Phase-modulated optical data storage

Abstract

A method for writing data to a transparent substrate includes forming a first voxel by focusing a first laser pulse at a first location within the transparent substrate, and forming a second voxel by focusing a second laser pulse at a second location within the transparent substrate. The first and second laser pulses have different amplitudes, thereby resulting in first and second voxels having different intensities. A system useful for implementing this method, an optical data storage medium obtained by this method, and a method for reading data from the optical data storage medium are also provided.

Description

Background Art

[0001] Background

[0001] There is a significant demand for data storage. Cloud storage providers are expected to require data storage capacities on the order of zetabytes (1 zetabyte is 1 trillion gigabytes (10 21 bytes)) in the near future. Much of the data needs to be stored for long periods of time.

[0002]

[0002] Examples of currently widely used data storage technologies include hard disk drives, magnetic tapes, flash memories, and optical disks. All of these technologies have the drawback that data needs to be periodically copied to a replacement medium. This is costly from both the perspectives of energy usage and hardware requirements.

[0003]

[0003] Magnetic storage media such as hard drives and magnetic tapes have the problem of gradually demagnetizing. Flash memories are subject to read disturb effects that cause failures in surrounding flash cells by repeatedly reading from a particular flash cell. The reflective materials used for data storage in optical media such as DVDs deteriorate over time.

[0004]

[0004] As a solution to these drawbacks, birefringent optical data storage media have been proposed. Birefringent optical data storage media include a substrate such as a quartz glass substrate. Data is encoded in three-dimensional nanostructures formed within the substrate. These nanostructures are called voxels.

[0005]

[0005] Voxels have optical properties different from those of the surrounding bulk substrate. Birefringent voxels exhibit different refractive indices depending on the polarization and / or direction of incident light. Birefringence can be controlled when writing voxels to the substrate and is used to encode data.

[0006]

[0006] The substrate is transparent in the sense that it is transparent to one or more wavelengths of light used to read and write voxels.

[0007]

[0007] Birefringent optical data storage media and the manufacture of the same are described, for example, in Anderson et al, Glass: A New Media for a New Era? 10th USENIX Workshop on Hot Topics in Storage and File Systems (HotStorage 18), 2018 and U.S. Patent No. 10,236,027B1. [Overview of the project]

[0008] overview

[0008] In one embodiment, a method is provided for writing data to a transparent substrate. This method includes forming a first voxel having a first intensity by focusing a first laser pulse to a first location in the transparent substrate, wherein the first laser pulse has a first amplitude, and forming a second voxel having a second intensity different from the first intensity by focusing a second laser pulse to a second location in the transparent substrate, wherein the second laser pulse has a second amplitude different from the first amplitude. By encoding the data by amplitude modulation, it is possible to write voxels using a single laser pulse, thereby increasing data throughput. Alternatively or additionally, this method can reliably form voxels on inexpensive substrates such as borosilicate glass.

[0009]

[0009] In another embodiment, an optical data storage medium is provided. The optical data storage medium comprises a transparent substrate containing a material having a bulk refractive index, a first voxel embedded in the transparent substrate, wherein the first voxel has a first intensity, and a second voxel embedded in the transparent substrate, wherein the second voxel has a second intensity different from the first intensity, thereby resulting in a first voxel and a second voxel that encode different data symbols. The transparent substrate can be obtained by a method as described herein.

[0010]

[0010] Another embodiment provides a system for writing phase voxels onto a transparent substrate. This system is useful for implementing a method such as that described herein. This system comprises a controller, a pulsed laser source, and a first amplitude modulator located downstream of the pulsed laser source in the optical path. The amplitude modulator is operably linked to the controller. The controller is configured to cause the system to perform a method such as that described herein.

[0011]

[0011] Further embodiments provide a method for reading data from an optical data storage medium as defined herein. This method includes capturing an image of voxels using a refractive index-sensitive microscope and processing the image using a processor to reconstruct the data. Portions of the image having different signal strengths encode different data symbols.

[0012]

[0012] The Specification also provides the use of amplitude modulation to encode multiple different data symbols as voxels within a transparent substrate.

[0013]

[0013] This summary is provided to introduce in a simplified form an excerpt of concepts that will be further described below in the detailed description. This summary is not intended to identify any major or essential features of the claimed invention, nor is it intended to be used to limit the scope of the claimed invention. Furthermore, the claimed invention is not limited to any implementation that solves any or all of the disadvantages described herein.

[0014] Brief explanation of the drawing

[0014] To aid in understanding embodiments of the present disclosure and to show how such embodiments may be carried out, attached drawings are referenced merely as examples. [Brief explanation of the drawing]

[0015] [Figure 1]

[0014] This is a flowchart illustrating an example method for writing data to a transparent substrate. [Figure 2]

[0014] This is a schematic cross-sectional view of an optical data storage medium obtained by the method shown in Figure 1. [Figure 3]

[0014] This is a schematic plan view of an optical data storage medium illustrating the modulation of voxel positions. [Figure 4]

[0014] This plot shows the contribution to the phase shift as a position function of three overlapping voxels. [Figure 5]

[0014] This is a block diagram of a system useful for writing data to a transparent substrate. [Figure 6]

[0014] This is a block diagram of the controller for the system shown in Figure 5. [Figure 7]

[0014] This is a flowchart showing an overview of the method for reading data from a transparent substrate obtained by the method shown in Figure 1. [Figure 8A]

[0014] This is a phase-contrast micrograph of the "positive" sublayer of the voxel group. [Figure 8B]

[0014] This is a phase contrast microscope photograph of the "negative" sublayer of the voxel group. [Figure 8C]

[0014] This is a differential image generated from the microscope photographs of FIGS. 8A and 8B. [Figure 9A]

[0014] This is a histogram of the image of the voxels written on the fused silica glass substrate. [Figure 9B]

[0014] This is a histogram of the image of the voxels written on the borosilicate glass substrate.

