Holographic Storage

Abstract

To provide a holographic data storage system and a method of exposing an area to a pattern over a sufficient sustaining duration (depending on an output of a beam).SOLUTION: In a method of performing a write operation in a holographic data storage system, a scheduler schedules at least one write operation across several discontinuous write intervals, and, in each of the discontinuous write intervals, a plurality of discontinuous write intervals, in which a region of a holographic recording medium is exposed to an interference pattern resulting from interference between a reference beam and an input beam carrying a set of data has a total duration sufficiently long to cause a sustained state change in the exposed region, thereby the set of data can be recovered from the region by the end of the last write interval of the plurality of discontinuous write intervals.SELECTED DRAWING: Figure 8A

Description

[Technical Field]

[0001] Technical Field

[0001] This disclosure relates generally to holographic storage. [Background technology]

[0002] background

[0002] Holographic storage is a form of computer storage in which information is recorded on a photosensitive holographic recording medium by exposing the medium to a light pattern. For example, a region (subvolume) of the medium can be exposed to an optical interference pattern resulting from the interference of an input beam, in which a set of data is embedded, with a reference beam. The beam can be, for example, a laser beam generated using a single laser and a beam splitter. To embed the set of data in the input beam, spatial light modulation (SLM) can be used (e.g., an image encoding the set of data can be spatially modulated and embedded in the input beam). For the avoidance of doubt, in this specification, the terms "light," "optics," and similar terms are not limited to visible light. Holographic storage can be implemented, for example, using infrared or ultraviolet beams in the non-visible portion of the electromagnetic spectrum.

[0003] With sufficient beam power and exposure time, the optical interference pattern produces a persistent state change within the sub-volume (at which point the interference pattern is said to be persistently recorded or written to the sub-volume herein). The state change of the sub-volume is such that, upon subsequent exposure of the sub-volume to a substantially matching reference beam, the interaction between the matching reference beam and the sub-volume produces an output beam that is essentially matching with the original input beam, in the sense that the set of data originally embedded in the input beam can be recovered from the output beam (this may be referred to herein as reading the recorded pattern).

[0004]

[0004] Rather than storing individual bits as discrete units, a single interference pattern can encode many (e.g., millions) of bits. For example, the data set could be a megapixel image embedded in an input beam. Moreover, by exploiting the sensitivity of certain forms of holographic recording media to small changes in the angle of a reference beam, it is possible to record many (e.g., hundreds or thousands) of such patterns in the same subvolume. For such media, once an interference pattern is generated with a reference beam at a given angle, the recorded pattern can only be read using a reference beam that closely matches the reference beam originally used to generate it. This effect can be exploited to record multiple patterns (encoding different data sets) in the same subvolume at different reference beam angles. Theoretically, data storage capacity is limited only by the wavelength of the beam, potentially hundreds of megabytes per cubic millimeter for red light and tens of gigabytes for ultraviolet light. In practice, there may be other limiting factors, but nevertheless, there is great potential for high-density data storage. Summary of the Invention

[0005] overview

[0005] This Summary is provided to introduce selected concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Nor is the claimed subject matter limited to implementations that solve any or all of the disadvantages described herein.

[0006]

[0006] The present disclosure relates to scheduling write operations in a holographic data storage system in an energy-efficient manner. In this specification, a write operation refers to a set of data, such as an image, carried by an input beam and recorded in a holographic recording area as an interference pattern caused by interference between the input beam and a reference beam. In order to store the data in a recoverable form, i.e., so that it can be read from the holographic recording area at a later time, it is necessary to expose the area to the pattern for a sufficient duration (which depends on the power of the beam).

[0007]

[0007] Surprisingly, it has been discovered that by "staggering" the write operation over multiple discontinuous write intervals, i.e., by exposing the holographic recording medium to the interference pattern as described above in each of the discontinuous write intervals except for the intervening time intervals separating the discontinuous write intervals ("pulsed" writing), the total exposure time required to record the pattern is less than the total time that would be required if the area were instead exposed to the pattern in a single continuous write interval ("continuous" writing).

[0008]

[0008] A first aspect of the present specification provides a method for performing a write operation in a holographic data storage system. A schedule schedules at least one write operation over a plurality of discontinuous write intervals, the write operation relating to a set of data to be stored in a region of the holographic recording medium. In each of the discontinuous write intervals, the region of the holographic recording medium is exposed to an interference pattern produced by interference between a reference beam and an input beam carrying the set of data. The plurality of discontinuous write intervals have a total duration long enough to produce a lasting state change in the exposed region, such that the set of data can be recovered from the region by the end of the last write interval of the plurality of discontinuous write intervals.

[0009] Brief description of the diagram

[0009] For a better understanding of the present disclosure and to show how embodiments of the disclosure may be put into practice, reference is made by way of example only to the following figures: [Brief explanation of the drawings]

[0010] [Figure 1A]

[0010] A schematic perspective view of a holographic recording medium is shown. [Figure 1B]

[0010] A schematic perspective view of a holographic recording medium is shown. [Figure 2A]

[0011] FIG. 1 shows a schematic perspective view of a holographic storage system including a set of waveguides that can be used to direct beams to / from different sub-volumes of a holographic recording medium to provide spatial multiplexing across the medium. [Figure 2B]

[0011] A plan view of the system during the write interval is shown. [Figure 2C]

[0011] A side view of one side of the system during the write interval is shown. [Figure 2D]

[0011] A side view of one side of the system during the write interval is shown. [Figure 2E]

[0011] A plan view of the reading section is shown. [Figure 2F]

[0011] A side view of one side of the reading section is shown. [Figure 2G]

[0011] A side view of one side of the reading section is shown. [Figure 3A]

[0012] 1 shows a schematic side view of an active light pipe in one configuration. [Figure 3B]

[0012] A schematic side view of an active light pipe in one configuration is shown. [Figure 3C]

[0012] A schematic side view of an active light pipe in one configuration is shown. [Figure 3D]

[0012] A schematic side view of an active light pipe in one configuration is shown. [Figure 3E]

[0012] A plan (cross-sectional) view of an active light pipe is shown. [Figure 3F]

[0012] A plan (cross-sectional) view of an active light pipe is shown. [Figure 4A]

[0013] 1 shows a side view of one side of a portion of an optical waveguide network. [Figure 4B]

[0013] A side view of one side of an optical waveguide network (part thereof) is shown. [Figure 5]

[0014] 1 shows a schematic diagram of an example of a multiplexed waveguide network used to multiplex across multiple pieces of holographic storage media. [Figure 6A]

[0015] 1 shows an example of a delivery system for providing input and reference beams in a holographic storage system. [Figure 6B]

[0015] A variation of the emission system with simplified optics is shown. [Figure 7]

[0016] An example data recovery system is presented that uses spatially coherent detection to measure the optical field of the output beam and uses signal processing to mitigate waveguide distortions in the measured optical field. [Figure 8A]

[0017] FIG. 1 shows a functional block diagram illustrating the functions performed within a holographic storage system. [Figure 8B]

[0018] 1 illustrates a schematic of a scheduling scheme for staging write operations. [Figure 9]

[0019] An alternative holographic storage system is presented that uses at least one waveguide network to spatially multiplex in two dimensions across a slab of holographic recording medium. [Figure 10A]

[0020] It is shown how spatial multiplexing can be achieved using passive inductive elements, where spatial multiplexing is achieved by modulating the beam characteristics. [Figure 10B]

[0020] We show how spatial multiplexing can be achieved using passive inductive elements, where spatial multiplexing is achieved by modulating the beam characteristics. [Figure 11A]

[0021] 1 shows a light pipe with passive optical filters having different frequency responses. [Figure 11B]

[0021] A light pipe with passive optical filters having different frequency responses is shown. [Figure 12]

[0022] 1 shows an example of a waveguide network with three hierarchical levels. [Figure 13]

[0023] 10 shows a graph of diffraction efficiency results with respect to writing time in pulsed and continuous writing modes. DETAILED DESCRIPTION OF THE INVENTION

[0011] Detailed Description of Example Embodiments

[0024] Embodiments that implement the scheduling techniques described above are described below. First, some useful context is provided. The described embodiments implement scheduling techniques in holographic storage systems that use a waveguide network to facilitate spatial multiplexing across one or more holographic recording media, without requiring mechanical movement of the media or equipment used to read and write therefrom. While this combination has additional advantages, it is important to note that the scheduling techniques can be implemented in any holographic storage context.

