shown is incident onto a first lens or system of lenses 1222a and onto second lens or system of lenses 1222b, which focusses the data beam onto the holographic medium 1230. The lenses me preferably achromatic. Between the two systems of lenses 1222a and 1222b, there may be provided a Fourier filter 1260. Since the SLM comprises a small grid of pixels which have a pitch that may have a similar order of magnitude to the wavelength of the signal beam, the SLM may effectively act as a diffraction grating, producing a plurality of diffracted orders. The Fomier filter 1260, further shown in Figure 14, may be used to filter some of these orders from the data beam. Following the Fourier filter 1260, a converging lens 1222b or system of lenses focusses the data beam onto the holographic medium 1230.

[0361] The holographic medium 1230 is stored on supply reel 1232 and take-up reel 1234. In addition to these reels is shown a secondary take-up reel 1258, which may be used if the holographic medium 1230 comprises a protective layer, for example wherein a protective polymer layer is provided over the photopolymer layer used for writing. In this case, the secondary take-up reel 1258 receives the protective layer of the holographic medium 1230 as the holographic medium is unwound from the supply reel 1232.

[0362] After being unwound from the supply reel 1232 and having protective layer 1258 removed, the holographic medium 1230 passes along rotating cylinder 1252. The rotating cylinder 1252 is a transparent cylinder (preferably made of glass, which may have surfaces coated with an anti-reflective material) such that the data and / or reference beams 1211 and 1212 (where the angle of the reference beam may be changed by prism 1254 and mirrors 1256) may pass through the cylinder onto the holographic medium (the beams may both pass through the cylinder in a transmission arrangement, or one beam may be incident from the other side in a reflection arrangement).

[0363] The cylinder 1252 rotates in sync with the reels 1232 and 1234, and rollers 1236 guide the holographic medium 1230 onto the rotating cylinder. For example, the tacky surface of a photopolymer material which is exposed by removal of the protective layer may temporarily adhere to the cylinder 1252, and the rollers 1236 may help to prevent the formation of air bubbles between the holographic medium 1230 and the cylinder 1252. The rotating cylinder 1252 is therefore an advantageous way of providing correct positioning and alignment of the holographic medium 1230, whilst allowing clear access of the writing beams to the holographic medium 1230.

[0364] The reference beam 1212 passes through a spatial filter 1216, and is reflected by a concave mirror 1249 towards an actuatable prism 1254. The position of the prism is adjustable; for example, it may be activated by a servodriven translation stage. The prism directs the converging laser beam onto one of the mirrors 1256, which direct it onto the holographic medium 930, onto the same region as the signal beam. The actuatable prism may be advantageously used to modify the angle of the reference beam incident on the holographic medium 1230, allowing a plurality of arrays to be stored on a region of the holographic medium at different angles through angular multiplexing. For example, the prism 1254 may be used to produce a first reference beam 1212a incident on the holographic medium at a first angle and a second reference beam 1212b incident on the holographic medium at a second angle. Further details of a holographic medium with a plurality of holograms angularly multiplexed thereon are shown in Figure 20. The prism 1254 may also be provided in the other embodiments described, such as in Figure 1, where the mirror 114 that reflects light onto the storage medium 130 may be replaced with the prism and mirrors may be placed adjacent to the holographic medium 130.

[0365] Figure 12b shows how the apparatus shown in Figure 12a may be mounted on a movable stage 1264 to allow the holographic medium 1230 to be moved relative to the signal and reference beams 1210 and 1212 in a second direction 1264b which is perpendicular to the first direction 1265a along which the holographic medium 1230 moves by rotation of the reels 1232 and 1234. The movable stage has fixed thereon rotatable supply and take-up reels 1232 and 1234, around which is wound holographic medium 1230 (in the form of a tape). The medium also passes along rotating cylinder 1252, secondary take-up reel 1258 for any removable layers of the storage medium 1230, and roller 1236 (further rollers not shown may be provided, particularly adjacent to the cylinder 1252 to avoid formation of air bubbles between the medium 1230 and the cylinder 1252).

[0366] The reels 1232, 1258, 1234 and cylinder 1252 are mounted on the movable stage 1264, and rotate together (either continuously or in a stepwise manner) to feed the holographic medium through the system. The cylinder 1252 is mounted on a housing 1253 that is driven to provide rotation of the cylinder. As a result, the holographic medium 1230 is movable in two directions relative to the writing beams. As the reels 1232 and 1234 rotate, the holographic medium moves in a first direction 1265a relative to the writing beams. This allows a plurality of holographic recordings to be made along the length of the holographic medium 1230. The movable stage 1264 is able to move in a second direction 1265b, perpendicular to first direction 1264a. By moving in direction 1265a between recordings, a plurality of holographic recordings can be made along the width of the holographic medium. Consequently, holographic recordings (each corresponding to a plurality of holograms recorded at a different wavelength) may be made in a grid comprising rows and columns of recordings, as shown in Figures 2, 9 and 11.

[0367] One method of recording grids of holographic recordings on the holographic medium 1230 is to produce a first row of holographic recordings along the length of the storage medium 1230 by rotating the reels 1232 and 1234 (preferably in a continuous motion without stopping for each recording) and moving the storage medium 1230 in second direction 1265a. When this column is finished (either by the end of the holographic medium 1230 being reached or when the desired number of recordings is made), the movable stage 1264 is moved in second direction 1265b by a distance equal to the desired column separation. The reels 1232 and 1234 may then be rotated again to record a second column of holographic recordings adjacent to the first column (the second column may be recorded in the same direction as the first column, in which case the reels are rewound prior to recording the second column, or the reels may not be rewound and the second column may be recorded in a direction opposite to the first column). The process may be repeated to produce further columns on the storage medium 1230.

[0368] Alternatively, holographic recordings may be made on the holographic medium 1230 by recording in rows along the width of the holographic medium. In this configmation, at each position along the length of the holographic medium, the movable stage 1264 may be moved in direction 1265b to produce a plurality of adjacent holograms along the width of the holographic medium 1230 (again, this is preferably in a continuous motion without stopping for each recording). Thereafter, at a fixed position of movable stage 1264, the reels 1232 and 1234 rotate to move the holographic medium 1230 by a distance equivalent to the desired hologram separation. Subsequently, the movement of movable stage 1264 is repeated, producing a second row adjacent to the first. The process is repeated to produce recordings row-by-row, rather than column-by -column as described above. This process may require less winding and rewinding of the reels than the method above, but may be slower as the holographic medium 1230 is able to be moved continuously for a shorter time along its width than along its length.

[0369] For increased compactness of the holographic recordings, the holographic recordings may be recorded diagonally, as shown in 1930b in Figure 19. To achieve this, the holographic medium may be moved in directions 1265a and 1265b simultaneously; i.e. the reels 1232 and 1234 and the movable stage 1264 moved simultaneously. This may produce a diagonal row 1976 as shown in Figure 19, and the medium is moved back (rewinding the reels by a small amount and moving back the stage 1264) before starting the next row.

[0370] Further details of the Fourier filter are shown in Figure 14. Light from a spatial filter 1416 is reflected by a collimating mirror 1448 onto an SLM (preferably a DMD) 1420. There may be some difficulty in reflecting light onto the DMD at a shallow angle (close to the normal of the DMD) without blocking the reflected light by mirror 1448. An aspheric collimating mirror may advantageously allow proximity of the mirror to the SLM. The collimating system may comprise a plurality of elements which in combination are effectively achromatic.

[0371] Since the pitch of the pixels of the DMD may have a similar order of magnitude to the wavelength of light used, the DMD may act as a diffraction grating. In particular, due to the pattern of the activated (tilted) pixels, the DMD may function as a partially blazed grating. This produces a plurality of orders of light reflected from the DMD, with the angle of each order being given by Bragg’s law: d sinO = n X where d is the pixel pitch, 0 is the angle of incidence of the light on the DMD, n is the diffraction order number, and X is the laser wavelength. There is therefore a plurality of orders of light reflecting from the DMD ; orders -1, and +1, corresponding to diffracted light, and order 0, corresponding to reflected light, are shown in Figure 14 (further orders may be present, such as +2 and -2). Barriers 1468 and 1460 are provided to filter the orders. The barriers comprise a screen with a small aperture; the aperture may have a fixed width or may be adjustable. Barrier 1468 is located in front of a converging lens 1124, which may be a single achromatic lens or a system of lenses comprising a plurality of elements which in combination are effectively achromatic. Barrier 1460 is located at the focal length of lens or lens system 1422b, such that the aperture of barrier 1460 coincides with the focal point of lens 1422a.

[0372] The barriers 1460 and 1468 filter most of the orders of light from the signal beam, leaving a small number - for example, one order of light or two orders of light. In Figure 14, only a single order, the Oth order, passes through the barriers. The signal beam with filtered orders passes through another converging lens or lens assembly comprising lenses 1422b and 1422c. These focus the data beam onto the holographic medium 1430.

[0373] However, it may be beneficial to include further orders, such as the +1, -1, +2, and / or -2 orders in the signal beam incident on storage medium 1430. This may allow improved brightness, increased sharpness and reduced noise. DMD patterns produce both a positive and negative image, with the negative image off-axis and the positive image on-axis. The negative image may be filtered, and the negative image information consequently lost.

[0374] Although an increased number of orders of diffraction may result in difficulties in resolving images, alignment of at least one of the higher orders of diffraction with the zero-order reflected light may provide additional brightness to the hologram. Therefore, allowing at least one higher order through the Fourier filter may be advantageous. In “Simulating digital micromirror devices for patterning coherent excitation light in structured illumination microscopy” by Mario Lachetta et al (Royal Society), it is described how in the field of optical microscopy configurations of certain laser wavelength whose various diffraction orders coincidentally align with zero order light to enable stronger array beams in a plurality of wavelengths sharing a single optical axis after reflection. Such techniques may be applied to embodiments of the present invention to achieve additional exposure power at the film.

[0375] Whilst the arrangement shown in Figure 14 uses an SLM which reflects the signal beam, such as a DMD, the same considerations apply to an SLM through which light is transmitted, such as an LCD. The pitch of the pixels in the transmissive SLM may cause the SLM to act as a transmissive diffraction grating, and produce diffracted orders of light from a beam transmitted through the SLM. Barriers such as 1468 and 1460 may be placed following such an SLM in a similar manner to that described above.

[0376] All of the embodiments shown previously have been suitable for writing reflection holograms, in which the signal beam and reference beam are incident on opposite sides of the holographic medium. However, such holograms can also be recorded as transmission holograms, in which the reference beam is incident onto the same side of the holographic medium as the signal beam. Apparatus 1500 for producing such a transmission hologram is shown in Figure 15. A signal beam 1510 and reference beam 1512 are produced as previously, and the signal beam is modulated with data corresponding to a multi-wavelength array by SLM 1520 to produce a data beam 1511. However, the reference beam 1512 is reflected by the mirrors 1514 onto the proximal side (the side facing the SLM 1220 and other signal beam producing optics) of the holographic medium 1530. As before, the reference beam 1512 and data beam 1511 form interference fringes at their intersection, and these are recorded on the holographic medium. A plurality of at least partially overlapping transmission holograms is stored at each region of the holographic medium, the plurality of holograms storing data as a multi-wavelength array.

[0377] To retrieve the stored array, the holographic medium 1530 is illuminated by the reference beam 1512 only from the same angle and at the same configmation as writing. The reference beam is transmitted through the holographic medium, which reproduces the array. The array may then be captured by detector 1540 and processed to retrieve stored data pages. The transmission hologram method may be used in combination with any of the previous embodiments shown, such as those with multiple SLMs shown in Figures 3, 4 and 5. It may be used to produce arrays including those shown in Figures 2, 9 and 11.

[0378] To produce a transmission hologram, the relative intensity of the reference beam may advantageously be increased relative to the signal beam; whilst for a transmission hologram the ratio of object to reference light may be close to equality (with the two having similar intensities), for a reflection hologram the reference beam may be brighter. Reflection holograms may also require an increased intensity of the reconstruction beam used to retrieve the recorded hologram.

[0379] Figure 16 shows apparatus 1600 for recording and viewing a reflection hologram, in which the imaging apparatus 1640 is positioned on the proximal side of the holographic medium, adjacent to the signal / data beam. The holograms are recorded as previously described, but to replay the stored array, the reference beam 1612 is passed through the holographic medium 1630, through lens 1622, and reflected from a beamsplitter 1672 onto detector 1640. Advantageously, this may allow the same optics, such as lens 1622, to be used for both reading and writing of the holograms. This may reduce the number of component arrays used and improve the compactness of the device. This arrangement may be used in combination with any of the embodiments, such as those with a plurality of SLMs, and to record arrays such as those shown in Figures 2, 9 and 11.

[0380] The beam splitter 1672 may be a polarising beam splitter, which may be provided in combination with a 1 / 4 wave plate between beam splitter 1672 and SLM 1630. This records the hologram in circular polarisation, and may allow the replayed hologram to reflect from the surface of beam splitter 1672 into the detector 1640 more efficiency. In addition, a 1 / 2 wave plate may be placed in the path of the reference beam 1612 to rotate the polarisation of the reconstructed hologram so that it reflects off beam splitter 1672 into the detector 1640.

[0381] Figure 17 shows apparatus 1700 for producing a coaxial hologram. Light from three laser sources 1702 is combined by dichroic filters 1706 to form a single beam 1710 comprising a plurality of wavelengths. Unlike previous embodiments, the combined beam is not separated into a signal beam and a reference beam, but a single beam is incident onto the recording optics. Instead of providing a separate signal beam and reference beam, a portion of combined beam 1710 is used as the data element and a portion is used as the reference element. For example, the central portion of the beam 1710 may be used as the reference element and not modified in any way as it passes through the SLM 1720, whilst the outer portion of the beam may be used as the data element, wherein the SLM 1720 encodes array data onto the other portion of the beam. Conversely, the outer portion of the beam may be used as the reference element and the inner portion as a data element; or the beam may be otherwise be partitioned into the reference and data portion (e.g. non-congruent portions of the beam may also be used for either portion). The interference between the data element and reference element records a plurality of holograms storing multiwavelength data on the holographic medium 1730. To retrieve the data, a reconstruction beam incident from the same angle and comprising the same wavelength(s) as the reference portion of the recording beam is directed onto the holographic medium 1730. This apparatus may be combined with other features of the apparatus shown in previous embodiments, such as the use of multiple SLMs, sequential writing, and recording arrays with 3 or more intensity values per pixel. Advantageously, the use of a coaxial hologram may allow a simpler apparatus with fewer optical components, and / or reduce errors that may result from inaccurate alignment of the reference beam with the signal beam on the holographic medium 1730 (see for example mirrors 114 in Figure 1, which may require careful alignment to ensure the array is written and reconstructed correctly at adequate quality).

[0382] Figure 18 shows an alternative embodiment of a system for producing reflective coaxial holograms. The apparatus 1800 is similar to apparatus 100 shown in Figure 1. However, instead of being incident on the holographic medium at an oblique angle, as in previous reflective embodiments, the data beam 1811 is incident on the holographic medium 1830 perpendicular to the storage medium. The reference beam 1812 has the same optical axis as the data beam 1811, but the data beam 1811 and reference beam 1812 are incident onto opposite sides of the holographic medium (i.e. to produce a reflection hologram). The reference beam 1812 is directed onto the holographic medium 1830 by circular aperture (or diffusion ring) 1868 and focussing lens 1838. The circular aperture filters a portion of the reference beam 1812 (see dotted line) and the focussing lens 1838 focusses this onto the holographic medium 1830, preferably where the holographic medium is positioned at the focal length of focussing lens 1838. Focussing lens 1838 is preferably achromatic and may be a system of lenses to achieve the focussing effect of a single achromatic lens. The data beam and reference beam interfere to record a plurality of holograms storing multi -wavelength data as described previously. Where possible, this system may be combined with other previous embodiments, for example with the embodiment shown in Figures 5a and 5b to provide a plurality of data and reference beams incident on multiple regions of the holographic medium. Such a system may be modified to provide a plurality of filters 1842 and 1838, one of each for each data beam.

[0383] Advantageously, a coaxial reflection system may allow improved hologram quality since colour (multiwavelength) holograms may be better recorded and reproduced as reflection holograms. A coaxial reflection arrangement may also allow easier reproduction of recorded holograms, since it may be more straightforward to provide a reconstraction beam perpendicular to the holographic medium 1530, rather than requiring a specific angle to be found.

[0384] To reconstruct the holograms stored on the holographic medium, a reconstruction beam is provided, which may be produced using reading / writing apparatus 1800 or using dedicated reading apparatus without writing components. The reconstruction beam is incident from the same angle as the reference beam 1812 used for writing onto the holographic medium - since the reference beam 1812 is perpendicular to the holographic medium 1830, this may be easier to achieve than providing a reconstruction beam at an oblique angle. The reconstruction beam may be directed onto the holographic medium using a similar set-up comprising diffusion filter 1842 and focussing lens 1838, or instead may simply be produced by a source perpendicular to the holographic medium, which may advantageously require fewer optical components. A reconstructed signal beam is produced comprising the stored multi-wavelength array, and this may be reconstructed signal beam may be read by detector 1840.

[0385] Figure 19 shows possible arrangements of rows and columns of holographic recordings in holographic storage media 1930a and 1930b, each holographic recording comprising a plurality of holograms stored in one or more overlapping regions of the holographic medium. Storage medium 1930a comprises rows of pluralities of holograms 1974 that are recorded by separately moving the holographic medium in a first direction and subsequently in a second direction relative to the recording beams, as previously described, where the holographic exposures may be provided whilst the holographic medium is moving in the first direction or in the second direction.

[0386] Storage medium 1930b shows an offset pattern of pluralities of holograms, wherein each column is offset from the adjacent column, producing diagonal rows 1976. Such an offset pattern may allow improved data density. It may be recorded column-by -column, where the storage medium is shifted between columns and each column is started at a slightly offset position to the previous column. Alternatively, it may be produced by providing exposures onto the holographic medium whilst the storage medium is moving in both the first direction and the second direction simultaneously (as described with reference to Figure 12b). Therefore, the pattern of recordings 1976 may allow improved medium and data throughput speed.

[0387] Figure 20 illustrates how angular multiplexing may be used in combination with any of the methods described above to achieve greater data density. The holographic medium 2030 has signal beam 2010 incident on a first side of the holographic medium. The reference beam is incident on the second side of the holographic medium 2030, and a plurality of recordings may be made, with each one being made at a different angle of the reference beam. For example, the reference beam may be switched between positions 2012a, 2012b, 2012c, 2012d, 2012e, 2012f. The angle of the reference beam in the x-y plane (corresponding to the plane of the storage medium) and the angle of the reference beam to the z-axis (corresponding to the normal to the storage medium) may be varied. For example, each beam position may be at a set angle to the normal to the medium (for example, 60°, 45°, 30° or less, 15°, or any intervening angles). The reference beam angles may then be separated by angles 0i, 02, 03, 04, 05 in the x-y plane. The angles may all be equal, or they may be different between each beam. For example, the multiple reference beams may be spaced by 45° or less, 30° or less, 20° or less, 15° or less).

[0388] This allows multiple arrays (each of which encodes different data) to be stored in the same (or substantially the same) region of the holographic medium, increasing data density. For example, two or more arrays, three or more arrays, or four or more arrays may be angularly multiplexed onto the same region of the storage medium in this way. Each array may be an array according to Figure 9 or a multi-wavelength array according to Figures 2 or 11. To retrieve each stored array, the holographic medium is illuminated by the reference beam at the same angle the respective array was recorded.

[0389] However, attempting to record multiple angularly multiplexed holograms or pluralities of holograms on the same region of the storage medium 2030 may limit the ability of the arrays to be reconstructed, resolved, and / or decoded for extraction of data. The addition of increasing numbers of microstructures formed by the stored interference patterns in the holographic medium, each of which modulates the refractive index of the medium, gradually decreases the fidelity of the diffraction grating formed by each recording as the active layer of the medium becomes overcrowded. Therefore, the reference beams are suitably appropriately distributed to avoid overcrowding of the medium. Radial distribution of arrays throughout a circle of possible incidence angles around the x-y plane may provide appropriate distribution to ensure clear separation of the arrays and prevent “ghosting” during the data recovery process, whilst still providing a high data storage density.

[0390] The angularly multiplexed arrangement shown in Figure 20 may be produced by apparatus such as apparatus 1200 shown in Figures 12a and 12b, where the position and angle of the prism 1254 may be adjusted to change the angle of the reference beam reflected by the mirrors 1256 onto the holographic medium 1230. Different positions of the prism thus correspond to different angles of incidence of the reference beam onto the holographic medium 1230.

[0391] Figures 21a and 21b show further details of the process of retrieving arrays from a holographic medium and reading the data recorded thereon. Figure 21a shows details of apparatus for reconstructing data from storage medium 2130 in a reflection arrangement. The holographic medium 2130 (or a photosensitive storage layer of the storage medium) has a first side 2178a and a second side 2178b. When recording data (such as a plurality of holograms according to previous embodiments) 2180 onto the holographic medium, the first side 2178a is illuminated by the reference beam, to produce a cured active layer comprising a hologram 2146 on or near the first side 2178a of the storage medium or layer. The holographic medium 2130 therefore has a region where a hologram or plurality of holograms storing data in the form of an array are recorded.

[0392] To retrieve the stored data, the region of the holographic medium 2130 comprising the hologram or plurality of holograms 2146 is illuminated by reconstruction beam 2144, wherein the reconstruction beam is incident onto the second side 2178b of the holographic medium 2130. This means the holographic medium 2130 is inverted from its original recording orientation. This produces a pseudoscopic real image of the array displaced from the second side 2178b of the holographic medium 2130. The reconstruction beam 2144 is positioned at the appropriate angle to the holographic medium 2130 corresponding to the angle of the reference beam.

[0393] For some materials, the imaging and curing process may cause a small reduction of the thickness of the active photopolymer layer (typically around l%-2%). This may cause the narrowing of the fringes of the recorded holograms and may result in a hypsochromic shift (which may be for example around 5nm-10nm, such as 8nm). As a result, the reconstruction beam may comprise light of an adjusted wavelength, or of adjusted plurality of wavelengths. The optimal angle of illumination for reconstruction of the hologram may also be affected by the change in the photopolymer layer and a small change in the angle of the reconstruction beam 2144 may be made relative to the original reference beam used for recording. The amount of angle and wavelength shift is dependent upon the inclination of the interference fringes in the layer, which in turn is determined by the original angle of the reference beam.

[0394] The reconstruction light source is preferably a laser, more preferably a diode laser (with an adjusted wavelength to account for hypsochromic shift, if appropriate). For example, a green hologram exposed at 532nm may be reconstructed with 525nm laser diode. Alternatively, since the reconstruction of a hologram does not require coherent light illumination, another light source, such as an LED, can be used to produce a reconstruction beam with a broad linewidth (and wavelength adjustment may not be required.

[0395] The reconstruction beam 2144 may be a collimated, converging, or diverging beam. The relative geometry of the reconstruction beam 2144 and the original reference beam may be used as a means to expand or contract the image of the array 2180; for example, a diverging beam may be used to expand the size of the image. The lens arrangement 2138a, 2138b is used to focus the projected array onto a detector 2040, such as a CCD or CMOS panel. The lens arrangement 2138a, 2138b may advantageously be of a configmation similar to the projection lenses used in the creation of the holograms to assist in the reduction of array aberration.

[0396] An additional holographic or printed mark 2186 may be made during the recording process on a region of the holographic medium 2130, such as on the edge of the holographic medium. Multiple marks 2186 may be provided at regular intervals corresponding to the stored holographic data on the medium. The marks 2186 may comprise an index of the data stored on the holographic medium, and the reader system may electronically check the marks to allow improved location of data pages for an improved speed of recovery. The marks may be an encoding layer; further details me described with reference to Figures 26-30.

[0397] As previously described, each region of the holographic medium may have a plurality of angularly multiplexed holograms stored thereon. It is possible for these holograms to be read simultaneously, with a plurality of reconstraction beams provided at a plurality of corresponding angles. It is also possible to use chopped light synchronised between the laser somce and detector to read these angularly multiplexed holograms.

[0398] Figure 21b shows details of an embodiment of the data recovery process in which the hologram or plurality of holograms is replayed in a transmission arrangement, wherein the reconstruction beam 2144 is incident onto the same first side 2178A of the storage medium 2130 as the reference beam used for recording. This produces a virtual image of the stored array 2180, located away from the second side 2178b of the holographic medium 2130. The detector 2140 and objective lens 2138c me used to view this virtual image. For a storage medium 2130 containing a plurality of holograms (which may be indexed by the marks 2186 previously described), the holographic medium is moved to bring a chosen hologram or plurality of holograms into alignment with the detector 2140 and reconstruction beam 2144, producing an image of an array 2180 stored on the hologram in the line of view of the objective lens 2138c and detector 2140. The objective lens 2138c is selected with focal length and numerical aperture to allow reconstraction beam 2144 to be incident on the storage medium 2130 without light contamination of the detector 2140. The detector converts image data into a data stream, which may be transmitted by data connection 2182 to computing device 2184. The method shown may advantageously reduce the amount of light from surface reflection of the reference beam that enters the objective lens and camera system as noise.

[0399] Tape

[0400] A holographic storage medium and associated componentry / systems will now be described. The holographic storage medium and associated componentry / systems is suitable for storing a plurality of multiplexed holograms storing data (in particular using a multi-wavelength array) as set out in relation to Figures 1 to 21, but may also be used with other holographic data storage methods / systems.

[0401] Referring to Figure 22, there is shown a holographic storage device 2200 comprising a holographic storage medium in the form of a holographic tape 2202 on a reel 2204.

[0402] A first end of the tape 2202 is attached to the reel 2204. A first end of the tape may be fixed to the reel, or it may be removably attached to allow the tape to be removed from the reel.

[0403] The tape has a height or thickness h, a width w, and a length x (where the tape 2202 is wound around the reel 2204 along its length).

[0404] The holographic tape 2202 comprises a photosensitive layer, where a hologram may be recorded as an interference pattern in the photosensitive layer. The photosensitive layer is preferably a photopolymer. The holographic tape also comprises at least one other layer. This may be a substrate layer, which is preferably a robust, optically clear polymer. At least one of the substrate layers may be removable.

[0405] Preferably, the photopolymer layer has a thickness of 15pm or less, preferably 12pm or less, more preferably 10pm or less, more preferably 7pm or less, more preferably 5pm or less, more preferably 3pm or less. The at least one substrate layer preferably has a thickness of 35pm or less, preferably 30pm or less, more preferably 20pm or less, more preferably 10pm or less, more preferably 5pm or less, more preferably 3pm or less.

[0406] The holographic tape preferably has a width of 5mm or more, 10mm or more, 15mm or more, 20mm or more, or 50mm or more. Preferably, the holographic tape has a width of 12.650mm, the same width as the Linear-Tape Open (LTO) standard for magnetic tape.

[0407] The holographic tape 2202 is suitable for storing holograms, and holograms storing data may be written onto and / or read from the tape 2202 in accordance with any of the embodiments shown in Figures 1 to 21 above.

[0408] Figure 23 shows a holographic storage device in which a reel 2304 and holographic tape 2302 attached to and wound around the reel 2304 are provided in a cartridge 2306. The reel 2304 with round tape 2302 is held inside a cartridge body 2306 which encloses the reel and tape. The reel 2304 is rotatably held within the cartridge 2306 such that it can rotate, allowing the holographic tape 2302 to be wound onto and unwound from the reel 2304. The cartridge 2306 comprises a reel rotation mechanism 2308, which is accessible from the outside of the cartridge and allows holographic storage equipment to engage with the reel rotation mechanism 2308 to rotate the reel 2304.

[0409] The cartridge body 2306 comprises an opening 2310 through the holographic tape 2302 may be accessed. The opening 2310 comprises an access gate 2312 which can close the opening, protecting the holographic tape 2302 when it is not in use. The access gate 2312 can be opened and closed by a mechanism integrated into holographic storage equipment.

[0410] A first end of the tape 2302 is attached to the reel 2304, whilst a second end of the tape comprises a leader pin 2314. The leader pin is a bar shaped body extending along an edge at the second end of the tape. The leader pin may be engaged by a / the holographic storage apparatus to allow the apparatus to withdraw the tape 2302 from the cartridge 2306. The leader pin may be used to attach the holographic tape to a second reel, as shown in Figures 25a- 25e. The leader pin may be larger than the opening 2310, thereby preventing the second end of the tape 2302 from entering the cartridge 2306, allowing the second end of the tape 2302 to remain accessible. The edges of the opening 2310 may comprise recesses to hold the leader pin when the tape is not in use.

[0411] The cartridge 2306 has a generally square shape, where the reel 2304 is positioned towards the middle of the cartridge. The cartridge 2306 also comprises a cut-out 2316 (located at a comer of the cartridge), which may be used to help insert and align the cartridge 2306 in holographic storage apparatus. The cut-out 2316 may be used to easily identify a correct orientation for inserting the cartridge 2306 into storage apparatus, and the apparatus may prevent insertion of the cartridge 2306 unless it is placed with the cutout 2316 in the correct orientation. The cartridge 2306 may comprise further cut-outs, as well as markings to assist the cartridge placement.

[0412] Preferably, the cartridge 2306 has the same format as a Linear-Tape Open (LTO) cartridge, and the properties of the cartridge 2306, such as the cartridge dimensions, tape width, and features of the cartridge 2306 such as the opening, leader pin, cut-outs, and reel rotation mechanism, correspond to features of an LTO cartridge. For example, the cartridge dimensions may be 102.0mm by 105.4mm by 21.5mm and the tape width may be 12.65mm to correspond to the dimensions of the LTO cartridge. However, unlike the magnetic tape in an LTO cartridge, the tape in a holographic cartridge 2306 is holographic tape, which comprises a photosensitive element to allow recording of holograms. In other words, the cartridge contains a holographic storage medium formed / shaped as tape (wrapped around a reel). Accordingly, the tape materials are different to those of an LTO tape. Accordingly, the tape thickness and tape length may also differ from the LTO standard.

[0413] By providing a holographic tape 2302 suitable for storing data in a cartridge 2306, a high-capacity, easy to store and standardised holographic storage format is provided. A cartridge with dimensions corresponding to the LTO format allows existing storage space designed for LTO cartridges to be re-utilised for holographic cartridges, and may also allow portions of existing storage apparatus (e.g. those for receiving cartridges and engaging the tape) to be reused. This can help to reduce the cost and effort of transitioning from existing magnetic storage to holographic storage. Holographic tape cartridges can advantageously be used for archival purposes, where a standardised, readily storable format is desirable.

[0414] Figure 24 shows holographic storage system 2400 for writing holograms onto and reading holograms from a reel-to-reel tape stored on a cartridge 2406. The system 2400 comprises a cartridge bay 2418 to receive the cartridge 2406. The cartridge bay 2418 may be motorised to allow motorised insertion and / or extraction of the cartridge 2406.

[0415] After receiving the cartridge 2406, the system 2400 is configured to engage the holographic tape 2402 to the second reel 2420, also referred to as a take-up reel. The system comprises an engagement mechanism to engage the end of the tape 2402 from the cartridge 2406, transfer the end of the tape towards take-up reel 2420, and affix the end of the tape onto the reel; further details of the engagement mechanism are shown in Figures 25a-25e. The engagement mechanism may engage with the tape via the leader pin 2314 shown in Figure 23, and the leader pin may be used to affix the end of the tape 2402 onto take-up reel 2420.