[0012]

[0025] The described embodiments combine scheduling techniques with waveguide architectures to achieve efficient spatial multiplexing across one or more holographic storage media in a manner that also reduces or eliminates the need for mechanical movement, and may use "active" light pipes, "passive" light pipes, or a combination of active and passive light pipes. Note that the terms "waveguide" and "light pipe" are used interchangeably herein.

[0013]

[0026] An "active light pipe" refers to a waveguide having one or more active switching or other guiding elements attached to the surface of the waveguide or within the bulk of the waveguide, and thus configurable (i.e., having changeable optical properties) to cause "one-to-many" light transfer (i.e., light is directed from a first surface region to one of multiple possible second surface regions (where the first surface region is an in-coupling region and the second region is an out-coupling region)) or "many-to-one" light transfer (i.e., light is directed from any one of the second surface regions (which would now be an in-coupling region) to the same first surface region (which would now be an out-coupling region)). The term "passive light pipe" refers to a light pipe with guiding elements having different light sensitivities (e.g., different wavelength and / or polarization sensitivities); a similar effect can be achieved by instead varying the optical properties or beam (e.g., using a tunable laser to change its wavelength, polarization, etc. so that it is guided along different routes by guiding elements with different wavelength / polarization responses, etc.). The term "passive optical guidance" is merely a convenient label to coincide with the fact that in this case the guidance element does not need to be active, but in this context active guidance elements or passive guidance elements with different light sensitivities (e.g., different wavelength and / or polarization sensitivities, etc.) can be used (i.e., the guidance elements can be both active and have different light sensitivities).

[0014]

[0027] A digital image (or data encoded as a digital image) can be propagated as a beam along an active or passive light pipe, whose guiding elements can be individually controlled to transmit or reflect the incident light beam.

[0015]

[0028] Many such light pipes (active, passive, or a combination of both types) can be combined in various geometries to create switching networks that can be used to steer beams and images to one of many addressable locations in one or more spatial dimensions. The input to the light pipe can be generated using a spatial light modulator (SLM), and the output is read, for example, on a CCD (charge-coupled device). Phase interference and noise can be corrected using a combination of optical and computational techniques, including machine learning techniques. Some embodiments use coherent detection in combination with such techniques to provide more effective waveguide distortion mitigation.

[0016]

[0029] In contrast to the types of optical switches and fibers traditionally used in optical data communications, the described embodiments use light pipes that can transmit entire images at once. This takes advantage of currently available high-resolution optical devices, such as SLMs and digital cameras. These devices have millions of pixels, allowing for the encoding and decoding of megabytes of data. This enables high-bandwidth transmission even with modest switching rates of the SLM, camera, and active light pipe elements (in the case of active light pipes) or beam optics (in the case of passive light pipes). In addition, applications such as holographic storage require interference between multiple beams, at least one of which is modulated with an image. In holographic storage, active light pipes can be used to efficiently steer beams and images to interfere at any desired location in the holographic storage medium. As described, simpler advantages can be achieved with passive light pipes, where switching is instead applied at the emitter stage.

[0017]

[0030] The light pipes described below are “multimode” waveguides in the sense that they have sufficient physical dimensions to support a wide range of “modes” (i.e., spatial paths through the waveguide for a given channel, e.g., corresponding to different propagation directions). This is in contrast to simple single-mode optical waveguides, such as thin optical fibers used in fiber optic systems, whose purpose is to essentially confine light entering the fiber to a single propagation mode. While single-mode optical waveguides can only carry data using amplitude, phase, or frequency modulation, multimode optical waveguides can carry much more data (e.g., potentially entire images of millions of pixels) through angular variations within the waveguide. Stated differently, multimode waveguides offer greater bandwidth through increased angular and / or spatial diversity by providing multiple optical paths through the waveguide from emitter to detector for any given channel (different paths corresponding to different propagation modes).

[0018]

[0031] Another aspect disclosed herein is a holographic data storage system that uses one or more waveguide networks to spatially multiplex (i.e., read from / write to different sub-volumes of the medium) across a holographic recording medium without requiring any relative mechanical motion between the medium and the waveguide network. Examples of such systems are described below and utilize active and / or passive light pipes. In the described examples, multimode waveguides can be used to simultaneously carry an entire digital image to / from the holographic recording medium or to carry a reference beam at one of several possible angles.

[0019] Active Light Pipe:

[0032] 3A-D show schematic side views of example configurations of active light pipes 300 having particular physical structures. As will be appreciated, this is merely one example of a suitable physical structure that can provide the desired optical configurability. Further examples are considered below.

[0020]

[0033] The active light pipe 300 is shown having at least a first surface region 300-0 and a plurality of active switches in the form of switchable Bragg gratings (SBGs), which may be surface or volume-embedded. In this example, two such SBGs 300-1, 300-2 are shown on the first surface 300-S1 of the waveguide 300, but it will be understood that more SBGs can be placed at suitable locations on the surface 300-S1 of the waveguide 300 and / or embedded within the bulk of the waveguide 300. Each SBG 300-1, 300-2 can be individually controlled to change its reflection / transmission characteristics to transmit or reflect an incident beam. The SBGs 300-1, 300-2 form respective surface regions of the active light pipe 300 where light can enter (incouple) or exit (outcouple) the waveguide 300, depending on how the waveguide 300 is used.

[0021]

[0034] The first surface region 300-0 is the end region of the waveguide 300 from which the first side surface 300-S1 of the waveguide extends along the axis 301 of the waveguide 300.

[0022]

[0035] 3E and 3F each show a cross-sectional view of waveguide 300, which in this example has a rectangular shaped cross-section, and can be seen to have four sides 300-S1, 300-S2, 300-S3, and 300-S4 extending along axis 301 of waveguide 300. In this example, as depicted in the figures, SBGs 300-1, 300-2 are all located along first side 300-S1, although in general such SBGs can be attached to multiple surfaces of waveguide 300 depending on the application.

[0023]

[0036] The SBGs 300-1, 300-2 are located at increasing distances from the first region 300-0 along the first side 300-S1 of the waveguide, with the first SBG 300-1 being located closest to the first region 300-0.

[0024]

[0037] 3A, 3B, and 3E depict a "one-to-many" use case, where the first surface region 300-0 serves as the in-coupling region and the second surface regions 300-1, 300-2 of the SBG serve as out-coupling regions. As an example, FIG. 3 shows a first light ray 304 being coupled into the waveguide 300 via the in-coupling region 300-0. In this example, the first surface region 300-0 is angled relative to the side surfaces 300-S1, ..., 300-S4, such that the first light ray 304 passes through the first surface region 300-0 and into the bulk of the waveguide 300 at an angle sufficient to achieve total internal reflection at each of the side surfaces 300-1, ..., 300-4 within the waveguide 300.

[0025]

[0038] Each of the SBGs 300-1, 300-2 can be configured to change between a reflective state and a transmissive state. Figure 3A shows a configuration in which the first SBG 300-1 is in a reflective state, where an incident light ray 300 reflects off the first SBG 300-1, back into the waveguide 300, and is guided along the waveguide 300 until it reaches the second SBG 300-2. SBG 300-2 is shown in a transmissive state, where light ray 304 diffracts out of the waveguide 300 through the second SBG 300-2, thereby extracting it from the waveguide 300 through the surface region of the second SBG 300-2. This configuration of the SBGs 300-1, 300-2 creates a "channel" through the waveguide 300 between the first surface region 300-0 and the surface region of the second SBG 300-2.

[0026]

[0039] 3B shows the first SBG 300-1 in a transmissive state. Thus, upon reaching the first SBG 300-1, the first light ray 304 is instead diffracted out of the waveguide 300 via the first SBG 300-1, and thereby instead extracted from the waveguide 300 via the surface region of the first SBG 300-1. This configuration creates a channel through the waveguide 300 between the first surface region 300-0 and the surface region of the first SBG 300-1.

[0027]

[0040] In this manner, a first light ray 304 can be guided through the waveguide 300 from the first region 300-0 until it exits the waveguide 300 at a surface region of the SBG 300-1 or 300-2. For simplicity, only two SBGs 300-1, 300-2 are described in relation to each other, but it will be understood that the same principles can be applied to many more SBGs.