[0416] Once the tape is engaged by the holographic storage system 2400, the tape therefore becomes part of a reel- to-reel arrangement in which the tape is positioned between the cartridge reel 2404, which acts as a supply reel, and the take-up reel 2420. This allows the tape to be easily moved through the system by rotation of the reels, causing the winding and unwinding of the tape from each respective reel. To move the holographic tape 2402 through the system, reels 2404 and 2420 rotate in direction 2424. The motion of the tape through the system is assisted by rollers 2422. The rollers 2422 guide the tape as it moves through the system, keeping the tape in the correct position for writing and / or reading. The rollers 2422 also keep the tape at tension, which can help prevent slippage and ensure smooth movement of the tape through the system. The engagement mechanism may help to position the tape onto the rollers; the rollers may also be movable to allow the rollers to move between a position suitable for loading the tape and a position suitable for writing and / or reading. Whilst only two rollers 2422 are shown, further rollers can be provided at different positions along the tape. Pairs of rollers 2422 may also be provided with a roller 2422 on either side of the tape 2402, with the tape moving between the two rollers; this can help provide improved guidance to the tape. The system 2400 also includes holographic apparatus 2430, which is located within the system 2400 so as to be able to operate on (i.e. read from / write to) tape moving through the system. In particular, the holographic apparatus may be located between rollers 2422, such that the rollers 2422 support the tape in the correct position for reading / writing (where this correct position may necessitate that parts of the holographic apparatus 2430 may access the tape from either side - i.e. from an upper surface and from a lower surface).

[0417] The reels are controlled electronically to ensure they move at the correct speed. The reels may also be linked mechanically to ensure they move together whilst allowing adjustments in speed; this may for example be achieved through an adjustable gear mechanism.

[0418] For writing holograms onto the holographic tape 2402, the holographic tape 2402 is illuminated by a data beam 2426 and a reference beam 2428, which are produced by holographic apparatus 2430. The data beam 2426 is incident on a lower surface of the tape, where the reference beam 2428 is incident on an upper surface of the tape. The data is preferably recorded as described previously with reference to Figures 1 to 20. The data beam 2426 comprises data encoded into the beam by a spatial light modulator and interferes with the reference beam 2428 (which does not comprise any data) to produce an interference pattern on the medium. This interference pattern is recorded in the holographic tape, thereby storing data on the tape in holographic format.

[0419] The data beam 2426 and reference beam 2428 may have any of the properties previously described. In particular, the data beam 2426 and reference beam 2428 may comprise radiation of a plurality of wavelengths, thereby allowing wavelength multiplexed holograms to be recorded. A plurality of signal and / or reference beams may be provided to record a plurality of holograms into the holographic storage medium simultaneously, which may each have different properties, such as wavelength (for wavelength multiplexing) or angle (for angular multiplexing, such as that shown in Figure 20). For example, multiple data beams 2426 each having a different wavelength may be incident on the holographic storage medium to simultaneously record individual monochromatic components of a multi-colour hologram, as in Figures 5a and 5b.

[0420] Whilst the reference beam 2428 is shown to be incident onto the holographic tape 2402 from the opposite side of the tape to the data beam 2426 (to record a reflection hologram), the system 2400 may also be arranged so that the reference beam 2428 is incident from the same side of the holographic tape 2402 as the data beam 2426 (to record a transmission hologram). Further arrangements such as coaxial arrangements shown in Figures 17 and 18 are also possible.

[0421] Preferably, recording occurs whilst the holographic tape 2402 is in continuous motion, and a pulse of the data beam 2426 and reference beam 2428 is provided onto the holographic tape 2402 at regular intervals corresponding to when the holographic tape 2402 has moved by a desired hologram pitch. As described, the holographic tape moves a distance no greater than 1 / 10 of the wavelength of the signal and reference beams (or the shortest wavelength if a plurality of wavelengths is used) . The pulses are provided by either a continuous radiation source (such as a continuous wave diode laser) with a shutter, or by a pulsed radiation source (such as a laser)

[0422] In an alternative embodiment, it is also possible for the holographic tape 2402 to be stopped (or at least slowed down) to record each hologram.

[0423] The holographic apparatus 2430 comprises optical and other equipment for generating the data beam 2426 and the reference beam 2428. This equipment may be as described with reference to any of the embodiments shown in Figures 1 to 21. This comprises one or more radiation sources (preferably monochromatic, coherent, collimated radiation sources such as a laser). The wavelengths of the radiation sources may be as previously described. Spatial filters and lenses may be provided to modulate one or more radiation beams from the one or more radiation sources, and one or more beamsplitters are used to divide the beams into the data beam and reference beam. If a plurality of radiation source having different wavelengths is used, one or more dichroic elements may be provided to combine the radiation beams into a single multi-wavelength beam, or the beams may be incident onto the holographic tape 2402 separately (as shown in Figures 5a and 5b).

[0424] As the holographic tape 2402 moves past the object and reference beams, a plurality of holograms are written onto the holographic tape at regular intervals, allowing a plurality of data pages to be stored in a row along the length of the holographic tape 2402. The holographic tape 2402 is unwound from the cartridge reel 2404 and wound onto take-up reel 2420 during this process. When the holographic recording process finishes for a row (such as when the end of the tape 2402 is reaches, or when all the desired data has been written onto the tape), the reels 2404 and 2420 may be rotated into the opposite direction to unwind the tape 2402 from the take-up reel 2420 and wind it back onto the cartridge reel 2404. It will be appreciated that a plurality of multiplexed holograms may be recorded at each location in the row (rather than a single hologram) - for simplicity (to avoid the need to discuss a plurality of plurality of holograms, the latter ‘plurality’ term referring to multiplexed holograms and the former ‘plurality’ term referring to differently located (multiplexed) holograms), in this section of the description reference is only made to recording a single hologram at each location in the row.

[0425] Subsequently, the recording process may be repeated to produce another row of holograms on the tape 2402. This is achieved by changing the position of the signal and reference beams on the holographic tape, such that the beams are incident on another position along the width of the tape. The reels then rotate to feed the tape through the device, producing another row of holograms on the tape. The process may be further repeated for further rows, producing a grid of rows and columns of holograms on the tape (as shown in Figures 2, 9 and 11). Movement of the beams along the width of the tape may be achieved by moving the beams or moving the tape.

[0426] For an arrangement in which the tape 2402 is moved, each of the cartridge 2404, take-up reel 2420, and rollers 2422 are provided on a movable translation stage. The stage is able to move relative to the signal and reference beams 2426 and 2428 and the holographic apparatus 2430 in the direction of the width of the holographic tape 2402. Once the recording process is complete for the plurality of rows, the holographic tape is moved back to its original position to allow removal of the cartridge 2406 from the system.

[0427] Alternatively, the signal and reference beams 2426 and 2428 are moved within the holographic system 2400 along the width of the holographic tape 2402. This may be achieved by moving at least a portion of holographic apparatus 2430. Preferably, to avoid moving the entirety of holographic apparatus 2430, only portions of the apparatus for directing the beams are moved, such as mirrors or lenses. It is also possible to move both the tape and the beams, for example to allow more efficient use of space. It will be appreciated that, in an alternative, holograms may be recorded on a column-by-column basis by moving the translation stage and / or the beams to record columns of holograms and then only moving the reels once a column has been recorded - however, in view of the benefit of the relative speed and ease of continuously moving the reels to perform ‘row-by-row’ recording the alternative ‘column- by-column’ recording is generally less effective.

[0428] It is also possible to avoid having to move the tape or beams by providing a plurality of reference and data beams, with each pair of data beams incident on a different position along the width of the tape, thereby allowing the plurality of beams to record multiple rows simultaneously. This can reduce the number of moving components in the system and increase writing speed. However, for a large tape width and small hologram pitch (i.e. the distance between successive holograms), the number of beams required may be very high; for example, for a tape width of 12.65mm and a hologram pitch of 1 ,00mm, at least 12 pairs of signal and reference beams are required, and this number is even greater for smaller holograms (or a wider tape). Therefore, to avoid having an excessive number of optical components, SLMs, etc., a plurality of pairs of reference and data beams may be provided to cover a portion of the width of the tape to write a plurality of rows simultaneously, with the beams or the tape moved after each set of rows is written (for the example above, 4 pairs of beams may be provided, and the tape or beams may be moved twice to repeat the recording process three times to write 12 rows of holograms onto the tape).

[0429] After the writing process is finished for all desired rows, wavelengths and other parameters, the tape 2402 is rewound onto the reel 2404 and the cartridge 2406 may be removed from the holographic system 2400. The tape engagement system shown in Figures 24 may be used to detach the tape 2402 from the take-up reel 2420 and return the end of the tape to the cartridge 2406. The cartridge 2406 may then be ejected, for example by an automatic ejection mechanism, and another cartridge 2406 may be inserted if required.

[0430] The holographic storage system 2400 may also be used to read holograms recorded onto holographic tape 2402. The holograms may be recorded by the same system 2400, or a different system with the same properties (such as the wavelength and angle of beams). To read holographic data from tape 2402, cartridge 2406 with data written thereon is loaded into the system 2400 as previously, the tape is engaged onto take-up reel 2420, and the reels rotate to move the tape past the beams. However, the data beam 2426 is not incident onto the holographic tape 2402, but only a reconstruction beam. The reconstruction beam comprises the same wavelengths of radiation as the reference beam and is incident onto the holographic tape at the same angle as the reference beam. Therefore, since the same equipment is used for both reading and writing, the same apparatus may be used to produce the reconstruction beam as the reference beam 2428.

[0431] However, the profile of the reconstruction beam may differ from that of the reference beam 2428. For example, the reconstruction beam may be larger than the reference beam 2428. For example, while for recording holograms a single pair of a data beam and a reference beam may be used to record only a single hologram; a single reconstraction beam may be used to read multiple holograms simultaneously. The reconstruction beam may therefore have a larger area than the reference beam; the width and breadth of the reconstruction beam are preferably an integer multiple of the hologram pitch on the medium (such as 2 or more times the hologram pitch, 5 or more times the hologram pitch, or 10 or more times the hologram pitch). A dimension of the reconstruction beam may be equal to the width of the holographic tape.

[0432] The reconstruction beam is incident onto the holographic tape and interacts with the interference patterns stored on the holographic tape, which reconstructs the data stored on the holographic tape by producing a reconstructed data beam comprising the data. The reconstructed data beam is recorded by the detector 2432. As shown in Figure 24a, the detector 2432 is positioned within the storage system 2400 above the tape 2402. The image data recorded by the detector 2432 is processed to reconstruct the data stored on the holographic tape 2402. The image data may be transmitted to a computer or other processing device in order to reconstruct the data.

[0433] As the reels 2404 and 2420 rotate during the writing or reading process, and the tape is wound and unwound from each reel, the effective diameter of the reel (that is, the diameter of the reel and the tape wrapped around the reel) changes: for example, reels which have a greater amount of wound tape therefore have a greater effective radius from which the tape is wound / unwound. Since a greater reel diameter results in a higher tape speed (at the outer edge of the tape) for a given rate of rotation of the reel, this may mean that, for example, the speed at which the tape is wound onto a take-up reel increases as more tape is wound onto the reel. This may lead to issues as a constant speed of tape is desirable for copying. Furthermore, where multiple tapes are used as master and copy media (as described elsewhere) this may further lead to the master and copy media moving at different rates, e.g. if the different (first and second) reels used with the master and copy media have different amount of wound tape. Therefore, it is desirable to adjust the speed of each medium / tape to ensure it remains constant through the copying process.

[0434] To allow monitoring of the motion of the tape(s), motion sensors 2434 may be provided at suitable positions in the holographic storage system 2400 to detect the motion of one or both tape(s). These sensors may comprise, for example, a radiation source that illuminates each medium with radiation and a detector that senses detected or transmitted radiation to determine the velocity of the medium. These may be used in combination with an encoding layer provided on each medium, as described further with reference to Figures 26-30 below. Such a layer comprises marks to allow the sensors 2434 to determine the velocity of the medium. This can be used to adjust the rate of rotation of the reels is adjusted to match the desired tape speed, for example to account for effects from changes in the effective diameter of the reels as tape is wound onto and unwound from the reels. The marks may also indicate the position of the tape (as an absolute encoder) and may provide, for example, an indication that the end of the tape is approaching. This may cause the system to end the copying process and, if required, begin rewinding the tape.

[0435] An exemplary system for engaging the tape 2502 from cartridge 2506 onto a take-up reel 2520 is schematically shown in Figures 25a-d.

[0436] Figure 25a shows an engagement system 2500 for receiving a tape cartridge 2506 and engaging the tape for reading and / or writing. The system comprises a cartridge mount 2536, which receives the cartridge 2506. The cartridge mount 2536 also acts to eject the cartridge at the end of the reading or writing process. The cartridge mount 2536 may be motorised, allowing motorised insertion and / or removal of the cartridge 2536 in response to a command or signal. Adjacent to the cartridge mount 2536 is an engagement arm 2540, which is configured to receive the tape from the cartridge and transfer it to the take-up reel 2520 via guide rail 2546.

[0437] Figure 25b shows a cartridge 2506 inserted into the cartridge mount 2536. The cartridge comprises the holographic tape 2502 on the reel 2504, with a leader pin 2514 at the free end of the holographic tape 2502.

[0438] Figure 25c shows the initial process of engaging the tape 2502 with the engagement system 2500. The engagement arm 2538 is moved towards the cartridge 2506, towards the access gate of the cartridge 2506; the access gate is opened by a mechanism within the engagement system 2500. Positioned within (on the end of) the engagement arm 2538 is tape coupler 2540; this couples to the leader pin 2514, for example by positioning a cavity or opening within the tape coupler 2540 onto the leader pin (where a suitable motorised or non-motorised mechanism may be provided in this regard). Alternate methods of engaging the tape may also be used; for example, a pin may be provided on tape coupler 2540 instead may be inserted into a suitable cavity on a leader block attached to the tape.

[0439] The tape coupler 2540 is attached to an end of take-up tape 2542, the other end of which is attached to takeup reel 2520. The take-up tape 2542 is not used for reading or writing and is not required to have holographic properties; instead, the take-up tape 2542 is used to assist transfer of the actual holographic tape onto the take-up reel 2520. However, the take-up tape 2542 may comprise markings to be used by a detector to determine the position and motion of the take-up tape 2542, for example to determine when to begin writing, or when to finish rewinding.

[0440] Figure 25d shows the process of transferring the holographic tape 2502 onto the take-up reel 2520. The takeup reel 2520 and the cartridge reel 2504 are rotated, causing the holographic tape 2502 to be unwound from the cartridge reel 2504 and the take-up tape 2542 to be wound onto the take-up reel 2520. As the holographic tape 2502 and the take-up tape 2542 are connected by tape coupler 2540 and leader pin 2514, the take-up tape 2542 pulls along the holographic tape 2502. The holographic tape 2502 passes through the engagement arm 2538, which has a channel to allow the tape to pass, and along tape guide 2546, which also comprises a channel or channels to direct the holographic tape 2502 (the tape guide 2546 may also comprise other components to guide the tape, such as rollers) and the take-up tape 2542 towards the take-up reel 2520 (in the direction indicated by the arrow). As the reels rotate, the tape moves through the apparatus. The take-up reel 2520 may comprise a slot or formation for receiving the tape coupler 2540 and leader pin 2514, allowing the holographic tape 2502 to be smoothly wound onto the take-up reel 2520.

[0441] Once the holographic reading or writing process is finished, the tape may be rewound and the cartridge 2506 disengaged from engagement system 2500, according to essentially the reverse of the process shown in Figures 25a- 25d. The reels 2504 and 2520 are rotated in the opposite direction to that shown in Figure 25d, rewinding the holographic tape 2502 onto the reel 2504. As all of the tape 2502 is rewound, the tape coupler 2540 with leader pin 2514 is disengaged from the take-up reel 2520 and passes back through the system, via tape guide 2546 back into the engagement arm 2538. Markings on the take-up tape 2542 may be used to determine when to stop the rotation of the reels. Once the tape coupler 2540 reaches the end of its track in the engagement arm 2538, the tape coupler 2540 is disengaged from the leader pin 2514, releasing the tape 2502 from the system. The engagement arm 2538 then rotates away from the cartridge 2506 to allow the cartridge 2506 to be released and removed.

[0442] Figures 25a-25d show only a schematic diagram of the engagement system 2500, and do not show any holographic apparatus. Figure 25e shows a system 2501 in which holographic apparatus is integrated with the engagement system 2500. In addition to the components shown in Figures 25a-25d, there is provided holographic apparatus 2530, for producing beams such as reference, object, and / or reconstraction beams, and a detector 2532. The holographic apparatus produces abeam or beams that are directed onto the holographic tape 2530, in accordance with any of the embodiments previously described. The holographic apparatus is provided along the path of the tape 2502 between the cartridge 2506 and the take-up reel 2520; the path of the beams produced by the holographic apparatus onto the tape 2502 (and onto the detector 2532) are kept clear of any tape guides, rollers, etc. to avoid the beam or beams being obstructed. The holographic apparatus is provided between two sections of tape guides 2546 as shown, but may also be in any other suitable position between the two reels, for example between the tape guide 2546 and the take-up reel 2520, or between rollers.

[0443] For writing, the holographic apparatus produces a data beam and a reference beam (which may be incident onto the same side of the holographic tape or onto opposite sides of the holographic tape and may be incident perpendicular to the holographic tape or at any angle, as previously described). The data beam, comprising data, interferes with the reference beam, and data is recorded onto the holographic tape. As the recording process takes places, the reels are rotated to move the holographic tape 2502 past the beams (either continuously or stopping and starting) to write holograms onto multiple regions of the holographic tape 2502. Holograms may also be multiplexed onto the tape in accordance with the principles described previously (such as wavelength, angle, and other multiplexing).

[0444] Alternatively, the beam may be a reconstruction beam, which is used to read data previously written onto the holographic medium. The reconstruction beam has the same properties (such as angle and wavelength) as a reference beam used for writing the data, though it may have a different size. If the apparatus is capable of both writing and reading, and the data has been written by the same apparatus or apparatus of a similar kind, the reconstruction beam may straightforwardly be produced by the apparatus in the same way as the reference beam used for writing (though, as previously mentioned, the aspect of the beam may also be changed, for example to make the beam wider). The reconstruction beam is incident onto the holographic tape 2502, reproducing the data that is holographically written on the tape 2502. This data is then read by detector 2532, which may transmit the data to a computer or other processing device to be decoded.

[0445] As shown, the apparatus 2501 comprises holographic apparatus for both reading from and writing onto the holographic tape 2502. It is also possible to provide holographic apparatus for only reading, in which case apparatus for producing an object beam is not required; and only writing, in which case a detector is not needed. Apparatus for only reading may also use radiation sources that have lower coherence properties than apparatus for writing, allowing more easily accessible radiation sources to be used.

[0446] Encoder

[0447] The holographic tape may also be provided with an encoding layer - this can be a physical or a holographic encoding layer. An encoding layer allows tracking of the motion of the medium. A relative encoder has a repeating pattern than can be used to track the speed and motion of the medium, whilst an absolute encoder has a unique pattern at each region of the holographic medium and can be used to determine the position of the medium as well as its motion. Both types of encoders can be used to detect the motion of the holographic to allow adjustment of the speed of the tape, whilst an absolute encoder may be used for position-dependent applications such as indexing. Many different implementations are possible, and several potential implementations of an optical encoder are shown in Figures 26-28.

[0448] Figure 26 shows a simple incremental encoder, which is a very basic relative encoder comprising an array of slits in a linear scale 2602. The linear scale 2602 is illuminated by a light source 2604 and a photodetector 2606 is positioned on the other side of the linear scale 2602. If a slit is illuminated, light passes through the encoder to the photodetector 2606, and otherwise the light is blocked. When the encoder is moving, the rate at which light flashes onto the photodetector 2606 (or a portion of the photodetector) can be used to determine the speed of the encoder.

[0449] An advanced version of an incremental encoder has a mask between the light source and array of slits or between the array of slits and the photodetector. This mask enhances the positional resolution of the encoder. A further improvement is to have multiple light sources and multiple photodetectors, which further enhances resolution.

[0450] An absolute encoder, as shown in Figure 27, has an additional array of non-repeating patterns. Light passing through this additional array triggers a separate row or section of the photodetector. A processor is used to convert this into a position.

[0451] In Figure 27, the absolute encoder comprises a scale grating 2708 and an index grating 2710. Light is produced by an LED 2704, passes through a lens 2712 and is received by a photoelectric device 2706. The signal from the photoelectric device 2706 is processed by a signal processor 2714, which transmits data to subsequent equipment 2716 according to a protocol 2718.

[0452] These above encoders can also be achieved in a reflection set up, as shown in Figure 28, wherein instead of light transmitted through slits the light is reflected from an array of mirrored surfaces / areas.

[0453] In Figure 28, the reflection encoder comprises a diffraction grating 2806. Light is produced by an LED 2804, is passed through collimating lens 2820, and is incident on the diffraction grating 2806. The light is reflected by the diffraction grating 2806 and passes through filtering optic 2822 before being detected by a detector array 2806.

[0454] Many of the optical encoder methods described above can be replicated using holographic techniques. This can be achieved by converting the slitted or mirrored surface into either a transmissive or non-transmissive, or a reflective or non-reflective hologram. Reading such holograms would be similar to reading a traditional optical encoder.

[0455] Holographic encoders may be produced by traditional reflection, transmission or Denisyuk holographic methods. One method of producing a holographic encoder along the length of a photopolymer tape includes using a master roller, into the outer surface of which a master encoder is imprinted. To copy the master encoder onto a holographic tape or material, a laser passes through the photopolymer to the roller and causes interference within the tape in a Denisyuk / reflection method.

[0456] Alternatively, an SLM, such as a DMD, can be used to record the encoding layer. In particular, the SLM may be used to record nonrepeating patterns into the photopolymer to produce an absolute encoder.

[0457] The holographic encoder pattern can be recorded onto the tape in such a way so that it is only visible at a specific angle (i.e. angularly multiplexing the encoder layer onto the medium). This is advantageous as data can be recorded in the medium at another, different angle, allowing both the encoder and data to be read at the same time in the same space without interfering with one another. This is shown in Figures 29 and 30. Figure 29 shows the holographic medium from an angle directly above the medium. In this view, data holograms are visible (and can be read by a reconstruction beam), but the encoding / indexing layer is not visible. Figure 30 shows the holographic medium from an oblique angle, offset from the normal to the medium. Here, the encoding / indexing layer is visible, and if illuminated by a suitable beam at this angle it can be read to determine the motion / position of the medium.

[0458] The holographic encoder may be formed in a manner that causes transmitted light passing through the hologram to be angularly deflected. This allows the position of the sensor to be altered in correspondence with the angle at which the light is transmitted; this may advantageously allow the sensor to be moved to a more convenient location away from other components, for example.

[0459] It is also possible to use a non-holographic encoder into a holographic medium such as a photopolymer. For example, optical patterns can be printed or etched onto / into the tape, a magnetic pattern can be bonded to an area of the tape dedicated to encoding, or a physical pattern can be formed into the tape (e.g. similar to 35 mm film).

[0460] An optical or holographic encoder as shown above may be implemented in combination with the motion sensors shown in Figures 24 and 38 to allow detection of the motion and potentially position of the holographic medium, where the motion sensors comprise the light source and photodetector for detecting motion of the encoder. The encoder may be used to detect and adjust the rate of rotation of the reels to ensure the tape moves through the system at a consistent speed and to account for changes in speed resulting from winding and unwinding of the media onto the reels. If an absolute encoder is used, this may further be used for detecting when a certain potion, such as the end of the medium, is reached or about to be reached. An absolute encoder may also be used for indexing as it can be used to detect and find specific portions of the tape, allowing specific data to be retrieved and / or written.

[0461] Material structure

[0462] Figure 31a shows a cross-section showing the structure of the holographic tape 3102. The holographic tape comprises a radiation-sensitive layer 3148. The radiation sensitive layer 3148 acts as the active layer of the holographic tape 3102, in which holograms may be stored by the interference of a signal and reference beam. The interference of the beams causes changes in the refractive index of the radiation sensitive layer 3148, producing a holographic grating in the layer.

[0463] The holographic tape 3102 also comprises two substrate, or cover, layers 3150 on either side of the radiationsensitive layer 3148. The substrate layers 3150 provide protection to the radiation-sensitive layer.

[0464] The thickness of the holographic tape is indicated as d in the figure. The radiation-sensitive layer 3148 preferably has a thickness of 15pm or less, preferably 12pm or less, more preferably 10pm or less, more preferably 7pm or less, more preferably 5pm or less, more preferably 3pm or less. The substrate layers 3150 preferably have a thickness of 35pm or less, preferably 30pm or less, more preferably 20pm or less, more preferably 10pm or less, more preferably 5pm or less, more preferably 3pm or less. The combined tape preferably has a thickness of 100pm or less, preferably 80pm or less, more preferably 60pm or less, more preferably 40pm or less, more preferably 20pm or less, more preferably 15 pm or less. A thinner tape can allow a greater amount of holographic tape to be stored on a single reel of a reel-to-reel system, and generally allows higher storage density (at the level of the system as a whole). In particular, thinner substrate layers are desirable as the volume of the substrate layers is not used for holographic storage, such that the substrate layers take up space on a reel that could otherwise be used for data storage. However, a thinner photopolymer material may also mean that angular sensitivity of the material is reduced, meaning the minimum angle between angularly multiplexed data is greater and the number of angularly multiplexed holograms in a region is reduced. Therefore, if a large amount of angular multiplexing is desired, a thinner material may not be desirable. A thinner photopolymer may also be less efficient as the reconstructed hologram may be dimmer than with a thick photopolymer, which may mean a brighter reconstruction beam or a higher detector sensitivity to be used to reconstruct the recorded hologram.

[0465] Figure 3 lb shows that the holographic tape may comprise a removable cover layer 3152, which is removable from the holographic tape 3102 to reveal the radiation-sensitive layer 3148. This layer can be removed during the holographic recording process to allow the radiation to impinge directly onto the radiation-sensitive layer 3148, allowing improved holographic recording without interference by the cover layer. The layer is therefore either not bonded or is weakly bonded to the radiation-sensitive layer 3148.

[0466] Copying

[0467] Figure 32 shows a system 3200 which may be used to copy data between a plurality of holographic data storage media, specifically from a master holographic medium 3202 to a copy holographic medium 3204. Both the master medium 3202 and the copy medium 3204 may be a holographic storage medium as described previously and may have any of the features previously mentioned, including a photopolymer layer, one or more protective polymer layers, and the dimensions previously described.

[0468] The master medium 3202 comprises stored data, which is written onto the master medium in holographic form. Specifically, the stored data may be written on the master medium in the form of a plurality of holograms, wherein each hologram may represent a data page. The holograms may be written onto the storage medium using the apparatus and according to the methods shown in and described with reference to any of Figures 1 to 21. Accordingly, the holograms may comprise holograms with are recorded at multiple wavelengths, multiple intensities, multiple angles of the reference beam, etc. Data may be multiplexed onto the master medium by any one or combination of: wavelength multiplexing, angular multiplexing, peristrophic multiplexing, shift multiplexing, spatial multiplexing, and any other types of multiplexing. As such, in this part of the description, references to a ‘hologram’ encompass a plurality of multiplexed holograms (being located at generally the same location in a medium).

[0469] The data may also be stored on the master medium in any other suitable holographic format, and the systems and methods described here allow copying of any format of holographic data. The data may be written onto the master medium using apparatus such as that illustrated in Figures 1 to 18, or any other suitable apparatus. When writing the holograms, a data beam and a reference beam are used; the data beam carries data, and the reference beam, which does not carry data, interferes with the data beam at the holographic medium to form an interference pattern. This interference pattern is recorded onto the holographic medium in the form of variations in the refractive index of the material of the holographic medium. The reference and data beam may be incident on the holographic medium in a reflective, transmissive, or coaxial configuration, as described previously.

[0470] The copy medium 3204 may be a blank holographic storage medium without any holographic data written thereon. It is also possible that the copy medium may have some holographic data written thereon but a blank portion of the copy medium is used for copying. Alternatively, the portion of the holographic storage medium may already have some holographic data written thereon, but at a different angle, wavelength, or other beam parameter to the data to be copied from the master medium 3202; the copying process therefore allows further data to be multiplexed in with the existing data by copying from the master medium 3202.

[0471] A portion of the master medium comprising data is placed adjacent to a portion of the copy medium, such that the two portions of the media are parallel to each other and are in contact or in close proximity. To copy data, the master medium 3202 and the copy medium 3204 are illuminated by suitable means for copying data - specifically, a copy beam 3206 produced by a laser or other suitable apparatus. The copy beam has the same or similar properties to the reference beam that was used to record the holographic data onto the master medium 3202. The copy beam has the same or a substantially similar wavelength to the original reference beam. However, the copy beam may suitably have a lower coherence length than the reference beam, since the distance from the master medium to the copy medium is small so a high coherence length is not required. The copy beam 3206 is incident on the holographic media at the same angle x relative to the media as the reference beam used to record data onto the master medium 3202 (the copy beam may also be incident at an angle of 90° - x, where x is the original reference beam angle; see Figures 35a, 35b and 35c for further details). The copy beam may have a larger size than a data and / or reference beam used to record the original data, preferably a slightly larger size, or may be the same size as the data and / or reference beam.

[0472] The copy beam 3206 comprises radiation of the same wavelength as that of the reference beam; if the reference beam was composed of radiation of a plurality of wavelengths, the copy beam 3206 may comprise the same plurality of wavelengths, only one of the plurality of wavelengths, or any other subset of the plurality of wavelengths corresponding to which holograms at which wavelengths are to be replicated. If the beam comprises a plurality of wavelengths, the characteristics of each wavelength component may be different: for example, the beam width, pulse time (if a pulsed beam is used) or beam intensity may be changed for each wavelength to account for factors such as the sensitivity of the holographic medium to each wavelength.

[0473] In some cases , the imaging and curing process may cause a small reduction of the thickness of the active photopolymer layer (typically around l%-2%) for certain photopolymer materials. This may cause the narrowing of the Bragg fringe planes of the recorded hologram and may result in a hypsochromic shift (for example, a shift between 5nm-10nm, such as 8nm). Therefore, the wavelength of the copy beam may be slightly adjusted from the wavelength of the original writing beams if required.

[0474] The copy beam (or each wavelength of the copy beam) is collimated and has a high coherence and monochromaticity. The copy beam is preferably produced by a laser source such as a diode laser, solid-state laser, or other suitable laser. The laser may be continuous or pulsed, as further discussed with reference to Figure 36.

[0475] The copy beam interacts with the holographic data stored on the master medium and is diffracted by the holographic data to produce a reproduced data beam. The reproduced data beam comprises the same data that was in the original data beam used to record holographic data on the master medium, and reproduces the properties of this original data beam (the copy beam will also reproduce any noise in the original hologram, so noise should be minimised when recording the original holograms). The data beam interferes with the copy beam 3206 to form an interference pattern on the copy medium 3204, which is recorded on the copy medium, thereby replicating the holographic data from the master medium 3202 to the copy medium 3204. Further details of the holographic replication process are shown in Figures 33, 34, and 35, and details of possible arrangements of the copy beam are shown in Figures 36a-36e.