[0028]

[0041] FIG. 3E shows, when viewed in cross section, how first light ray 304 may propagate through TIR from some or all of sides 300-S1, ..., 300-S4 depending on the angle of first light ray 304.

[0029]

[0042] It is equally feasible to use the depicted active light pipe 300 for many-to-one light transfer, as depicted in Figures 3C, 3D and 3F.

[0030]

[0043] Figure 3C shows the same SBG configuration as Figure 3A. The only difference is the use of the waveguide 300; here, a second light ray 308 is shown entering the second SBG 300-2 from an external source (not shown). With the second SBG in a transmissive state, the second light ray 308 is diffracted through the second SBG into the waveguide 300 (here providing incoupling at its surface region) and from there is guided through the waveguide 300 to the first surface region 300-0 (here the outcoupling region). This includes reflection from the first SBG 300-1, which is now in a reflective state. The reflective state of the first SBG 300-1 prevents the light ray 308 from exiting the waveguide through the first SBG 300-1. Moreover, external light rays 309 that happen to be incident on the first SBG 300-1 are essentially reflected away from the first SBG 300-1 and therefore do not enter the waveguide 300.

[0031]

[0044] 3D shows the same configuration as FIG. 3B, but here, a second light ray 308 is incident on the first SBG 300-1 from an external source. With the first SBG 300-1 in a transmissive state, a third light ray 310 enters the waveguide 300 at that location by diffraction and is guided to the first surface region 300-0.

[0032]

[0045] FIG. 3F shows in cross section how a second light ray 308 may propagate within the waveguide 300, with the same explanation as in FIG. 3E being true, but with the light ray direction reversed.

[0033]

[0046] The above description assumes perfect reflectivity / transmittance of the SBGs in the transmission / reflection state. As will be appreciated, this is not an absolute requirement in practice, and more typically, systems have some tolerance for imperfections in the SBGs 300-1, 300-2 and the waveguide 300. Suitable signal processing techniques for compensating for distortions introduced in the waveguide 300 are described below.

[0034]

[0047] Although SBGs 300-1, 300-2 are depicted as separate elements, in fact separate, independently controllable regions of a single larger SBG may extend across all or most of first side 300-S1.

[0035]

[0048] SBGs are just one possible form of active switching element. For example, with polarized light beams, the same effect can be achieved using controllable polarizing filters attached to the surface of the waveguide 300 or embedded within the bulk of the waveguide. SBGs and controllable polarizing filters are examples of non-mechanical active switches that can change the optical properties of the waveguide 300 through non-mechanical effects. Other examples of inductive elements include controllable mirrors such as micromirror devices or other microelectromechanical systems (MEMs), which are examples of mechanical inductive elements.

[0036]

[0049] When using polarizing filters as the steering elements, SBGs 300-1, 300-2 can be replaced with passive diffractive elements, with the polarizing filters operating to controllably steer beams towards or away from the passive diffractive elements as needed, without the need to reconfigure the diffractive elements.

[0037]

[0050] Note that even though the inductive elements themselves are mechanical, this still avoids the need for mechanical movement of the waveguide 300 as a whole.

[0038] Active Light Pipe Network

[0051] As used herein, a "waveguide network" can take the form of a single waveguide or a network of multiple interconnected waveguides. Waveguide networks comprising multiple active light pipes have particular advantages in terms of flexible optical data transmission.

[0039]

[0052] 4A and 4B show side views of one side of a waveguide network (portion thereof) including first and second active light pipes 400, 420. The second light pipe 420 has a first surface area 420-0 located adjacent to a corresponding surface area of ​​the first light pipe 400 to receive a beam from or direct a beam to the second waveguide 420 via the first surface area 400-0. Purely by way of example, a light ray 404 is shown propagating through the first waveguide 400 to a corresponding surface area of ​​the first waveguide 400, which is adjacent to the first surface area 420-0 of the second waveguide 420. A light ray 404 is extracted from the first waveguide 400 via SBG 400-1 attached to an adjacent surface region of the first waveguide 400 and is coupled into the second waveguide 420 via first surface region 400-0. From there, the light ray 404 can be directed to any one of multiple SBGs 420-1, 420-2 of the second waveguide 420 in a one-to-many manner. The same arrangement can also be used to reverse the direction of the light ray and direct the beam in the other direction from the second waveguide 420 to the first waveguide 400 in a many-to-one manner.

[0040]

[0053] Although this example considers two interconnected waveguides 400, 402, the principles can be applied to many more interconnected waveguides to enable flexible data routing through a waveguide network.

[0041]

[0054] More generally, the surface region of the medium may be optically coupled to a corresponding surface region of the waveguide in another manner, such as via an air interface or one or more other optical components (which may themselves be waveguides and may or may not provide active or passive switching functionality).

[0042] Holographic Storage

[0055] Here we describe the application of active light pipes to holographic storage.

[0043]

[0056] 1A and 1B show schematic perspective views of a holographic recording medium 102 (which for simplicity may be referred to simply as medium 102), which is a volume of relatively thick photosensitive material capable of persistently storing a light pattern as a “hologram” embodied within the holographic recording medium 102. A hologram is generated by exposing a subvolume 110 (region) of the medium 102 to a light pattern, such that a persistent state change occurs within that subvolume 110. The hologram generated in the subvolume 110 by that state change records the light pattern into the medium 102, from which the light pattern can be reproduced at a later time. The hologram is persistent in that, once generated, the medium 102 requires no power to maintain it. The composition and structure of the medium 102 can be such that the hologram cannot be erased once generated (thus providing a form of “write-once-read-many” (WORM) storage) or such that the hologram can be erased and replaced (but persists unless and until erased).

[0044]

[0057] A single hologram can record a light pattern that encodes a very large number (e.g., millions) of bits, allowing very large amounts of data to be written / read in parallel (simultaneously) to / from the holographic recording medium 102 through the exposure / regeneration of the light pattern. Another advantage of holographic storage is that many holograms can be written to the same sub-volume 110 of the holographic recording medium 102, thereby greatly increasing the data storage capacity per unit volume of the holographic recording medium 102.

[0045]

[0058] More specifically, FIG. 1A shows how an input beam 104 and a reference beam 106 are directed toward a subvolume 110 via first and second sides 102-4 and 102-6 of the medium 102, respectively, to write a set of data to the medium 102. This generates a light pattern in the form of an interference pattern caused by interference between the input beam 104 and the reference beam 106. If the beams 104 and 106 have sufficient power and the subvolume 110 is exposed for a sufficient duration, the interference pattern generated by the interfering beams 104 and 106 will be persistently recorded in the subvolume 110 as a hologram. As explained below, the set of data can be embedded in the input beam 104 and recovered from the resulting hologram. In this manner, an encoded set of data is written to the subvolume 110. In the following example, the set of data is encoded as a digital image and then embedded in the input beam 104 via spatial modulation.

[0046]

[0059] 1B, to read data from subvolume 110, a matched reference beam 116 is directed into subvolume 110 via second side 102-6 of medium 102, where it interacts with the hologram to produce output beam 108, which is essentially matched to the input beam 104 used to write the hologram, to the extent that the embedded data can be recovered from output beam 110. Output beam 108 propagates out of subvolume 110 via third side 102-8 of medium 102.

[0047]

[0060] The reference beam 116 used to read the data is substantially aligned with the reference beam 106 originally used to write the data, and in particular is oriented at an angle (or, more generally, a direction) that closely matches the angle of the original reference beam 106. This is because the ability to read a hologram (i.e., to generate an output beam 108 from which the data can be recovered) is sensitive to angular deviations between the reference beam 106 used to write the hologram and the reference beam 116 used to read the hologram. This sensitivity can be exploited to record multiple holograms in the same subvolume 110, each hologram generated using a different reference beam angle, and two entirely different holograms can be generated using only slight differences in the reference beam angle. In this way, many (e.g., hundreds or thousands) holograms can be written to the same subvolume 110, each encoding many (e.g., millions) of bits.