[0476] Whilst Figure 32 shows the copy beam incident on the master medium 3202, and passing through the master medium 3202 to the copy medium 3204 (a transmission arrangement, which is shown further in Figure 34a), the copy beam may alternatively be incident on the copy medium 3204 first, and pass through the copy medium 3204 to the master medium 3202 (a reflection arrangement, shown further in Figure 34b). To have the copy beam incident onto the copy medium 3204 first, the positions of the master medium 3202 and the copy medium 3204 may be swapped, or the copy beam may be provided from the opposite side of the system.

[0477] During the copying process, the master medium 3202 and the copy medium 3204 are transported relative to the copy beam 3206 by suitable means for transporting. Specifically, the master medium 3202 and the copy medium 3204 move in direction 3208 relative to the copy beam 3206. The master medium and copy medium are moved such that the region of each medium that is exposed to the copy beam 3206 continuously changes. This allows holograms that are written along the length of the master medium to be copied along the length of the copy medium. For example, the master medium may have written thereon regularly spaced holograms that have a specific pitch; as the master and copy media move past the copy beam, each of these holograms is copied as the region of the master medium comprising each hologram is exposed to the copy beam 3206.

[0478] The portions of the master medium 3202 and copy medium 3204 that are illuminated by the copy beam 3206 are parallel to each other, such that all surfaces of both media in these portions are parallel. The master medium and copy medium are preferably in contact at this region or are in close proximity, being almost in contact and separated from one another by a very small distance. Having the media in close proximity ensures the size and shape of the reproduced holograms is as close as possible to the original holograms, and allows a high data density to be maintained on the copy medium. The reproduced data beam produced by illuminating the medium is a diverging beam (especially so if optics with a high numerical aperture (NA) are used to record the original data), producing a large angle of expansion on the beam when replaying the hologram - this may mean that even a small distance between the media may result in the copied holograms being larger than the holograms on the master medium, reducing data density on the copy medium. This is offset by having the media in contact. Contact between the media may also allow a greater proportion of the energy from the copy beam to be transferred to the copy medium 3204, thereby increasing the quality and clarity of the copied hologram. Since the master and copy are in contact and moving together as one, a cheaper continuous wave laser may be used (rather than, for example, a pulsed laser). Having the media in contact or in close proximity also means that additional optics between the master medium 3202 and copy medium 3204 are not required, simplifying the apparatus layout.

[0479] However, having the media in contact may cause issues relating to friction between the two media, such as wear or scratching of the media, which may be worsened by having a greater area of the media in contact. It may also be difficult to ensure the media stay in contact throughout the region used for copying, and any separation of the media may distort and disturb the copying process. These issues can be at least partially mitigated by having guides and rollers for the media, such as rollers 3220 (further described in Figures 38a and 38b).

[0480] To ensure high-quality copying and to avoid any smearing of the copies or other issues, the master medium and copy medium move past the copy beam at the same speed. Preferably, the two media move continuously, such that both media move past the copy beam 3206 at a constant speed throughout the copying process without stopping or slowing down. Alternatively, it is possible in an embodiment that motion is not constant and the media are moved in a stepwise manner, stopping and starting during the copying process (for example, the media may stop or slow down when the copy beam overlaps each hologram to provide a prolonged exposure for that hologram, then may continue moving to align the copy beam which the next hologram).

[0481] Both the master medium 3202 and copy medium 3204 are on a reel-to-reel arrangement, the components of which serve as means for transporting the media relative to the copy beam. Master medium 3202 is provided on a first supply reel 3210 and a first take-up reel 3212, whilst copy medium 3204 is provided on a second supply reel 3214 and a second take-up reel 3216. The media are provided on the reels as tapes as previously described (though it will be appreciated that other forms / shapes of media could also be used). The two tapes are brought together into contact or close proximity in the region where the copy beam is incident on the two tapes; this is done using rollers 3220, which are positioned to received the respective tape from the reels and guide the two tapes towards each other.

[0482] At the beginning of the copying process, the master medium 3202 and the copy medium 3204 are wound around the first supply reel 3210 and second supply reel 3214 respectively. The first and second supply reels are preferably removable, for example with each removable reel being part of a cartridge, as shown in Figure 23, and the master and copy media are detachable from the take-up reels 3210 and 3214 respectively. Further details of a copying set-up with removable cartridges are shown in Figures 38a and 38b. Means to attach the respective holographic media to the first and second take-up reels 3210 and 3214 may be provided according to the systems shown in Figures 25a- 25e. Take-up reels 3212 and 3216 may be removable instead of or in addition to supply reels 3210 and 3214 being removable; in this case, means for attaching and detaching the holographic storage media to and from supply reels 3210 and 3214 may be provided. Each pair of supply reel and take-up reel may also be removable together from the system; for example as a cartridge which comprises first supply reel 3210 and first take-up reel 3212, and a similar cartridge for second reels 3214 and 3216 (this may also be referred to as a cassette) - where the cartridges may be cartridges as previously described and as shown in Figure 23.

[0483] During the copying process, the reels rotate in direction 3220, moving the master and copy media in direction 3208. As supply reels 3210 and 3214 rotate, the master and copy media respectively are unwound from the supply reels and are wound onto take-up reels 3212 and 3216 as the take-up reels rotate. After being unwound from the supply reels 3210 and 3214, each portion of the master and copy media moves past the copy beam 3206, allowing data on the master medium 3202 to be copied to the copying medium. The master and copy media are then wound onto first take-up reel 3212 and second take-up reel 3216 respectively. As shown in Figure 32, all reels may rotate in the same direction 3218; however, depending on the desired direction of winding / unwinding on each reel the reels may spin at different rates.

[0484] The rotation of the reels allows the tape to move past the copy beam and holographic data stored along the length of master medium 3202 to be copied along the length of the copy medium 3210. After a period of rotation of the supply reel, each tape may be fully unwound from its respective supply wheel and the end of the tape may be reached. When this occurs, the copying process is stopped, and the copy beam may be switched off. The supply reels and take-up reels may rotate in the direction opposite to 3220 to unwind the tape from the take-up reels 3212, 3216 and wind it back onto supply reels 3210, 3214. This can allow the supply reels to be removed from the system (e.g. on cartridges as shown in Figures 38a and 38b). Alternatively, the tape may not be wound back onto the supply reels, but kept on the take-up reels, which may be removable.

[0485] The rotation of the reels may be provided by motors. For each pair of reels (for each of the first pair of reels 3210, 3212 and the second pair of reels 3214 and 3216), both of the reels may be powered, or only one reel may be powered and the other rotates with tension from the tape; for example, only the take-up reels may be powered during normal copying, and only the supply reels may be powered during rewinding.

[0486] To guide the tape through the system, provide tension in the tape, and ensure the tape aligns with the copying system, rollers 3220 are provided. Rollers 3220 are provided for both the master medium 3202 and the copy medium 3204; two rollers are shown, though more rollers may be provided (for example, pairs of rollers may be provided adjacent to each reel to guide material being wound onto and unwound from the reels, as shown in Figures 38a and 38b). The rollers are positioned to ensure that the portions of the master and copy media that are exposed to the medium are correctly positioned and aligned, such as ensuring that they are parallel and correctly separated. The rollers are positioned adjacent to the region in which the master and copy media are in contact (or close proximity) and in which copying takes place. For a system with two rollers for each medium, one roller for each medium receives the medium from its respective supply reel, whilst another roller for each medium provides the medium towards its respective take-up reel. Between the rollers, the media are in contact or in close proximity, and the pairs of rollers are separated by a distance equal to the sum of the thicknesses of the media, or the sum of thicknesses and the desired separation. The rollers may be movable to allow for different thicknesses of material or desired separation, or to assist with guiding the tape into the system (further details of movable rollers are shown in Figures 38a and 38b).

[0487] Figure 33 shows the process of writing original holograms onto the master medium 3302. The master medium 3302 is illuminated by a reference beam 3328 and a data beam 3530. The reference beam does not comprise any data, and may comprise radiation of a single wavelength or of a plurality of wavelengths.

[0488] The data beam 3330 is incident onto the same region of the master medium 3302 as the reference beam. The data beam 3330 comprises radiation with the same wavelength or plurality of wavelengths as the reference beam, and carries data that has been modulated into the data beam, by a spatial light modulator or otherwise. Following data modulation, the data beam 3330 passes through a focussing lens (or lens array) 3354, which produces a diverging beam, thereby focussing the data beam 3330 onto a portion of the master medium 3302. This lens and other optics used to record data may have a high numerical aperture (NA), thereby producing a highly converging beam and providing a high data density in the medium. Whilst the data beam 3330 is shown here incident at a right angle to the master medium 3302, the data beam may also be incident from any other suitable angle onto the medium.

[0489] The reference beam 3328 and data beam 3330 overlap within the volume of the master medium 3302 at a region 3315, which is a three-dimensional volume of the holographic medium. Within this region 3315, the reference and data beams form interference fringes (which may also be referred to as Bragg fringes). These are recorded into the photosensitive material of the medium, storing the data carried by the data beam 3350 in the region 3315 of the holographic medium. The fringes stored in the region 3315 of the medium form a three-dimensional diffraction grating.

[0490] The data beam 3330 is shown incident onto the same side of the master medium 3302 as the reference beam 3328 (to produce a transmission hologram), but it may also be incident onto the opposite side of the master medium 3302 to the data beam 3330 (to record a reflection hologram). Whether the hologram recorded is a transmission hologram or a reflection hologram, the same method may be used to copy data from the master medium to the copy medium, and the copy beam may be incident onto the same side of the master medium 3302 as the reference beam 3328 or onto the opposite side.

[0491] Potential arrangements for the data beam and reference beam, including spatial light modulators and wavelengths of the data beam, are shown in further detail in Figures 1 to 18, and the data beam and reference shown here may be produced and directed onto the master medium 3302 to those arrangements or any others suitable for recording data in a holographic format on the master medium 3302.

[0492] Figure 34a shows details of a copying process from a master medium 3402 to a copy medium 3404 in a transmission arrangement, in which the copy beam 3406 is incident on the master medium 3402 before the copy medium 3404. The master medium 3402 and the copy medium 3404 are positioned parallel and adjacent to one another; they may be in contact (as shown in Figure 34a) or may be separated by a small separation distance as described above. A copy beam 3406 is provided from the direction proximal to the master medium 3402 and is incident on the master medium 3402. The copy beam is incident onto a first side of the master medium 3402. The copy beam 3406 is incident from the same angle x (or may be incident from an angle 90° - x, see Figures 35) to the master medium 3402 as the reference beam used for recording data onto the master medium and comprises the same wavelength(s) or a subset of the wavelengths of the original reference beam.

[0493] The copy beam 3406 interacts with the region 3415 of the master medium 3402 having data written thereon in the form of interference fringes. The copy beam 3406 is partially diffracted by the this region 3415 of the master medium 3402 to produce a reconstructed data beam 3426. The reconstructed data beam 3424 is produced by the interaction of the incident copy beam 3406 with the holographic data stored on the region 3415 of the master medium 3402. The reconstructed data beam is a reconstruction of the original data beam used to record holographic data onto the master medium and comprises the holographic data (such as data stored in holograms as described in relation to Figures 1 to 21). The reconstructed data beam 3424 emanates from the master medium 3402 in two directions, with a diffracted portion that passes out of the master medium into the copy medium from a second side of the master medium facing the copy medium, and a reflected portion that is reflected out of the master medium 3402 in the opposite direction from the first side of the master medium facing away from the copy medium.

[0494] A portion of the copy beam 3306 is not diffracted by the master medium 3302 but passes through the medium without interfering with the holographic data (the copy beam may be refracted by the optical index of the holographic media).

[0495] At the copy medium 3404, the reconstructed data beam 3424 (comprising holographic data) and the copy beam 3406 (comprising no holographic data) overlap and interfere, forming interference fringes (these interference fringes may have slight variation from those in the master medium as they are formed from a different section of the reconstructed data beam, and as there may be some added variations from noise created during the copying process). The interference fringes are recorded in a region 3417 of the copy medium 3404 in the form of variations to the refractive index of the medium. This replicates the holographic data from the master medium 3342 to the copy medium 3404 and produces a copy of the data in region 3415 of the master medium 3402 in region 3417 of the copy medium 3404.

[0496] During the copying process, both the master medium 3402 and the copy medium 3404 are moving in direction 3408 past the copy beam 3406. As the copy beam 3406 is incident on different regions of the master medium 3402 containing different holographic data, the data beam 3424 changes according to the holographic data that the copy beam 3406 is incident onto. The transmitted copy beam 3426 does not change as the media moves, since it does not contain data. As the copy medium 3404 moves with the master medium 3402, the beams are incident onto new regions of the copy medium 3404 and new data corresponding to the data in the corresponding section of the master medium 3402 is written onto the copy medium 3404. Each region of the copy medium is illuminated by the beams for a duration corresponding to the time taken by the region of the medium to move through the area illuminated by the beams; this time provides an exposure time during which the data is written onto the copy medium. To read the copied data from the copy medium 3404, the copy medium may be illuminated with a reconstruction beam having the same or similar properties to the copy beam (and consequently the reference beam originally used to record holographic data onto the master medium). The reconstruction beam contains the same wavelength(s) or a subset of the wavelengths of the copy beam 3406. The reconstruction beam interferes with the fringes stored on the copy medium 3404 to reproduce the data stored on the copy medium 3404. This data may be recorded with a detector (a camera, a CMOS sensor, a CCD sensor, etc.) as image data and the image data may be sent to a computer or processor to be decoded or read.

[0497] To account for refraction of the copy beam 3406 through the master medium 3402 during copying, the angle of the copy medium 3404 may be adjusted by a small amount, typically about 1°. This may be achieved by moving the source of the reconstruction beam by a small amount, or by tilting the copy medium 3404 when reading. The position of the copy medium may also be adjusted by an offset distance equal to the distance between the photosensitive layers in the two media during copying - for example, if both media comprise a 5pm photopolymer layer sandwiched between two 5 pm substrate layers, the pitch between the photopolymer layers of the master and copy media is 15 pm if the media are in contact (which would increase if there is a small separation between the media). As a result, the copy medium may be offset from the position of the master medium by approximately 10pm to 20pm. Whilst the angle of the reconstruction beam is preferably in very close correspondence to the angle of the copy beam, it is permissible that the wavelength or wavelengths of the reconstruction beam may have some variation from the wavelength or wavelengths of the copy beam, and that the reconstruction beam may not be as strongly coherent. This may allow less sophisticated radiation sources, such as cheaper diode lasers, to be used for reading data, increasing the ease of reading the data.

[0498] The reconstruction beam may not be strictly collimated but may be diverging or converging to allow larger or smaller images of the stored data to be reproduced. For example, for a converging copy beam (and consequently a converging reproduced data beam), since the cross-section of the beam is reduced from the master medium to the copy medium, the resulting copies made in the copy medium are smaller than the original holograms in the master medium (i.e. miniatures of the original holograms). This can allow a higher data density in the copy medium than in the master medium. This may be useful if, for example, data density that can be achieved for writing the master medium is constrained, such as through a limited resolution of an SLM used for writing or through optical constraints. The converse applies for a diverging copy beam, which can produce copy holograms that are larger than the original master holograms.

[0499] Figure 34b shows details of a copying process from a master medium 3402 to a copy medium 3404 in a reflection arrangement, in which the copy beam 3406 is incident on the copy medium 3404. The master medium 3402 and the copy medium 3404 are positioned parallel and adjacent to one another in contact (as is shown in Figure 34b) or may be separated by a small distance. A copy beam 3406 is provided from the direction proximal to the copy medium 3404 and is incident on the copy medium 3404. The copy beam is incident onto a first side of the copy medium 3404. The copy beam 3406 is incident from the same angle x (or 90° - x) to the media as a reference beam used for recording data onto the master medium and comprises the same wavelength(s) or a subset of the wavelengths of the original reference beam.