[0048]

[0061] 2A shows a schematic perspective view of an example holographic storage system 200 incorporating certain principles of the present disclosure. In this particular example, three separate waveguides 204, 206, and 208 are used to carry the input beam 104, the reference beams 116 and 126, and the output beam 118, and the three separate waveguides 204, 206, and 208 may be referred to individually as the input waveguide 204, the reference waveguide 206, and the output waveguide 208. As noted, the terms "optical waveguide" and "light pipe" are used interchangeably herein. Each of the waveguides 204, 206, and 208 provides spatial multiplexing in the sense that it can direct signals to any one of multiple subvolumes within the holographic recording medium 102 (in the case of the input and reference waveguides 204 and 206) or from any one of multiple subvolumes within the holographic recording medium 102 (in the case of the output waveguide 208). This provides spatial multiplexing across the volume of the holographic recording medium 102 without requiring any mechanical movement of the waveguides 204, 206, 208 relative to the holographic recording medium 102. To avoid the need for such mechanical movement, inductive elements are located on or within each waveguide 204, 206, 208, and can be configured to vary the optical properties of the waveguides 204, 206, 208 in order to direct signals to or from different sub-volumes of the medium 102, i.e., to create different channels within the waveguides 204, 206, 208 as desired. In this particular example, the inductive elements take the form of active optical switching elements (switches). The forms that the switches can take are varied. In this example, the switches take the form of SBGs located on different surface regions of the waveguides 204, 206, 208 in the same general arrangement as in FIGS. 3A-E. That is, each waveguide 204, 206, 208 takes the form of an active light pipe, and each waveguide 204, 206, 208 has the same general physical structure as active light pipe 300 of Figures 3A-E.

[0049]

[0062] Each waveguide 204, 206, 208 is positioned such that its first surface (i.e., the surface on which its SBG is located) is adjacent to a different side of the medium 102, thereby causing its SBG to extend along that side of the medium 102. The first and second SBGs of each waveguide 204, 206, 208 are indicated by reference numerals 204-1, 204-2, 206-1, 206-2, 208-1, and 208-2, respectively, and are all configurable in the manner described above. Additional SBGs are depicted without reference numerals, and the number of SBGs can be selected to accommodate any size of holographic recording medium 102. The following description will refer to the first and second SBGs of each waveguide 204, 206, and 208 for simplicity, but it will be understood that the description applies to more SBGs.

[0050]

[0063] 2B-2D illustrate how input and reference waveguides 204, 206 are used to write data to medium 102 in a one-to-many fashion. FIG. 2B shows a schematic plan view of system 200, while FIGS. 2C and 2D show side views of one side showing input and reference waveguides 204, 206, respectively. Input waveguide 404 is used to direct input beam 104 to any one of multiple subvolumes of medium 102 via any one of SBGs 404-1, 404-2 of input waveguide 204 in the manner described above. Reference waveguide 406 is configured to simultaneously direct reference beam 106 to the same subvolume, generating the desired interference pattern to be written in that subvolume. In the depicted example, both the input waveguide 204 and the reference waveguide 206 are configured to direct the input and reference beams 104, 106, respectively, to the subvolume indicated by reference numeral 110 via the second SBGs 204-2, 206-2 of each waveguide 204, 206, respectively.

[0051]

[0064] Figures 2E-2G show how reference and output waveguides 206, 208 can be used to read data from medium 102. Figure 2E is a plan view, and Figures 2F and 2G show side views of one side showing reference and output waveguides 204, 206. Reference waveguide 206 is used in exactly the same way as depicted in Figures 2B-2D, except here, reference beam 116 is directed into the subvolume where the hologram is to be read (in this case, subvolume 110). Output waveguide 208 is used in a one-to-many fashion to direct the resulting output beam 108 from subvolume 110 through waveguide 208 for subsequent detection.

[0052]

[0065] Each sub-volume 110 may have, for example, a height and width of a few millimeters as measured along any side, which would generally be sufficient to store several million pixels per data "page" (e.g., multiplexing angle). In this case, the sub-volume volume is sufficient to store (millions of pixels) * (number of multiplexing angles).

[0053]

[0066] The inductive elements of the input waveguide 204 and reference waveguide 206 (SBGs in this example) are configured to provide channels for the input beam 104 and reference beam 106, 116, as needed, from the beam source (emission system) to the sub-volume 108 being read. With an SBG, this is a case of setting the SBG to a transmissive or reflective state as needed to create the channels. Similarly, the inductive elements of the output waveguide 208 (also SBGs in this example) are similarly configured to provide channels from the sub-volume 108 being read to the detector. To provide additional context, this is described in more detail below with reference to the multiple waveguide network depicted in FIG. 5. However, the principles described in connection with the specific example of FIG. 5 apply more generally to other waveguide network topologies, including both simpler networks (e.g., a single waveguide) and more complex waveguide networks.

[0054]

[0067] As noted above, this allows spatial multiplexing across medium 102 without mechanical movement of medium 102 relative to waveguides 204, 206, 208. This is true whatever form the inductive elements take (which, as noted above, can themselves be mechanical or non-mechanical).

[0055] Holographic storage using multiple waveguide networks.

[0068] FIG. 5 shows an example of a holographic storage system incorporating a multiplexed waveguide network of the type shown in FIG.

[0056]

[0069] The input waveguide network is shown to include a first input light pipe 203 (a "parent" waveguide) to which multiple second input light pipes 204A, 204B ("child" waveguides) are coupled. An input beam 104 from an emitter system 504 is coupled into the first input waveguide 203 through its incoupling region, from which it can be directed to a second input waveguide 204A or 204B.

[0057]

[0070] The reference waveguide network is shown to include a first reference waveguide 205 to which multiple second reference waveguides 206A, 206B are coupled. Reference beams 106, 116 from an emitter system 504 can similarly be coupled into the first reference waveguide 205 and directed into any of the second reference waveguides 206A, 206B.

[0058]

[0071] The output waveguide network is shown to include a first output waveguide 207 to which a plurality of second output waveguides 208A, 208B are coupled.

[0059]

[0072] The depicted arrangement allows beams to be directed to / from different sub-volumes of multiple pieces 102A, 102B of holographic storage media.

[0060]

[0073] Although Figure 4 shows input beam 104, reference beams 106, 116 and output beam 108, it will be understood that the sub-volumes will typically be written and read at different times in the manner described above with reference to Figures 2A-G.

[0061]

[0074] A first group of second waveguides 204A, 204B, 204C (one each for input, reference, and output) are positioned around a first piece 102A (first medium) of holographic storage media, and a second group of second waveguides 204B, 206B, 208B are positioned around a second piece 102B (second medium), each in the same general arrangement as in Figures 2A-G. Thus, input and reference beams 104, 106, 116 can be directed to any subvolume of any media piece 102A, 102B by first directing those beams into the desired second waveguides of the input and reference networks, respectively, and then directing them into the desired subvolume of the media piece adjacent to the desired waveguide.

[0062]

[0075] The output waveguide network can be used to direct output beam 108 from any subvolume of any media piece 102A, 102B from the applicable second output waveguide 208A, 208B to the first output waveguide 207, and then from the first output waveguide 207 to the detector via the outcoupling region of first output waveguide 207-detector 508. To read from a particular subvolume, the SBGs are configured to provide a channel from that subvolume to the detector; thus, in this case, SBGs 204A-2 and 207A-1 are set to a transmissive state, and other SBGs of the output waveguide network are set to a reflective state as needed to provide a channel for output beam 108 to detector 508 (e.g., in this case, SBG 207-2 of first output waveguide 207 is set to a reflective state to prevent propagation of output beam 104 to waveguide 207-2). Other SBGs in the output waveguide network can be set to be reflective to the extent necessary to prevent unwanted light transmission (i.e., "leakage") from other areas of the same media piece 102A or from different media pieces 102B (e.g., in this example, SBG 204A-1, which is close to the subvolume being read, is shown to be set to be reflective to prevent unwanted leakage).

[0063]

[0076] Although the above example uses three separate waveguide networks for the input, reference, and output beams 104, 106, 116, 108, this is not necessary. For example, the same waveguide network could be used to carry both the input beam 104 and the reference beams 106, 116, and / or the same waveguide network could be used to carry the input beam 104 and the output beam 108, and / or the same waveguide network could be used to carry the output beam 108 and the input beam 104. Generally, having three separate networks is expected to provide optimal performance, although there are nevertheless perfectly viable implementations using only one or two waveguide networks.

[0064]

[0077] Although not depicted in any of the figures, a fourth waveguide network can be used to deliver beams to the remaining sides of the media pieces 108A, 108B. For example, a fourth network can be used to deliver an erase beam to a desired sub-volume that is at least suitable for erasing a hologram therefrom (in the case of erasable holographic storage).