[0500] The copy beam 3406 passes through the copy medium 3404, and since the copy medium 3404 does not contain any data written thereon, the beam passes unimpeded and unmodified (except for an angular deviation due to refraction). The copy beam passes through to the master medium 3402 through the copy medium 3404 from a second side of the copy medium facing the master medium, the second side of the copy medium being opposite to the first side of the copy medium.

[0501] As with the transmission copying arrangement shown in Figure 34a, the copy beam 3406 is partially diffracted by the master medium 3402. The copy beam 3406 interferes with holographic data stored on the master medium 3402 in the form of interference fringes in region 3415 and is diffracted to produce a reconstructed data beam 3424. The reconstructed data beam 3424 is produced by the interaction of the incident copy beam 3406 with the holographic data stored on region 3415 of the master medium 3402. The reconstructed data beam is a reconstruction of the original data beam used to record holographic data onto the master medium and comprises the holographic data. The reconstructed data beam 3424 emanates from the master medium 3402 in both directions. A portion of the reconstructed data beam 3424 is transmitted through the master medium 3402 and passes out of the medium, but a portion of it is also reflected from the master medium 3402 back into the copy medium 3404.

[0502] The reflected portion of the reconstructed data beam 3424 interferes with the copy beam passing through the copy medium 3404. As the beams interfere, they form interference fringes on region 3417 of the copy medium, which are recorded in the copy medium 3404 in the form of variations to the refractive index of the medium. This replicates the holographic data from the master medium 3402 to the copy medium 3404 in region 3417 of the copy medium.

[0503] During the copying process, both the master medium 3402 and the copy medium 3404 are moving in direction 3408 past the copy beam 3406. This allows different regions of the master and copy media to be exposed to the copy beam 3406 and data stored throughout the length of the master medium 3402 to be copied onto the length of the copy medium 3404, as with the transmission arrangement shown in Figure 34a.

[0504] To read the copied data from the copy medium 3404, the copy medium may be illuminated with a reconstraction beam having the same or similar properties to the copy beam, including the same angle and the same or substantially similar wavelength or wavelengths. Since the copy beam is incident onto the copy medium directly during copying, changing the angle of the reconstruction beam to account for refraction is not required, but the copy medium may still be offset slightly to account for any changes in position. This reconstructs the data copied into the copy medium 3402 and allows the data to be read by a camera or other detector.

[0505] Figures 35a, 35b and 35c show how the angle of the copy beam 3506 is determined from the angle 3532 at which a reference beam 3528 is used to record data onto the master medium 3502.

[0506] Figure 35a shows the process of recording holographic data onto a master medium 3502. The master medium 3502 is illuminated by a reference beam 3528 and a data beam 3530, which is focussed onto the medium by focussing lens 3554. The reference beam does not comprise any data, and may comprise radiation of a single wavelength or of a plurality of wavelengths. The reference beam is incident onto the master medium at an angle 3532 of x°.

[0507] The data beam 3530 is incident onto the same region of the master medium 3502 as the reference beam. The data beam 3530 comprises radiation with the same wavelength or plurality of wavelengths as the reference beam, and carries data that has been modulated into the data beam, by a spatial light modulator or otherwise. Whilst the data beam 3530 is shown here incident at a right angle to the master medium 3502, the data beam may also be incident from any other suitable angle onto the medium. The data beam 3530 interferes with the reference beam 3528 at region 3515 of the master medium 3502, producing interference fringes which are recorded in region 3515 to thereby store holographic data in the master medium 3502.

[0508] The data beam is shown incident onto the same side of the master medium 3502 as the reference beam 3528 (to produce a transmission hologram), but it may also be incident onto the opposite side of the master medium 3502 to the data beam 3530 (to record a reflection hologram). Whether the hologram recorded is a transmission hologram or a reflection hologram, the same method may be used to copy data from the master medium to the copy medium, and the copy beam 3506 may be incident onto the same side of the master medium 3502 as the reference beam 3528 or onto the opposite side.

[0509] To record a plurality of holograms along the length of the master medium 3502, the master medium may be moved in direction 3534 relative to the signal and reference beams. Direction 3534 may be the same as direction 3508 in which the medium moves relative to the copy beam 3506 and the movement may be continuous, but note that the nature or direction of motion may also be different to that of the medium relative to the copy beam 3506: the medium may not move continuously but may start and stop, and it may move in the direction opposite to direction 3508.

[0510] Potential arrangements for the data beam and reference beam, including spatial light modulators and wavelengths of the data beam, are shown in further detail in Figures 1 to 18, and the data beam and reference shown here may be produced and directed onto the master medium 3502 to those arrangements or any others suitable for recording data in a holographic format on the master medium 3502.

[0511] Figure 35b shows a copying arrangement in which the angle 3536 of the copy beam 3506 relative to the master (and copy) medium is the same angle x as the angle between the reference beam 3528 and the master medium 3502. Whilst the copy beam 3506 is shown incident onto master medium 3502 (transmission copying), the beam may also be incident onto the copy medium 3504 in a reflection arrangement. Data is copied from region 3515 of the master medium 3502 to region 3517 of the copy medium 3504.

[0512] Figure 35c shows a copying arrangement in which the angle 3536 of the copy beam 3506 relative to the master (and copy) medium is 90° - x, i.e. 90° minus the angle at which the reference beam 3528 was incident onto the master medium 3502 during recording. The remaining arrangement is the same as in Figure 35b. This angle also allows copying of holographic data from region 3515 of the master medium 3502 to region 3517 of the copy medium 3504. Since the reconstructed data beam 3524 interferes with the transmitted copy beam 3526 in both cases, both arrangements are suitable for copying data.

[0513] The master medium 3502 may be in the same orientation when copying as when writing, with the reference beam 3528 and copy beam 3506 incident onto the same side of the medium and angled away from the same end of the medium, or the medium may be in any orientation mirrored from the orientation used when recording (the copy beam may be incident onto a different side of the medium and / or the beams may be angled away from the opposite end of the medium).

[0514] Figures 36a, 36b, 36c, 36d and 36e show possible arrangements of the copy beam 3606 used for copying data, including different dimensions of the copy beam and a plurality of copy beams. These figures show a master medium 3602 (in the form of tape) with a plurality of holographic recordings 3640 (also referred to simply as ‘holograms’) stored thereon in rows and columns of a grid (similar to the arrangements shown in Figures 2, 9, and 11). The holograms have an even size and are evenly spaced in each direction at a defined hologram pitch (such that regular rows and columns are formed). Whilst only two rows of holograms 3640 are shown, it will be appreciated that in reality a greater number of rows may be stored on the medium, according to the hologram size and pitch and the width of the medium. The master medium 3602 and copy medium 3604 move past the copy beams in direction 3608 (as in previous figures, whilst the master medium 3602 is shown facing the copy beam, alternatively the copy medium 3604 may face the copy beam).

[0515] Figure 36a shows a copy beam 3606a with a very narrow beam (represented schematically in Figure 36a as having no width) with width 3638a, which is much less wide than the hologram size and the hologram pitch. The beam has a breadth 3642 that is equal to or slightly greater than the width of the holographic media. Since the beam 3606a is so narrow, it is incident onto each region of the media for a short time, so this arrangement preferably has a high copy beam intensity. Each hologram is copied as the beam is continuously moved along the hologram, since the hologram is larger than the beam. For example, the width of the copy beam may be 0.5mm or less, 0.25mm or less, 0.1mm or less, 0.05mm or less, or 0.01mm or less. For a hologram width or pitch of 0.5mm, this corresponds to a width of 100%, 50%, 20%, 10%, and 2% of the hologram width or pitch, and similar percentages of other hologram widths or pitches may be used.

[0516] Figure 36b shows an arrangement with a wider beam 3606b, which has a larger width 3638b comparable to the hologram pitch. Preferably, the beam width 3638b is equal to the hologram diameter, or is equal to the hologram pitch, or has a width between the hologram diameter and the hologram pitch. This allows the beam to illuminate a column of entire holograms simultaneously, and ensures that each pulse copies only entire holograms, so that holograms are not left partially copied after each pulse. The beam width may also be greater than the hologram pitch, and may be equal to an integer multiple of the hologram pitch to allow a plurality of columns of holograms to be covered by the beam simultaneously. Such a wider beam may be easier to produce than a narrower beam.

[0517] Since each region of the holographic storage medium is illuminated by the beam 3606b for a longer period, a lower copy beam intensity is possible than for the arrangement in Figure 36a.

[0518] The copy beam 3606b may be continuous or it may be pulsed. For a pulsed beam, the beam 3606b is activated and directed onto the media when the beam is aligned with each column of holograms and is switched off after a specified pulse time. The pulsed beam may be produced using a pulsed radiation source, such as a pulsed laser, or using a constant radiation source and a shutter.

[0519] Preferably, the master medium 3602 and copy medium 3604 move past the copy beam in a continuous motion without stopping, but it is also possible to pause the holographic media whenever the beam overlaps each column of holograms to provide a longer copying exposure, and subsequently restart the motion to align the copy beam with the next set of holograms. If a pulsed beam is used, the pausing of the motion may be matched to the beam pulses.

[0520] Preferably, the beam width is similar to the hologram pitch, more preferably it is between 1 and 1.5 times the hologram pitch, between 1 and 1.25 times the hologram pitch, or between 1 and 1.1 times the hologram pitch. The width of the beam may be 0.5mm or greater, 1mm or greater, 2.5mm or greater, 5mm or greater, 10mm or greater, 15mm or greater, or 20mm or greater. For a set hologram width or pitch the width of the copy beam may be 1, 2, 5, 10, 20, 30, or 40 times the hologram pitch or width.

[0521] Figures 36a and 36b show a single beam with a single width, but if a copy beam with a plurality of wavelengths is used it is possible to change the characteristics of each beam for each wavelength, such as beam width, pulse time (if a pulsed beam is used) or beam intensity to account for factors such as the sensitivity of the holographic medium to each wavelength.

[0522] Figure 36c shows an arrangement in which a plurality of copy beams 3606c is provided incident onto the master and copy media. The beams are incident onto separate non-overlapping portions of the media, though the beams may also be incident onto overlapping portions of the media. Preferably, the spacing between each of the beams corresponds to the hologram pitch (or an integer multiple of the hologram pitch, for example 2 times the hologram pitch, 5 times the hologram pitch, 10 times the hologram pitch, 50 or more times the hologram pitch, or any intervening values). The beam width is equal or less than the beam spacing and is preferably equal to an integer multiple of the hologram pitch or the hologram diameter.

[0523] The beams 3606c are preferably pulsed (although they may also be a continuous). Preferably, each pulse copies a section of the tape illuminated by the beams 3606, and the tape is moved after each pulse to copy holograms from a different section of the tape. The width of each beam may be the same or slightly greater than the pitch of the holograms, and the medium may be moved by a distance equivalent to the total combined width of the beams between each pulse.

[0524] If the holograms 3640 are recorded at a plurality of recording wavelengths, the beams 3606c may have a different wavelength corresponding to the plurality of recording wavelengths, for example a unique wavelength per beam or a plurality of that is a subset of the recording wavelengths per beam. The beam characteristics may be varied for each wavelength, such as beam width, pulse time (if a pulsed beam is used) or beam intensity.

[0525] The copy beam width and breadth are modified optically, such as by being passed through an aperture having the appropriate dimensions to give the beam its desired width and breadth. For a narrow beam such as that shown in Figure 36a, this aperture is smaller, whilst for a larger beam, such as that shown in Figure 36b it is larger. Following the aperture, the beam may be passed through a focussing lens, which focusses the beam from the aperture onto the tape and produces a sharp image of the aperture on the medium, reducing any noise that may arise from the edge of the aperture and thereby reducing noise in the copies. A spatial filter may also be used before the aperture to produce a diverging lens, and a collimating lens may be used to produce a collimated beam from the diverging beam from the spatial filter.

[0526] The properties of the aperture, spatial filter and / or lenses, such as focal length and separation, are adjusted in accordance with the desired beam shape; for example, for a larger beam, a greater separation distance between a spatial filter and lens may be used.

[0527] To produce a plurality of copy beams such as those shown in Figure 36c having the same wavelength, a beamsplitter may be used to produce a plurality of beams, each of which is then modulated by appropriate optics such as a spatial filter, collimating lens, and aperture. Alternatively, a single large beam may be produced and passed through a filter comprising a plurality of apertures to produce the plurality of beams. For a plurality of copy beams having different wavelengths, eachbeam is produced individually by a different radiation source or plurality of sources and the beam profile is adjusted for each beam separately.

[0528] The speed of the media may be set in according to the laser power and the copy beam width. For example, for a hologram recorded with a green laser at a wavelength of 532nm, an energy density of 0.45mJ / mm2- 0.65mJ / mm2is preferable for copying. The laser power may be lOmW or greater, 50mW or greater, lOOmW or greater, 250mW or greater, 500mW or greater, 1 W or greater, or 5W or greater. The energy density U on the holographic material is the product of the power density of the beam S and the exposure time t: U = tS. In turn, the exposure time is equal to the width of the beam w divided by the speed of the tape: t = w / v; whilst the power density of the beam is given by the power p divided by the area of the beam (width w multiplied by breadth h of the beam): S = p / wh. Taken together, this yields the energy density on the holographic material as:

[0529] U = ts = - x — = — v wh vh

[0530] As can be seen, the energy density is not dependent on the beam width. This formula may be used to calculate the suitable velocity of the holographic media for a given desired energy density and a given laser power. Whilst it is particularly useful for copying, since only a single copy beam is used, it is also useful to calculate a suitable tape velocity at a given energy density for a pair of reference and data beams when writing a hologram, where the formula may be used for the signal or reference beam in isolation, or for a combination of the reference and data beam (in this case, U and p would be the sum of the energy density and the power respectively of the data beam and reference beam - where the power in particular may be the power of the laser source before it is split by a beamsplitter to form the reference and data beams).

[0531] From the above equation, it canbe calculated that for a laser power of 50mW and abeambreadth of 12.65mm, to achieve a desired energy density of 0.45mJ / mm2a tape speed of 8.8mm / s is required. For a wider beam with width 10mm, this is equivalent to an exposure time of 1.14s for each hologram, and for a narrower beam with width 0.5mm this is equivalent to an exposure time of 0.057s for each hologram.

[0532] The laser power may be varied to allow for a different tape speed. For example, a higher-power laser of 500mW allows an increased speed of 88mm / s. The properties of the apparatus may be varied to provide a tape speed of lOmm / s or greater, 50mm / s or greater, lOOmm / s or greater, 500mm / s or greater, or Im / s or greater.

[0533] For a holographic tape cartridge holding holographic tape with a length of 1000m, a laser power of 50mW allows copying of the contents in about 32 hours, a laser power of 500mW in about 3.2 horns, and a laser power of 5W in about 19 minutes. For a capacity of 20 TB per tape, this is equivalent to copying speeds of 186MB / S, 1860MB / S and 18600MB / S respectively. It can therefore be seen how important laser power is to copying speed. It canbe seen that a high laser power may allow copying speeds greater than LTO tape of comparable dimensions (where LTO tape has a maximum copying speed of approximately 400MB / s).

[0534] The above calculations are given for air transmission holography with continuous-wave lasers. Contact copy transmission or reflection may advantageously require lower laser power.

[0535] It may be advantageous to have a wider beam with a size equal or larger than the diameter or pitch of the holograms. However, for contact copying, a wider beam (or a plurality of beams) also increases the size of the copying area where the holographic tapes are in contact with one another. For example, it may be difficult to ensure the two areas remain in constant contact with each other without separating. Any issues from a large area of contact can be addressed through mechanical constraints, such as by providing guides and rollers (including the movable rollers 3450 shown in Figures 38a and 38b).

[0536] As described above, the breadth 3642 of the copy beams 3606a, 3606b, 3606c may be equal to or slightly greater than the width of the holographic media (if the master and copy media have different widths, then the breadth of the beam may be equal to or slightly greater than the width of either medium, preferably the copy medium 3604, or preferably whichever medium is wider). It is also possible for the breadth 3642 of the copy beam to be less than the width of the holographic media. For example, the breadth of the copy beam may be equal to an integer multiple of the width or pitch of the holograms in the direction of the width of the tape. This may require multiple passes of the copy beam along the media to copy all data stored on the master medium 3602 to the copy medium 3604, with the position of the copy beam along the width of the holographic medium changed for each pass.

[0537] Any of the methods described above in Figures 32-35c may also be used to copy shift multiplexed holograms which partially overlap each other. An example is shown in Figures 36d and 36e.

[0538] Figure 36d show a plurality of recordings 3460 on a master medium, with each of the recordings overlapping at least one other. This is shown in further detail in Figure 36e, which shows a plurality of holograms recorded in a plurality of regions 3615a-d in the master medium 3602. The regions are partially overlapping; for example, a portion of region 3615b overlaps with a portion of region 3615a and a portion of region 3615c, and may overlap with portions of other regions. The shift multiplexed holograms each store different data, thereby increasing the data density on the medium. The holographic medium may store a continuous set of evenly spaced shift multiplexed holograms, each of which partially overlaps at least one other shift multiplexed hologram, or may store the holograms in separate “stacks” of shift multiplexed holograms, wherein each stack comprises a group (for example 3 to 10) of shift multiplexed partially overlapping holograms, and a plurality of stacks is provided on different portions of the medium (the stacks being spatially multiplexed with one another, where the stacks may or may not overlap, but the spacing between subsequent stacks is greater than the spacing between each shift multiplexed hologram in a stack).

[0539] A wide copy beam 3606d is used to copy a plurality of shift multiplexed holograms simultaneously. As shown in Figure 36e, the copy beam 3606d is incident on a portion the master medium 3602 comprising regions 3615a-d. The copy beam interacts with the interference fringes stored in each region to produce a plurality of reconstructed data beams 3624a-d, each reconstructed data beam corresponding to a holographic recording on a region: for example, reconstructed data beam 3624a is produced by diffraction from region 3615a, etc.

[0540] Each of the reconstructed data beams 3624a-d interacts with the copy beam 3606d to produce an interference pattern on a corresponding region of the copy medium 3604. For example, reconstructed data beam 3624a interacts with the copy beam to produce interference fringes at region 3617a of the copy medium, and similarly for the other reconstructed data beams. This reproduces data from regions 3615a-d of the master medium 3602 onto corresponding regions 3617a-d of the copy medium.

[0541] The copy beam 3606d preferably has a width equal to an integer multiple of the pitch of the holograms. If spatially multiplexed stacks of shift multiplexed holograms are used, the width of the copy beam is preferably equal to the width of each stack. The beam may also be slightly wider than an integer multiple of the pitch of the holograms or the width of each stack to ensure the entire area of each hologram is captured each time. The beam is preferably pulsed onto the media, and the media shifted after each pulse to copy a new set of holograms for each pulse.

[0542] Figure 37 shows how multiple copy beams 3706a, 3706b, 3706c may be provided to copy multiple angularly multiplexed holograms stored on the master medium 3702. Three copy beams 3706a, 3706b, 3706c are incident on the master medium 3702 (the beams may also be incident on the copy medium 3704 first, in which case the positions of the master medium 3702 and copy medium 3704 are swapped). Each beam is incident on the medium from a different angle relative to the medium. Each beam may be angled away from the normal to the medium in the direction of the width of the medium and / or in the direction of the length of the medium; the beam may come from anywhere in the hemisphere above the medium (although a very high angle relative to the medium may not be optimal for copying due to difficulties in reconstructing data from the master medium and / or writing the data to the copy medium).

[0543] Each copy beam at each angle corresponds to the angle of a reference beam used to record angularly multiplexed data onto the master medium 3702. When recording data onto the master medium 3702 (e.g. through the process shown in Figure 35a), a plurality of reference beams was provided onto each region of the master medium 3702 at a plurality of angles (either sequentially or simultaneously), and a plurality of different holograms storing different data was recorded on a single region of the master medium 3702, increasing the storage capacity of the medium. Further details of a holographic storage medium to store a plurality of angularly multiplexed holograms is shown in Figure 20, and apparatus that may be used to record the angularly multiplexed holograms is shown in Figure 12.

[0544] By illuminating the master medium 3702 with the plurality of copy beams 3706a, 3706b, 3706c, the plurality of angularly multiplexed holograms is reconstructed. Each beam therefore produces a reconstructed data beam, which interferes with the copy beam to produce an interference pattern that is recorded onto the copy medium 3704. This allows the plurality of angularly multiplexed holograms to be replicated onto the copy medium 3704. Since the copy beam is incident onto the media at the appropriate angle for each angularly multiplexed hologram, each of the copied holograms may be reproduced after copying by illuminating the hologram with a reconstruction beam at the appropriate angle corresponding to the angle of the appropriate copy beam (and hence the original reference beam onto the master medium 3702). Each of the copy beams also has the same wavelength as a reference beam used to record the original holograms at that angle. If the original reference beam comprised a plurality of wavelengths, the appropriate copy beam comprises the same plurality of wavelengths or one or more of the wavelengths. As described previously, each of the angularly multiplexed holograms may be recorded at the same wavelength or wavelengths or at different wavelengths; therefore the different copy beams may also have different wavelengths or the same wavelength or wavelengths.

[0545] The copy beams may have properties as described previously. In particular, the width of the beams 3738 may be as described for the copy beams with reference to Figures 36a-e; it may be smaller than the hologram width or pitch, equal to the hologram width or pitch, or greater than the hologram width or pitch. Preferably, the width of each copy beam is an integer multiple of the hologram width or pitch. The breadth of the beam may also be as described previously (such as equal to or slightly greater than the width of the media). The beams may be pulsed or continuous. The beams may have different widths, intensities, pulse times, etc., for each wavelength, whether this is within a single copy beam or between a plurality of copy beams.

[0546] The beams 3706a, 3706b, 3706c shown to be nonoverlapping, but they may also overlap partially or entirely. A plurality of beams may be provided at each angle, as described with reference to Figure 36c above.

[0547] Figures 38a and 38b show apparatus 3800 that may be used to copy between a master medium 3802 and a copy medium 3804 that are both mounted in respective cartridges 3842. The master medium 3802 is mounted on a first supply reel 3810 which is provided inside a cartridge 3842, and the copy medium is mounted on a second supply reel 3814 which is also provided inside a cartridge 3842, both in the manner described previously with reference to Figure 23. The copying apparatus 3800 comprises cartridge bays 3844 to receive these cartridges, which are received into the bays through openings. As previously, the positions of the master medium 3802 and the copy medium 3804 may be swapped; it is also possible to have a copy system with a copy medium in the place of the master medium shown and vice versa. As will be appreciated, apparatus 3800 is similar to the holographic storage system 2400 described previously and shown in Figure 24 - optionally, apparatus 3800 used the same cartridge bays and reel-to- reel system as the holographic storage system 2400.

[0548] The apparatus 3800 comprises a system, for example such as that shown in Figures 25a-25d for engaging with the tape on the cartridges 3842 and transferring the end of the tape onto the take-up reels. The tapes are brought through the apparatus such that the master medium 3802 is brought onto first take-up reel 3812 and the copy medium 3804 is brought onto second take-up reel 3816. A leader pin such as that shown in Figures 23 and 25a-25e may be used to assist the apparatus in taking up the tape and engaging it onto the take-up reels. Between the two sets of reels is positioned a beam forming apparatus 3848, which produces the copy beam and directs the copy beam onto the master and copy media.

[0549] Rollers 3820a, b,c are provided to hold the tape in position and to guide the tape. The rollers hold the tapes together in the correct position for copying, and apply tension to the tapes to avoid any slippage between the media. Preferably, to improve the application of tension on the tapes, the rollers may be sprung or dampened, provided with a soft or high-grip surface, have sprung or dampened contacting surfaces, and / or have an increased contact angle or surface area. Rollers 3820a are provided adjacent to each cartridge 3844 and adjacent to each of the take-up reels 3812 and 3816 to guide the tape being unwound and wound onto the reels. Two rollers 3820b are provided between rollers 3820a to guide the master medium 3802 and two rollers 3821c are provided between rollers 3820a to guide the copy medium 3804. The rollers 3820b, c are positioned near the regions of the media onto which the copy beam is incident, thereby ensuring the media are in the correct position for copying.

[0550] The two rollers 3821b are movable, as shown in Figure 38b. The rollers 3820b are positioned further away from rollers 3820c when the tapes are being loaded into the apparatus, as shown in Figure 38a, allowing a large gap between rollers 3820b and 3820c for the tapes to pass through and be engaged by engagement apparatus. Once the tapes are engaged, rollers 3820b are moved towards rollers 3820c to bring the media into contact. The rollers 3820b are moved past rollers 3820c to “pinch” the tape between the rollers, providing a single path between the rollers for the two media to follow. The rollers 3820b are provided at different horizontal positions (as shown in the figures) to rollers 3820c but are moved into a vertical position that overlaps with the vertical position of rollers 3820c. This presses the copy and master media together.

[0551] Alternatively, the movable rollers 3821a may simply position the two media close together if physical contact is not desired.

[0552] Alternatively, instead of only having rollers 3820b movable, rollers 3820c may be movable instead of or in addition to rollers 3820b. Rollers 3820a may also be movable. Additional rollers may be provided in the system, which may be movable or static. Instead of a single roller at each point on only one side of the tape, each roller shown in Figures 38a and 38b may be replaced with a pair of rollers, one on each side of the medium. This may allow greater control of the medium and reduce the risk of the tapes deviating from their paths.

[0553] Once the tapes are engaged, the copy beam 3806 is switched on and directed onto the master and copy media. The copy beam is produced by copy beam producing apparatus 3848. The copy beam producing apparatus 3848 comprises at least one radiation source of a suitable wavelength (preferably a laser) and optics to produce the copy beam from the light source. The light source produces a collimated, coherent beam of the appropriate wavelength. Optics such as lenses, spatial filters and mirrors is used to produce appropriate beam properties (such as beam size) and direct the beam onto the media at the appropriate angle. As described previously, each beam may comprise a plurality of wavelengths, so a plurality of radiation sources with different wavelengths may be provided (alternatively, a single tunable light source may be used). A plurality of copy beams may be produced (as shown in Figures 36c and 37), and a plurality of light sources and appropriate optics are also provided if this is the case.

[0554] The copy beam 3806 is shown to be incident onto the copy medium 3804 first (reflection copying). As described previously, transmission copying is also possible, in which the copy beam 3806 is incident onto the master medium 3802 first. The same apparatus 3800 may allow both transmission and reflection copying, simply by swapping the two cartridges with the media. Alternatively, the apparatus may require a specific configmation of either reflection or transmission copying; in this case separate apparatus may be provided to allow each.

[0555] During the copying process, the supply and take-up reels to rotate (in direction 3818, though they may also rotate in different directions, as described previously) to move the master and copy media past the copy beam 3806, thereby replicating holograms from the master medium 3802 to the copy medium 3804 along the length of both media.

[0556] Once the copying process is finished (once the end of either tape has been reached or whenever the desired number of holograms have been copied), the reels are rotated in the opposite direction to rewind the media back onto the supply reels. The engagement mechanism may then disengage the tapes from the take-up reels and the rollers, allowing the cartridges to be removed.

[0557] Alternatively, it is possible for take-up reels 3812 and 3816 to also be removable as part of a cartridge; in this case the tape is not rewound onto the supply reels and each reel is provided on a separate cartridge. It is also possible for the reels to be provided on cartridges that comprise both a supply reel and a take-up reel (also called cassettes); the reels are then removable from the apparatus together.

[0558] As the reels rotate and the tape is wound and unwound from each reel, the effective diameter of the reel changes: for example, reels which have a greater amount of wound tape therefore have a greater effective radius from which the tape is wound / unwound. Since a greater reel diameter results in a higher tape speed for a given rate of rotation of the reel, this may mean that, for example, the speed at which the tape is wound onto a take-up reel increases as more tape is wound onto the reel. This may lead to issues as a constant speed of tape is desirable for copying, and may further lead to the master and copy media moving at different rates if the first and second reels have different amount of wound tape. Therefore, it is desirable to adjust the speed of each medium to ensure it remains constant through the copying process.

[0559] To allow monitoring of the motion of the media, motion sensors 3822 may be provided to detect the motion of one or both media. These sensors may comprise, for example, a radiation source that illuminates each medium with radiation and a detector that senses detected or transmitted radiation to determine the velocity of the medium. An encoding layer, as described above with reference to Figures 26-30, may be provided on each medium to allow the sensors 3822 to determine the motion of the medium. This can be used to adjust the speed of the reels, for example to account from effects from the changing effective diameter of the reels as described above. The encoding layer may also be an absolute encoding layer, in which case it also indicates the position of the tape; this may be used for purposes such as indexing and determining whether the end of the tape is about to be reached.

[0560] A detector such as a camera may also be provided in the path of the copy beam 3806 (a plurality of detectors may also be provided if a plurality of copy beams are used). The detector may detect the reconstructed data beam and may be used for reading the data at the same time as copying, or for tracking the data being copied. The detector may also be used for tracking the speeds of the media instead of or in addition to the motion sensors 3822 above, or for detecting when the end of either medium is reached or about to be reached.

[0561] Figure 39 shows an arrangement for copying to a plurality of copy media simultaneously. As previously, a master medium 3902 comprising holographically stored data is provided on first supply reel 3910 and first take-up reel 3912, but instead of a single copy medium a plurality of copy media 3904a, 3904b, 3904c is provided. Each copy medium is provided on a set of supply and take-up reels - first copy medium 3904a is provided on a second supply reel 3914a and a second take-up reel 3916a, second copy medium 3904b is provided on a third supply reel 3914b and a third take-up reel 3916b, and third copy medium 3904c is provided on a fourth supply reel 3914c and a fourth takeup reel 3916c.

[0562] A portion of each medium is aligned with a portion of the master medium such that the copy beam 3906 is incident onto all of the master and copying media (wherein the media are parallel to each other in this portion). This is assisted by rollers; the three copy media share rollers 3920a as shown, but separate rollers may be provided to guide and direct each medium instead of or in addition to the shared rollers. The copy media are preferably in contact with each other and copy medium 3904a is preferably in contact with the master medium 3902; although there may also be a small separation between each of the copy media and the copy medium 3904a and the master medium 3902.

[0563] All of the reels rotate (in direction 3918, although they may rotate in other directions if appropriate) to move all master and copy media past the copy beam 3906 in direction 3908 at the same speed. As the copy beam 3906 is incident onto the master medium, it is partially diffracted to produce a reconstructed data beam which interferes with the copy beam to produce interference fringes. The reconstructed data beam is transmitted through all copy media, as is the (unmodified) data beam. The interference between these beams produces the fringes at each medium, and the holographic data is copied from the master medium 3902 onto each copy medium by the fringes being recorded in all copy media.

[0564] Whilst a transmission copying arrangement is shown in which the copy beam 3906 is incident onto the master medium 3902 first, a reflection copying arrangement may also be used. In this arrangement, all copy media are placed in the path of the copy beam 3906, and the master medium 3902 is placed behind them, such that light is incident onto it last. It is also possible to have an arrangement in which a master medium is “sandwiched” by copy media, with a first copy medium (or multiple copy media) on a first side of the master medium facing towards the copy beam and a second copy medium (or multiple copy media) on the second side of the master medium facing away from the copy beam. Holographic data is therefore copied from the master onto the copy media both by reflection (onto the first copy medium) and transmission (onto the second copy medium).