[0065]

[0078] Figure 9 shows an alternative physical structure in which a single "slab" of holographic medium 102 is used in place of the individual pieces 102A, 102B of Figure 5. An input waveguide network is depicted, having essentially the same physical configuration, but here second input waveguides 204A, 204B are configured to direct input beams 104 into different subvolumes of the same slab 102. While in Figure 5 each of the second input waveguides 204A, 204B provides multiplexing in one dimension along the length of a different single piece of medium 102A, 102B, in Figure 9 the second waveguides 204A, 204B provide spatial multiplexing in two dimensions across the slab of holographic medium 102 (each waveguide individually provides one-dimensional multiplexing, but there is 2D multiplexing across the slab 102 as a whole).

[0066]

[0079] The system of Figure 9 is limited to a maximum of two waveguide networks (one on each side of the slab 102). As noted above, this is still a viable arrangement because the same network can be used to carry multiple beams.

[0067] Data Encoding

[0080] 6A shows an example of an emitter system 504 that provides both the input beam 104 and the reference beam 106. The input beam is an expanded, spatially modulated laser beam. A laser 600 emits a coherent, narrow laser beam, which is split using a beam splitter 604.

[0068]

[0081] A portion of the beam from beam splitter 602 is used as reference beam 106. In this example, controllable reference beam steering element 612 is used to steer reference beam 106 to enter reference waveguide 106 at a desired angle. By changing the angle of reference beam 106 before it is introduced into reference waveguide 206, different holograms can be written to / read from the same sub-volume of media in the manner described above.

[0069]

[0082] As an alternative or in addition to beam angle multiplexing, multiple patterns can be stored in and read from the same sub-volume with different phases of the reference beams 106, 116 (phase multiplexing). Thus, a logical address may correspond to a particular reference beam angle and / or phase characteristic. All discussion regarding modulation of the reference beam angle applies equally to phase modulation.

[0070]

[0083] The other portion of the beam from the beam splitter 602 is expanded using a beam expander 604, and the expanded beam passes through a spatial light modulator (SLM) 606. An encoder 610 receives the set of data to be encoded and encodes the set of data as a digital image, which is then modulated via the SLM 606 and embedded in the expanded beam. An incoupling optic (in this case, a Fourier lens 608) positioned so that the plane of the SLM 606 is substantially in the focal plane of the Fourier lens 608 is used to separate the expanded beam into distinct propagation modes, where each mode corresponds to a unique propagation direction, where each mode corresponds to a specific point in the plane of the SLM 606. The distinct propagation modes are introduced into the input waveguide 202 and guided therefrom in the manner described above. Using the incoupling optics 608, data is "angularly encoded" within the reference beam in the sense that points in the digital image essentially correspond to unique propagation directions (i.e., unique propagation modes of the input beam 104). This is analogous to a ray of light from a distant object viewed as infinity. The angle-encoded input beam 104 of Figure 6A is an example of multiple propagation modes (i.e., components propagating in different directions) of a "multimode" optical signal; this arrangement provides a form of angular diversity.

[0071]

[0084] Note that the term "multimode" does not necessarily imply the use of such incoupling optics 608, nor does it necessarily require that all image points uniquely correspond to a given propagation direction. That is, multimode does not necessarily imply a one-to-one correspondence between propagation modes and image / data points. For example, FIG. 6B shows an alternative viable emitter system in which the spatially modulated beam is directly coupled into the input waveguide 204. In this case, there are still multiple modes (i.e., multiple spatial paths through the waveguide for any given channel), but there is no one-to-one correspondence between propagation directions and image points, and possibly no one-to-one correspondence between image / data points and modes. This provides a form of spatial diversity based on a form of MIMO (multiple-input, multiple-output) transmission through the multiple pixels of the SLM 606 and the detector array of the spatially coherent detector 508.

[0072] Data Decryption

[0085] 7 illustrates a spatially coherent detector 508 used to measure the optical field of the output beam 108. In contrast to conventional "direct detection," the spatially coherent detector 508 includes an array of pixels (or, more generally, detector elements), each configured to measure both the amplitude and phase (rather than just intensity) of the optical field at that pixel's location. These can be measured, for example, using the spatially coherent detector's 508 local oscillator 712. The array of pixels can thus measure the phase and amplitude variations of the optical field in both time and space, and thus provide an analog or digital representation of the measured optical field. In this case, the optical field being measured is the optical field of the output beam 108 directed to the spatially coherent detector 208 via the output waveguide 208.

[0073]

[0086] Although only a single array is depicted, there may in fact be multiple physical arrays working together as a single "logical array." For example, this logical array may be split across two physical cameras.

[0074]

[0087] The physical detector array can take the form of a single camera (each detector element is a pixel or set of pixels in the camera) or multiple cameras. In the extreme case, each detector element can be a separate camera, in which case the logical detector array can potentially be split across a large number of physical detectors.

[0075]

[0088] As described, the route from a particular incoupling region where a beam enters a waveguide network to a particular outcoupling region where the beam exits the waveguide network (these regions may be in the same or different waveguides) may be referred to herein as a "channel." As described, in a multimode waveguide network, a single channel encompasses multiple spatial paths. The output beam 108 will be directed through a particular channel of the output waveguide network (i.e., from that particular incoupling region to the outcoupling region of the output waveguide 208). Moreover, the output beam 108 will be generated from a hologram generated using an input beam directed from the incoupling region of the input waveguide network to that particular outcoupling region. The hologram will be generated and read using a reference beam similarly directed through a particular channel through a reference waveguide network. The input beam 104, the reference beams 106, 116, and the output beam 108 are all subject to distortions within the associated waveguide network specific to the channel through which they are directed. To compensate for such distortions, the signal processing component 700 applies analog and / or digital signal processing to the representation of the measurement field. The signal processing component 700 does so using a channel model associated with the subvolume currently being read (i.e., from which the output beam 108 was generated). The channel model associated with a particular subvolume models not only the channels through which the output beam 108 is directed to the detector 508, but also the channels through which the input beam 104 used to write the hologram was directed to that subvolume and the channels through which the reference beams 106, 116 used to write / read the hologram were directed to that subvolume.

[0076]

[0089] Each channel model can take the form of, for example, a transfer function (which models the channel directly) or an inverse transfer function (which models the channel with its approximate inverse). Note that the transfer function is applied to a representation of the measured light field (i.e., both its measured phase and amplitude at different spatial points), not just the light intensity. Spatially coherent detection offers greater scope for removing or reducing such channel distortions, with the goal of accurately reconstructing the original digital image enough to allow the decoder 704 to easily decode the encoded data from the reconstructed image.

[0077]

[0090] The signal processing 700 can correct for phase interference and noise, for example, using a combination of optical and computational techniques (which may include, for example, machine learning techniques).

[0078]

[0091] Although described in the context of holographic storage, the use of such signal processing 700 in combination with spatially coherent detection is not limited in this respect and may be applied in other contexts such as optical communications or optical computing, or any other context in which a received output beam is susceptible to distortions introduced in one or more waveguide networks.

[0079]

[0092] Figure 7 shows outcoupling optics 715 arranged to essentially reverse the effect of incoupling optics 608 of Figure 6B (i.e., to essentially resolve each propagation mode to a single point in the plane of spatially coherent detector 508). Again, this is not required, and outcoupling optics 715 can be omitted by using the alternative emission system of Figure 6B.

[0080]

[0093] Although not depicted in Figures 6A or 6B, some level of pre-processing can be applied to the digital image before it is modulated and embedded into the input beam 104. This can reduce the level of compensation required at the detector side. Even with such pre-processing, some degree of detector-side processing can be applied to account for different distortion effects between different channels.

[0081] Dynamic Scheduling

[0094] 8A shows a controller in the form of a scheduler 800 that can schedule read and write operations within a holographic storage system of the type described above. To facilitate effective scheduling, sub-volumes within the medium 108, or within each media piece 108A, 108B, are assigned unique addresses. This provides a form of addressable holographic storage that is similar to more conventional forms of addressable electronic storage. However, there are many features that go beyond conventional addressing.

[0082]

[0095] First, as described above, a single subvolume can store multiple holograms at different reference beam angles. To accommodate this, each address uniquely corresponds to a specific subvolume in combination with a specific reference beam direction (i.e., each available tuple is assigned a unique address), indicates a specific subvolume within media 102 or one of media pieces 102A, 102B, and indicates a specific reference beam direction (e.g., an angle or set of angles defining a beam direction; the term "angle" can be used as shorthand to refer to the direction of the reference beam, but it will be understood that a direction can effectively be defined by multiple angles depending on the system configuration). Thus, a subvolume can be associated with potentially many addresses corresponding to different reference beam angles. A tuple defines a logical storage location, and the same subvolume at different reference beam angles provides multiple logical storage locations at the physical level. Each logical storage location has a unique address (ADDR). This notation is used as shorthand to mean an address corresponding to a subvolume and a reference beam angle, but it will be understood that this does not imply a specific address representation. Any address space and addressing mechanism that uniquely identifies logical storage locations of this nature can be used.

[0083]

[0096] Second, in contrast to conventional storage devices, each logical memory location can store an entire image, and thus a single logical memory location can potentially store a very large number (e.g., thousands or millions) of bits.

[0084]

[0097] Scheduler 800 operates at the logical storage level and schedules incoming read and write operations for different addresses within appropriate time intervals.

[0085]

[0098] Reference numerals 804, 806, and 808 are used to denote input, reference, and output optical waveguide networks, respectively. As described above, each can be a single waveguide network or a multiple waveguide network (e.g., similar to FIG. 5), with one or more configurable inductive elements (e.g., SBG or other active switching elements) that can be used to create channels to different sub-volumes of a piece (or pieces) of holographic storage media.

[0086]

[0099] During intervals when a write operation for a particular address is scheduled (write intervals), the inductive elements in the input and reference waveguide networks 804, 806 are configured to create channels for the input beam 104 and the reference beam 106 from the emitter system 504 through the input and reference networks 804, 806, respectively, to the corresponding sub-volumes. In addition, the reference beam steering element 612 is configured to steer the reference beam 106 in a corresponding direction toward the reference network 806. This generates a desired interference pattern at the reference beam angle within the sub-volume, and therefore, if the sub-volume is exposed to the interference pattern for a sufficiently long duration, the interference pattern will be persistently stored as a hologram.

[0087]

[0100] During intervals when a read operation for a particular address is scheduled (write intervals), the steering elements in the reference and output networks 806, 808 are similarly configured to create a channel for the reference beam 116 through the reference network 806 to the subvolume, and a channel for the output beam 108 from the subvolume through the output network 808 to the detector 508. The reference beam steering element 612 is similarly configured to steer the reference beam 116 to the reference network 806 in a corresponding direction to read the intended hologram in the subvolume at the reference beam angle.

[0088] Optimal Scheduling

[0101] One problem with holographic storage is that write operations require a relatively long time to complete. This is because, to generate a stable (persistent) hologram, the subvolume must be exposed for a relatively long time interval compared to the time required to read an existing hologram. For example, some systems may require exposure times on the order of 10 or 100 milliseconds to persistently generate a hologram. In contrast, a read operation need only expose the subvolume for the time required to measure the resulting light field of the output beam 108. For example, some systems may achieve exposure times for read operations (i.e., duration of media exposure and / or camera shutter open time for reading the beam) on the order of 100 microseconds to 1 millisecond, and write operations may be segmented, for example, over intervals of similar duration.

[0089]

[0102] However, it is recognized herein that the write operation need not occur in a single, uninterrupted time interval. The exposure can be “staged,” as long as the sub-volume is exposed to the interference pattern for a sufficient total time. To perform a write operation, the sub-volume can be exposed to the interference pattern to be recorded for a time that does not cause the interference pattern to be recorded persistently. The exposure can then be terminated to perform some other operation and resumed at a later time. The time between write intervals can generally be arbitrarily long, and it is contemplated that the holographic data cannot be read from the media until the complete exposure is complete, so the data can be held elsewhere (e.g., in a non-volatile buffer) during that time. However, this can result in significant write latency. Accordingly, scheduler 800 seeks to balance the amount of such buffering required with the need to avoid head-of-line blocking during reads.

[0090]

[0103] 8B schematically illustrates a scheduling scheme that exploits this effect to efficiently schedule a set of read and write operations. For illustrative purposes, a single write operation 814 and three read operations 818a, 818b, and 818c are depicted, but the scheduling scheme can be applied to any number of read and write operations.

[0091]

[0104] Similar to energy efficiency, an additional benefit of staggering write operations is that read operations can be scheduled in a way that reduces "head-of-line blocking," which otherwise would have a "naive" scheduling scheme, which simply schedules the write operation for a long enough time interval to complete, and then performs the read operation once the write operation is complete. A "naive" scheduling scheme is how write operations are typically scheduled in conventional storage systems. In such systems, this has the effect of significantly increasing the latency of read operations 818a-c, which are forced to wait until write operation 814 has completed.

[0092]

[0105] By staging the write operation 814 over multiple discontinuous time intervals and scheduling the read operations 181a-c in read intervals that are time-interleaved with the multiple write intervals, the dual benefits of increased energy efficiency and reduced read operation latency are achieved.

[0093]

[0106] In each write interval, the sub-volume involved in the write operation 814 is exposed to the same interference pattern that encodes the image to be stored. The input and reference networks 804, 806 are configured to provide the necessary channels in the same way in each write interval, and the reference beam 106 is pointed in the same direction.

[0094]

[0107] In the intervening readout section, the reference and output networks 806, 808 can be reconfigured and the reference beam 116 can be redirected as needed to read any desired hologram in the holographic storage system.

[0095] Energy Optimization

[0108] Surprisingly, it has been found that when a write operation is staged in the manner described above, the total time required to persistently generate a hologram (i.e., the sum of the individual durations of multiple discontinuous write intervals) is less than the exposure time that would be required to write a hologram in a single, uninterrupted write interval for a given power of the input and reference beams 104, 106. For example, it has been found that a write operation that may require approximately 10 ms to complete in a single, uninterrupted write interval can, in the same system, be performed over multiple discontinuous time intervals having a total duration of only approximately 7 ms. This, in turn, means that by staging the write operation in the manner of FIG. 8B , the amount of energy required to complete the write operation (which is essentially directly proportional to the total duration required to complete the write operation with a beam of fixed power) can be reduced.

[0096]

[0109] Figure 13 shows a set of results illustrating this effect for one particular setup. The results for pulsed and continuous writing are indicated by reference numerals 1302 and 1304, respectively. In this particular example, it can be seen that for each write time greater than about 30 ms, pulsed writing with a total duration of that amount achieves a higher diffraction efficiency than continuous writing with the same total duration. It can be seen that the diffraction efficiency increases with increasing write time. In this context, diffraction efficiency indicates the total energy diffracted into the medium 102, which is proportional to the incident energy. Thus, it can be seen that better energy efficiency is achieved with pulsed writing compared to continuous writing.

[0097]

[0110] 8A illustrates an energy estimation function 804 that can be used to estimate the amount of energy required to perform a set of operations according to a particular schedule, taking the above-described effects into account. Scheduler 800 attempts to determine a substantially optimal schedule that substantially minimizes the total estimated energy required to complete a given set of operations. That goal is fully compatible with the goal of mitigating the effects of head-of-line blocking on write operations. That is, staging write operations 814 in the manner described above not only frees up intervening time intervals to allow read operations 818a-c to complete with lower latency in the intervening read intervals, but also reduces the total amount of energy required to complete write operations 814, for the reasons discussed in the previous paragraph.

[0098]

[0111] Scheduling can be dynamic in the sense that the write operation 814 can be dynamically suspended in response to an incoming read operation 818a, 818b, 818c that occurs or is received after the write operation 814 has been initiated but before it has completed. This ability to dynamically suspend the write operation 814 allows the scheduler 800 to respond to incoming read operations with minimal latency.

[0099] Cost Function Based Scheduling

[0112] We now describe one possible scheduling scheme based on cost function optimization. In this case, scheduler 800 seeks to optimize a defined cost function that seeks to balance competing factors such as two or more of write latency, read latency (e.g., expressed in terms of read buffer time), and energy (taking into account the energy-saving effects from staging write operations).

[0100]

[0113] For example, a set of read operations R={r i}, a set of write operations W={w j} and a schedule S that schedules these operations in a particular way (i.e., assigns each operation to one or more time intervals, which may include discontinuous time intervals for write operations), the cost function is

number

[0101]

[0114] This cost function considers three competing factors, although any number of cost terms (eg, any two of the above and / or other cost terms) may be present.