[0565] This process allows very high throughput copying and the production of multiple copied simultaneously. However, since each copy medium may absorb a portion of the copy and reconstructed data beam or otherwise interact with the beams, this may reduce the hologram quality on the later media the beams are incident onto. Therefore, the properties of the beams and media must be carefully selected; for example a high laser power and a high medium transparency may be desirable. The number of tapes which can be copied to simultaneously can be increased by reducing the tape thickness and by minimising the separation between the tape (preferably to have all tape in contact).

[0566] The copy beam may be as described previously, particularly with reference to Figures 36 and 37; for example, a plurality of copy beams may be provided, for angular multiplexing or otherwise. The copying process shown in Figure 39 may be carried out in apparatus as shown in Figure 38, wherein each of the copy media is provided on a separate cartridge.

[0567] Another method of producing further copies is to use the copy medium produced in a copying process as described above as a master medium to produce further copies. For example, a master medium (primary) comprising original data canbe used to produce a first copy or set of copies (secondaries). This first set of copies can then be used as a master medium to produce a second copy or set of copies (tertiaries). This process can be repeated to produce further copies, although the quality and legibility of the holograms may be degraded to an unusable extent if the process is repeated excessively.

[0568] Figure 40a-f show how data recorded using shift-peristrophic multiplexing may copied using the above methods.

[0569] Figures 40a and 40b show how shift-peristrophic multiplexing may be used to increase the density of data written to a master medium 4002. Figure 40a shows the principle of peristrophic multiplexing, involving a data beam 4030 incident on a master medium 4002 via a focussing lens 4054. The data beam 4030 interferes with a reference beam 4028 to record data on the master medium 4002. The data beam 4030 is preferably normal to the medium and the reference beam 4028 is preferably angled 46° from the data beam, but other angles may be used. Following producing a first recording with the reference beam in a first position, the reference beam is rotated around the normal to the master medium 4002. As a result, whilst the angle between the reference beam and data beam remains the same as previously (preferably 46°), and the reference beam is incident onto the same region of the master medium 4002, the angle of the reference beam in a plane parallel to the master medium 4002 is changed. This is shown on the right- hand side of Figure 40a, which shows a set 4029 of positions 4028a-4028e for the reference beam in the plane of the medium 4002. Any integer number n of positions of the reference beam may be used, with the angle between subsequent positions is 3607n. In an alternative, the data beam may be used in different positions while the reference beam remains static.

[0570] At each different position of the reference beam, the medium 4002 is illuminated by a data beam 4030 carrying different data (such as a plurality of arrays 4056). The data may be in the form of arrays as shown in Figures 2, 9 and 11. This allows different data to be recorded as a hologram for each reference beam position, and thereby allowing a plurality of data pages to be multiplexed onto a single region of the medium by peristrophic multiplexing of a plurality of holograms. The data beam 4030 and reference beams 4028 are shown as being incident on the same side of the medium 4002 in a transmission arrangement; a reflection arrangement is also possible and shown in Figures 41.

[0571] Figure 40b shows how multiple sets of peristrophically multiplexed holograms may be shift multiplexed onto a holographic medium. Each set of holograms is recorded by a set of reference beams 4029a-f, each of which comprises a plurality of positions of reference beams as shown in Figure 40a. Each set of peristrophically multiplexed holograms is recorded at the corresponding set of reference beam positions, and the medium 4002 is moved relative to the copy beam in direction 4034. This exposes a new region of the medium to the data and reference beams (where the region may partially overlap with the previous region), into which another set of peristrophically multiplexed holograms is recorded. The process may be repeated as many times as required, producing a high density of data in the holographic medium.

[0572] As an alternative to the use of n fixed positions for the reference beam, the reference beam may be moved continuously, which may be beneficial as this may allow for recording to take place while a medium is moved at a constant velocity. The continuous movement involved in this alternative would mean that each set of peristrophically multiplexed holograms would not be in exactly the same physical space.

[0573] Figures 40c and 40d show how data may be copied from the master medium 4002 to the copy medium 4004. Instead of a copy beam which is incident on the media from the direction of the original reference beam (which would require a plurality of positions of the copy beam), the copy beam 4006 is incident onto the media from the direction of the original data beam 4030. The copy beam is the same as the data beam but does not carry any data - it has the same profile and same wavelength or plurality of wavelengths as the data beam 4030 and may be focussed by the same focussing lens 4054 as the data beam. The copy beam is incident on each set of peristrophically multiplexed holograms 4040 shown in Figure 40d. The copy beam 4406 reproduces the data contained in the original set of data beams used to record the holograms along the angles of the original reference beams, and these resultant beams interfere with the copy beam to produce a copy of the set of peristrophically multiplexed holograms in the copy medium 4004. The copy beam 4006 therefore simultaneously copies all holograms stored in the set of peristrophically multiplexed holograms, regardless of the angle of the original reference beam used to record each hologram. The storage medium and / or copy beam may subsequently be moved to copy a different set of peristrophically multiplexed holograms from a different region of the master medium, thus copying the shift-peristrophically multiplexed data onto the copy medium. Whilst Figures 40c and 40d show copying in a transmission arrangement, in which the copy beam 4006 is incident on the master medium 4002 before the copy medium 4004, it is also possible to copy in a reflection arrangement, where the copy beam 4006 is incident on the copy medium 4004 before the master medium 4002 (e.g. as shown in Figure 34b)

[0574] Figures 40e and 40f show two different methods of reading shift-peristrophically multiplexed data from the copy medium. The first method, shown in Figure 40e, comprises illuminating the copy medium 4004 with a reconstraction beam 4058a from the direction of the reference beams 4028 used to record data onto the master medium 4002. The wavelength of the reconstruction beam is the same as or similar to the wavelength or plurality of wavelengths of the original data and reference beams. Since each hologram of a set of peristrophically multiplexed holograms was recorded at a different angle of the reference beam 4028, a different angle of the reconstruction beam 4058a is required to retrieve each of the holograms. The reconstruction beam 4058a is diffracted by the corresponding interference pattern on the copy medium to produce a reconstructed data beam 4024, which takes the same path as the original data beam. The reconstructed data beam carries the data stored in the hologram and is recorded by detector 4060 to retrieve the data; the data 4056 stored in each of the holograms is retrieved one-by-one, independently for each hologram.

[0575] An alternative, preferred method of retrieving shift-peristrophically multiplexed data is shown in Figure 40f. Instead of using a reconstruction beam that is incident from the direction of the original reference beams, this method uses a reconstruction beam 4058b that is the same as the data beam 4030 but that does not carry any data. The reconstruction beam 4058b is produced along the same path as the data beam 4030 (which is also the direction of the copy beam 4006) and has the same wavelength or plurality of wavelengths as the data beam. The same focussing lens 4054 may be used to focus the reconstruction beam onto the medium.

[0576] The reconstruction beam is diffracted by each of the holograms in a set of peristrophically multiplexed holograms, producing a reconstructed data beam for each hologram carrying the data stored on the hologram. Each reconstructed data beam is produced at the angle of the original reference beam used to record that hologram, so each reconstructed data beam for each hologram of the set is produced at a different angle. These beams may be read by detector 4060 simultaneously (a plurality of detectors at a plurality of positions corresponding to the beam angles may be provided, or a single large detector). The reconstructed data beam is transmitted out of both sides of the medium, so may be read from either direction, but preferably from the direction opposite the source of the copy beam to avoid the detector interfering with the copy beam. This allows all peristrophically multiplexed data 4156 stored in a set of holograms to be read simultaneously using a single reconstruction beam, without requiring multiple reconstruction beams at multiple angles or movement of the beam between each hologram.

[0577] The reconstruction beam 4058b and / or the copy medium 4004 can be moved to expose a different region of the medium 4004 to the copy beam 4006 and retrieve a different set of peristrophically multiplexed holograms. This allows shift-peristrophically multiplexed holograms to be read.

[0578] Figures 41a-d show a similar method of recording and copying shift-multiplexed holograms, but with the holograms recorded as reflection holograms rather than transmission holograms, as shown in Figures 40a-f. Reflection holograms may be more suitable for colour holograms (holograms comprising light of a plurality of wavelengths) such as those shown in Figures 2 and 11, whilst transmission holograms may be more suitable for recording singlewavelength (grayscale) holograms.

[0579] Figure 41a shows a method of recording shift-multiplexed reflection holograms onto a master medium, similar to Figure 40a, but with the reference beams incident onto the opposite side of the master medium 4102 to the data beam 4130. The angle between the data beam and reference beams is preferably 136°, but other angles may be used. A set of peristrophically multiplexed holograms is recorded at each region with a different reference beam position used for each of the holograms, and multiple sets may be shift multiplexed to produce shift-peristrophic multiplexing. Data 4156 recorded in this way is preferably multi-wavelength data and the data and reference beam preferably comprise a plurality of wavelengths. Figure 41b shows a method of copying shift-peristrophic multiplexed reflection holograms from the master medium 4102 to the copy medium 4104 using copy beam 4106. The same method and arrangement may be used as shown for copying transmission holograms in Figure 40c, with the copy beam being incident from the path of the original data beam 4130. If the data and reference beams comprised a plurality of wavelengths, the copy beam preferably comprises the same plurality of wavelengths, although a subset of the plurality of wavelengths (such as a single wavelength) may also be used for copying, which would produce copies of data stored at only certain wavelengths in the copy medium.

[0580] Methods of reading copied shift-multiplexed holograms from the copy medium 4104 are shown in Figures 41c and 41d. Figure 41c shows a similar method to Figure 40e, with multiple reconstruction beams 4158a being provided along multiple angles corresponding to the original reference beam paths, but with the reconstraction beam being incident onto the same side of the copy medium as the original reference beam (the opposite side to the data beam). The preferred method of reading the holograms shown in Figure 41d is the same as shown in Figure 40f: the reconstruction beam 4158b is incident on the medium 4104 from the same path as the original data beam, and the reproduced data beams are produced in both directions out of the copy medium 4104, allowing them to be captured by the detector 4160 in the same way as shown in Figure 40f. The reconstruction beams 4158a and 4158b comprise the same wavelength or plurality of wavelengths as the data beam 4130 and reference beams 4128; since ...

Claims

Claims:

1. A holographic data storage medium shaped as tape and having a thickness of 60pm or less.

2. A holographic data storage medium according to claim 1, comprising a photosensitive layer and at least one substrate layer; preferably wherein the photosensitive layer comprises a photopolymer.

3. A holographic data storage medium according to claim 2, wherein the photosensitive layer has a thickness of 15pm or less, preferably 12pm or less, more preferably 10pm or less, more preferably 7pm or less, more preferably 5pm or less, more preferably 3pm or less.

4. A holographic data storage medium according to claim 2 or 3, wherein the at least one substrate layer comprises a polymer, preferably an optically clear polymer.

5. A holographic data storage medium according to any of claims 2 to 4, wherein the holographic data storage medium comprises a first substrate layer attached to a first side of the photosensitive layer and a second substrate layer attached to a second side of the photosensitive layer, preferably wherein the second substrate layer is removable from the holographic data storage medium.

6. A holographic data storage medium according to any of claims 2 to 5, wherein the at least one substrate layer has a thickness of 35pm or less, preferably 30pm or less, more preferably 20pm or less, more preferably 10pm or less, more preferably 5pm or less, more preferably 3pm or less.

7. A holographic data storage medium according to any preceding claim, wherein the holographic data storage medium has the same width as Linear Tape-Open (LTO) tape; preferably wherein the holographic data storage medium has a width of 12.65mm.

8. A holographic data storage medium according to any preceding claim, wherein the holographic data storage medium has a thickness of 40pm or less, more preferably 20pm or less, more preferably 15pm or less, more preferably 10pm or less, more preferably 5.2pm.

9. A holographic tape cartridge for use with a holographic data storage system comprising: a holographic data storage medium, preferably according to any preceding claim; a reel, wherein a first end of the holographic data storage medium is attached to the reel; and a casing to hold the reel and holographic data storage medium.

10. A holographic tape cartridge according to claim 9, wherein the holographic tape cartridge has a single reel.

11. A holographic tape cartridge according to claim 9 or 10, wherein a second end of the holographic storage medium comprises an attachment means, preferably wherein the attachment means is a leader pin.

12. A holographic tape cartridge according to any of claims 9 to 11, further comprising at least one of: an opening for accessing the holographic data storage medium; and a mechanism allowing the reel to be rotated from to the exterior of the casing.

13. A holographic tape cartridge according to any of claims 9 to 12, wherein the cartridge has the same outer dimensions as a Linear Tape-Open (LTO) cartridge.

14. A system for receiving a holographic tape cartridge comprising a holographic data storage medium, the holographic tape cartridge preferably being according to any of claims 9 to 13, the system comprising: a further reel to receive a second end of the holographic data storage medium; and means for recording holograms and / or reading holograms on the holographic storage medium within the holographic tape cartridge.

15. A system according to claim 14, further comprising an attachment arm to receive the second end of the holographic data storage medium, preferably by means of a leader pin, and means for directing the second end of the holographic data storage medium onto the second reel, preferably wherein the second end of the holographic data storage medium attaches to the second reel by means of the leader pin.

16. A system according to claim 14 or 15, further comprising at least one rolling element in contact with the holographic data storage medium.

17. A system according to any of claims 14 to 16, wherein the means for recording holograms and / or reading holograms is configured to record and / or read holograms on a part of the holographic data storage medium extending between the reel and the further reel; preferably a part of the holographic data storage medium extending between a rolling element and a further rolling element.

18. A system according to any of claims 14 to 17, wherein the means for recording holograms and / or reading holograms is configured to record and / or read holograms on the holographic data storage medium while the holographic data storage medium is in motion between the first reel and the second reel, preferably wherein the holographic data storage medium is in motion at a continuous speed, more preferably wherein a distance the holographic data storage medium moves during recording and / or reading of a hologram is equal to or less than one tenth of a wavelength of radiation used for recording and / or reading.

19. A system according to any of claims 14 to 18, adapted to write data on the holographic data storage medium, further comprising: a transport mechanism for transporting the holographic tape; a motion detector configured to detect motion of the tape caused by the transport mechanism; a controller; a laser; and a spatial light modulator, SLM; wherein the controller is configured to receive a signal from the motion detector indicating that the tape has moved a set distance and further configmed to cause the activation of the laser and SLM as a result.

20. A system according to any of claims 14 to 19, adapted to read data on the holographic data storage medium, further comprising: a transport mechanism for transporting the holographic tape; a motion detector configured to detect motion of the tape caused by the transport mechanism; a controller; a laser and a sensor; wherein the controller is configured to receive a signal from the motion detector indicating that the tape has moved a set distance, and further configmed to cause the activation of the laser and sensor as a result.

21. A system comprising the system of claim 19 and the system of claim 20, wherein the SLM is configured to be deactivated during reading of data and wherein the sensor is configmed to be deactivated during writing of data.

22. The system of any of claims 19 to 21, wherein the motion detector is a rotational motion detector, or an optical motion detector configmed to read markings on the tape.

23. A holographic tape library comprising: a plurality of slots each for holding a respective one of a plurality of holographic tape cartridges, each holographic tape cartridge comprising holographic tape; and a transport mechanism for transporting each such cartridge between its respective slot and a system for reading and / or writing data to the holographic tape on each such holographic tape cartridge.

24. The holographic tape library of claim 23, further comprising the system for reading and / or writing data to the holographic tape on each such holographic tape cartridge, preferably wherein the system for reading and / or writing data to the holographic tape on each such holographic tape cartridge is a system according to any of claims 14 to 22.

25. The system of any of claims 14 to 22, or the holographic tape library of claim 23 or 24, further comprising at least one holographic tape cartridge according to any of claims 9 to 13 and / or at least one holographic data storage medium according to any of claims 1 to 8.

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