[0102]

[0115] Using the second and third terms, staggering write operations can reduce the associated energy penalty of the third term, but may increase the latency penalty of the second term. The optimizer finds an optimal schedule S that balances these competing requirements. * will try to find out. S * =argmin S C(S)

[0103]

[0116] Namely, Schedule S * is the actual minimum cost.

[0104]

[0117] Although the above uses the term "cost," this is merely a convenient label: optimizing a reward function (e.g., one that rewards reduced latency and / or energy) is synonymous here with optimizing a cost function.

[0105] Alternative Waveguide Networks:

[0118] 10A and 10B show an alternative system in which spatial multiplexing is achieved instead by modulating one or more optical properties of the input and reference beams 104, 106, 116. In such a system, passive (non-switchable) inductive elements can be used in place of the active (switchable) inductive elements of the previous figures.

[0106]

[0119] The example in Figure 10A considers frequency (or equivalently wavelength) modulation. In this case, the light pipes themselves are passive, with static wavelength-dependent outcoupling (such as continuous long-pass dichroic interference filters or variable center wavelength bandpass filters).

[0107]

[0120] FIG. 11A shows a light pipe 1100 having an outer surface 1100-S, along which are located multiple passive filters 1100-1 and 1100-2. The configuration of the light pipe 1100 is the same as that shown in FIGS. 3A-D, except that the filters 1100-1 and 1100-2 replace the SGBs 300-1 and 300-2. The filters 1100-1 and 1100-2 have different frequency responses (i.e., they act as frequency filters). More specifically, each filter 1100-1 and 1100-2 essentially transmits a relatively narrow range of optical frequencies and is essentially reflective to frequencies outside that range. FIG. 11A shows an incoupling beam 1104, whose frequency is within the range of the second filter 1100-2 but outside the range of the first filter 1100-1. Thus, beam 1104 reflects off first filter 1100-1 but transmits through second filter 1100-2 (thus exiting light pipe 1100 at that location). Figure 11B shows a different frequency beam 1104', where the frequency is within the range of first filter 1100-1 and is therefore transmitted through first filter 1100-1.

[0108]

[0121] Such a light pipe 1100 can be used in place of the active light pipe described above, and the above description applies equally to the following system variations.

[0109]

[0122] 10A shows a scheduler 800 communicatively coupled to the laser 600 of the emitter system for varying the frequency (or equivalently, wavelength) of the input and reference beams 104, 106, 116. In this case, either beam can be directed to a desired holographic storage region by setting the frequency accordingly. Here, different beam frequencies correspond to different routes through the waveguide network (defined by different frequency characteristics of the passive filters), and the frequencies can be set to correspond to any desired route.

[0110]

[0123] In this case, wavelength is used as the switching dimension. Laser 600 is a high speed tunable laser that functions as the active element.

[0111]

[0124] In such a realization, switching can be in only one spatial dimension (i.e., along a single pipe). However, with a laser having sufficient range and narrow linewidth, the first light pipe can filter coarsely (i.e., over a relatively wide wavelength range), while subsequent light pipes sample more finely (i.e., over a narrower wavelength range). Another factor limiting the linewidth is the requirement for a relatively long coherence length, so that in any event, the linewidth can be sufficiently narrow. In the context of holographic storage, this implementation can be combined with a second implementation using, for example, a different switchable parameter (e.g., polarization) to achieve replication of an input field to addressable locations across a 2D output space.

[0112]

[0125] In connection with the readout operation, the frequency of the output beam 108 is matched to the frequency of the reference beam 116 used to read the particular sub-volume, and the same principles can be applied to return it to the detector using an appropriate filter in the output waveguide network 808.

[0113]

[0126] Figure 10B shows an example of such an implementation with a controllable polarizing element 601 that can be used to change the polarization of the input and reference beams 104, 106, 116. This can be combined with a passive polarizing filter on or within the light pipe. This passive polarizing filter can be implemented as an alternative to or in addition to the passive frequency filter of the example of Figure 10A. Such polarization modulation provides two independent routes and can be useful in combination with, for example, passive wavelength filtering and / or an active light pipe. Beam polarization modulation can also be combined with an active polarizing filter.

[0114]

[0127] It should be noted that the various "passive" and "active" implementations described above can all be implemented separately or in combination (e.g., a combination of active and passive inductive elements can be used), i.e., a waveguide can have both passive and active elements and / or it is possible to combine active and passive waveguides in the same network.

[0115] Additional hierarchy levels:

[0128] The above example considers a waveguide network having two hierarchical “levels”: a parent waveguide and a child waveguide. However, a multiplexed waveguide network may have three (parent, child, grandchild) or more levels. Note that the terms “child,” “parent,” and “grandparent” do not necessarily imply a direct hierarchical relationship. That is, the term child or grandchild may refer to any waveguide at any hierarchical level below the parent or child waveguide, respectively. That is, a child / grandchild waveguide may be optically coupled to a parent / child waveguide, for example, not only via an air interface (direct descendants) but also via one or more of its other child / grandchild waveguides (indirect descendants).

[0116]

[0129] 12 shows an example of a waveguide network with three hierarchical levels: a parent waveguide 1200 has two direct child waveguide networks 1202A, 1202B optically coupled to the parent waveguide 1200 in the manner described above, and each of those child waveguides 1202A / 1202B has two grandchild networks 1204A-A, 1204A-B / 1204B-A, 1204B-B optically coupled to the child waveguide 1202A / 1202B in the same manner.

[0117]

[0130] An extreme example is a "binary tree" architecture where every waveguide has exactly two direct children, potentially with more than three levels of waveguides, but in practice there may be situations where it is preferable to increase the number of direct children in order to reduce the number of hierarchical levels required.

[0118]

[0131] More generally, the described scheduling techniques can be implemented in any holographic storage system (with or without waveguides) with the same energy-saving advantages, including, for example, systems without spatial multiplexing or systems in which spatial multiplexing is achieved using mechanical means (moving media or moving read / write heads).

[0119]

[0132] The scheduler 800 and energy estimation function 804 shown in Figures 8A, 10A, and 10B are functional components of the system. Similarly, the encoder 610, decoder 704, and signal processing component 700 are functional components. Such components may be implemented in software (i.e., as program code executing on one or more programmable hardware processors, such as a CPU, accelerator, or GPU) or using other forms of processor hardware, such as field programmable gate arrays and / or application-specific integrated circuits. The signal processing performed by the signal processing component 700 may be analog or digital signal processing, or any combination thereof. Such program code and other data (e.g., channel model 702) may be encoded in computer-readable storage devices. Examples of computer-readable storage devices include optical, magnetic, and / or solid-state storage devices, capable of storing code, data, and the like in a non-transitory form. This is in contrast to a transitory medium, such as a transient signal carrier wave.

[0120]

[0133] A first aspect of the present specification provides a method of performing a write operation in a holographic data storage system, the method comprising: scheduling, by a scheduler, at least one write operation over a plurality of discontinuous write intervals, the write operation relating to a set of data to be stored in a region of the holographic recording medium; and in each of the discontinuous write intervals, exposing the region of the holographic recording medium to an interference pattern produced by interference between a reference beam and an input beam carrying the set of data, the plurality of discontinuous write intervals having a total duration long enough to cause a last state change in the exposed region, such that the set of data can be recovered from the region by the end of the last write interval of the plurality of discontinuous write intervals.

[0121]

[0134] In an embodiment, the scheduling step may further include the scheduler scheduling at least one further operation in at least one interval interleaved with the plurality of non-contiguous write intervals.

[0122]

[0135] The at least one further operation may include at least one read operation.

[0123]

[0136] The at least one further operation may include at least one further write operation scheduled across a further plurality of non-contiguous write intervals interleaved with the plurality of non-contiguous write intervals.

[0124]

[0137] The scheduler may perform said scheduling using an energy estimation function, the energy estimation function for estimating an amount of energy required to complete at least one write operation.

[0125]

[0138] The scheduler may perform said scheduling by essentially optimizing a cost function that penalizes increases in the estimated amount of energy required to complete at least one write operation.

[0126]

[0139] The cost function may also penalize an increase in the estimated latency for at least one write operation.

[0127]

[0140] Alternatively or additionally, the cost function may penalize an increase in the estimated latency for at least one read operation, and / or an increase in the estimated latency for at least one further write operation and an increase in the estimated amount of energy required to complete the at least one further write operation.

[0128]

[0141] The scheduler may dynamically schedule at least a later one of the plurality of discrete time intervals in response to a read operation during or after an earlier one of the plurality of discrete time intervals.

[0129]

[0142] A second aspect of the present specification provides a holographic storage system comprising: at least one processor configured to implement a scheduler, the scheduler configured to schedule at least one write operation over a plurality of discontinuous write intervals, the write operation relating to a set of data to be stored in an area of ​​the holographic recording medium; an emitter system configured to emit, in each of the plurality of discontinuous write intervals, a reference beam and an input beam in which the set of data is embedded; and a holographic recording medium, the holographic storage system being arranged to expose, in each of the plurality of discontinuous write intervals, an interference pattern resulting from interference between the input beam and the reference beam, thereby performing the scheduled write operation.

[0130]

[0143] The scheduler may be configured to schedule at least one further operation in at least one interval interleaved with the plurality of discontinuous write intervals, and the system may further include one or more optical waveguide networks arranged to direct the input beam and the reference beam to a region of the holographic recording medium in each of the plurality of discontinuous time intervals, and the system may be configured to use at least one of the one or more optical waveguide networks in the interleaved interval to convey the reference beam to another region of the holographic recording medium or to another holographic recording medium, with or without changing the angle and / or phase characteristics of the reference beam, or to the same region of the holographic recording medium but with different angle and / or phase characteristics.

[0131]

[0144] The at least one optical waveguide network may have at least one configurable inductive element, and the at least one processor may be coupled to the at least one inductive element and configured to reconfigure the at least one inductive element to direct the reference beam to another holographic recording region in the interleaved section.

[0132]

[0145] The at least one optical waveguide network may have at least one static guiding element responsive to controllable optical properties of the reference beam, and the at least one processor may be coupled to the emitter system to change the optical properties of the reference beam to direct the reference beam to other holographic recording regions in the interleaved sections.

[0133]

[0146] The write and further write operations may be associated with respective logical addresses, each logical address defining a holographic storage area and a reference beam angle and / or phase characteristic for the associated operation.

[0134]

[0147] A third aspect of the present specification provides a scheduler embodied as executable program code stored on a computer-readable storage medium, the scheduler being configured to perform any of the methods described above when executed on at least one processor of a holographic data storage system.

[0135]

[0148] It will be understood that the above-described embodiments are described by way of example only. Other variations or uses of the disclosed techniques will be apparent to those skilled in the art given the disclosure herein. The scope of the present disclosure is not limited by the described embodiments, but only by the appended claims.

Claims

1. 1. A method for performing a write operation in a holographic data storage system, comprising: scheduling, by a scheduler, write operations over a plurality of non-contiguous write intervals, the write operations relating to data to be stored in regions of a holographic recording medium, the scheduler scheduling the write operations using an energy estimation function, the energy estimation function for estimating an amount of energy required to complete the write operations; exposing the area of ​​the holographic recording medium in each of the discontinuous writing intervals to an interference pattern produced by interference between a reference beam and the data-bearing input beam; wherein the plurality of non-contiguous write intervals have a total duration long enough to cause a sustained state change in the exposure area, so that the data can be recovered from the exposure area within a last write interval of the plurality of non-contiguous write intervals.

2. 2. The method of claim 1, wherein the scheduler performs the scheduling of the write operations by substantially optimizing a cost function that penalizes increases in the estimated amount of energy required to complete the write operations.

3. The method of claim 1 , wherein the scheduler performs the scheduling of the write operations by substantially optimizing a cost function that penalizes increases in estimated latency for the write operations.

4. The scheduler Increased estimated latency for read operations, An increase in the estimated latency for further write operations, or an increase in the estimated amount of energy required to complete the further write operation; 2. The method of claim 1, wherein the scheduling of the write operations is performed by substantially optimizing a cost function that penalizes at least one of:

5. The method of claim 1 , further comprising scheduling, by the scheduler, read operations in read intervals interleaved with the plurality of non-contiguous write intervals.

6. The method of claim 1 , wherein further write operations are scheduled over a further plurality of non-contiguous write intervals interleaved with the plurality of non-contiguous write intervals.

7. 2. The method of claim 1, wherein the scheduler dynamically schedules at least a later one of the plurality of non-contiguous write intervals in response to a read operation during or after an earlier one of the plurality of non-contiguous write intervals.

8. 1. A holographic data storage system comprising: a processor configured to implement a scheduler, the scheduler configured to schedule write operations over a plurality of non-contiguous write intervals, the write operations relating to data to be stored in regions of a holographic recording medium, the scheduler configured to schedule further operations in intervals interleaved with the plurality of non-contiguous write intervals; an emitter system configured to emit a reference beam and an input beam in which the data is embedded in each of the plurality of discontinuous writing intervals; a holographic recording medium, wherein the holographic data storage system is arranged to expose, in each of the plurality of discontinuous write intervals, a region of the holographic recording medium to an interference pattern resulting from interference between the input beam and the reference beam, thereby performing the scheduled write operation; an optical waveguide network arranged to direct the input beam and the reference beam to the region of the holographic recording medium in each of the plurality of discontinuous writing sections, wherein the system uses the optical waveguide network to direct the reference beam in the interleaved sections: to another region of the holographic recording medium or to another holographic recording medium, with or without changing at least one of the angle or phase characteristics of the reference beam; or at least one of different angles or different phase characteristics, but in the same region of the holographic recording medium. an optical waveguide network configured to carry 1. A holographic data storage system comprising:

9. 9. The system of claim 8, wherein the at least one optical waveguide network has at least one configurable inductive element, and the processor is coupled to the at least one inductive element to reconfigure the at least one inductive element to direct the reference beam to other regions of the holographic recording medium in the interleaved section.

10. 9. The system of claim 8, wherein the at least one optical waveguide network has at least one static guide element responsive to a controllable optical property of the reference beam, and the processor is coupled to the emitter system to change the optical property of the reference beam to direct the reference beam to another region of the holographic recording medium in the interleaved section.

11. 9. The system of claim 8, wherein the write operation is associated with a logical address, the logical address defining the region of the holographic recording medium and at least one of the angle or phase characteristics of the reference beam.

12. The system of claim 11 , wherein the scheduler is configured to schedule read operations in read intervals interleaved with the plurality of non-contiguous write intervals.

13. 12. The system of claim 11, wherein the holographic data storage system is arranged to expose the region or another region of the holographic recording medium to the reference beam in the interleaved sections, thereby generating an output beam for reception by a detector, thereby performing a scheduled read operation.

14. The system of claim 11 , wherein the at least one optical waveguide network comprises an active light pipe.

15. A scheduler embodied as executable program code stored on a computer-readable storage medium, which when executed on a processor of a holographic data storage system: scheduling, by the scheduler, write operations over a plurality of non-contiguous write intervals, the write operations relating to data to be stored in regions of a holographic recording medium, the scheduler scheduling the write operations using an energy estimation function, the energy estimation function for estimating an amount of energy required to complete the write operations; exposing the area of ​​the holographic recording medium in each of the discontinuous writing intervals to an interference pattern produced by interference between a reference beam and the data-bearing input beam; configured to perform a write operation in a holographic data storage system, comprising: A scheduler, wherein the plurality of discontinuous write intervals have a total duration long enough to cause a sustained state change in the exposure area, so that the data can be recovered from the exposure area within the last write interval of the plurality of discontinuous write intervals.

16. 16. The system of claim 15, wherein the scheduler performs the scheduling of the write operations by substantially optimizing a cost function that penalizes increases in the estimated amount of energy required to complete the write operations.

17. 16. The system of claim 15, wherein the scheduler performs the scheduling of the write operations by substantially optimizing a cost function that penalizes increases in estimated latency for the write operations.

18. The scheduler Increased estimated latency for read operations, An increase in the estimated latency for further write operations, or an increase in the estimated amount of energy required to complete the further write operation; 16. The system of claim 15, wherein the scheduling of the write operations is performed by substantially optimizing a cost function that penalizes at least one of:

19. The system of claim 15 , wherein the scheduler is configured to schedule read operations in read intervals interleaved with the plurality of non-contiguous write intervals.

20. The system of claim 15 , wherein the scheduler is configured to schedule further write operations over a further plurality of non-contiguous write intervals interleaved with the plurality of non-contiguous write intervals.

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