Mode for Carrying Out the Invention

[0016] Detailed Description

[0015] For convenience of explanation, in this specification, terms indicating directions such as "uppermost", "lowermost", "left", "right", "above ~", "below ~", "horizontal", and "vertical" are used, referring to the directions shown in the relevant drawings. To avoid misunderstanding, this terminology is not intended to limit the directions in the external coordinate system.

[0017]

[0016] In this specification, ordinal numbers ("first", "second", "third", etc.) are used as labels for distinguishing different elements. For example, the description of forming a "first" voxel includes forming one or more identical voxels.

[0018]

[0017] In this specification, the "strength" of a voxel is the magnitude of the phase change that occurs when light of one or more predetermined wavelengths passes through the voxel. The one or more predetermined wavelengths are one or more wavelengths of the light used to read the voxel. The magnitude of the phase change is a function of the refractive index of the voxel and the optical path length of the light passing through the voxel, and the latter is determined by the thickness of the voxel.

[0019]

[0018] Two voxels are considered to have "different" strengths if the voxels produce phase changes different enough to be useful for encoding different data symbols.

[0020]

[0019] The difference exceeds the noise floor of the writing system, so random variations (e.g., in laser power output) do not cause incorrect symbols to be written to the substrate. The difference is also large enough to be resolved by a phase-sensitive microscope. Minor variations in intensity due to random noise are not considered “differences” in the context of this application.

[0021]

[0020] As will become clear from the following discussion, the magnitude of the intensity difference can be controlled by changing the amplitude of the laser pulse used to form the voxel. This difference can be appropriately selected based on the characteristics of the writing and reading systems. For example, a refractive index of at least 10 -4 Pairs of voxels of different but equal sizes may be considered to have different strengths. Future developments in writing and reading systems may allow for a reduction in this limit.

[0022]

[0021] Birefringent optical data storage enables the long-term storage of large amounts of data. Nevertheless, birefringent voxels have some limitations. Forming a birefringent voxel requires at least two laser pulses, which limits the data throughput (i.e., the rate at which data can be written). High-purity glass substrates, such as fused silica, are generally required to ensure that birefringent voxels are reliably formed.

[0023]

[0022] Furthermore, recovering data from birefringent voxels requires capturing multiple images (3 to 5 images in many implementations) using a polarization-sensitive microscope with illumination using light of different polarizations.

[0024]

[0023] This specification provides a method for writing data to a transparent substrate which can enable the writing of data symbols using a single laser pulse and / or enable the use of a wider range of substrate materials. According to this method, the data symbols are encoded by amplitude modulation in contrast to the orientation of birefringent voxels.

[0025]

[0024] Related embodiments provide an optical data storage medium obtained by the present method, a system useful for implementing the present method, and a method for reading data from the optical data storage medium.

[0026]

[0025] Now, with reference to Figure 1, an example method for writing data to a transparent substrate will be described. Figure 1 is a flowchart showing an overview of this method.

[0027]

[0026] In block 101, a first voxel is formed by focusing a first laser pulse to a first location within the transparent substrate.

[0028]

[0027] By focusing the laser to a first location, a permanent change occurs in the structure of the substrate material at that first location. The interaction between the laser and the transparent material can result in a variety of structural changes. Examples include the formation of high-density areas, low-density areas, localized glass matrix changes, crystallization, amorphous formation, or nanoscale void formation. The nature of the change may vary depending on the laser parameters (e.g., energy, wavelength, pulse duration) and the properties of the transparent substrate material.

[0029]

[0028] A change in the structure of the material alters the refractive index of the material at a first location relative to the refractive index of the unaltered bulk material. The altered region at the first location is called a voxel. More specifically, a voxel is sometimes called a "phase voxel" because electromagnetic waves (i.e., light) passing through the voxel undergo a phase shift. The amount of phase shift caused by a phase voxel is sometimes called its intensity.

[0030]

[0029] In many materials, a laser causes heterogeneous changes to the material's structure in the direction of laser propagation. These heterogeneous changes include regions where the refractive index is increased and regions where the refractive index is decreased. These regions are sometimes called "sublayers".

[0031]

[0030] Perpendicular to the laser propagation direction, the change in refractive index follows a radial profile corresponding to the laser beam profile. For example, the change in refractive index may have a Gaussian radial profile.

[0032]

[0031] The first laser pulse may be a single laser pulse. By forming each voxel with a single laser pulse, it is possible to write the voxels at a rate that approximates the repeat rate of the laser source used to write the voxels. In principle, two or more laser pulses may be used. For example, by writing the voxels using a GHz burst laser, it is possible to improve the quality of the voxels.

[0033]

[0032] After the first voxel is formed, a second voxel is formed in block 102.

[0034]

[0033] The second voxel is formed at a second location in the transparent substrate using a second laser pulse.

[0035]

[0034] The second location is different from the first location. The point at which the laser is focused can be changed by moving the transparent substrate relative to the laser and / or by adjusting the laser's focusing position.

[0036]

[0035] The degree to which the structure of the transparent substrate is altered depends on the intensity of the laser pulse. Intensity is sometimes called fluence, energy, amplitude, or peak power. Therefore, the intensity of the phase voxel can be controlled by modulating the energy of the laser pulse.

[0037]

[0036] Voxels formed using laser pulses of different energies may have different refractive indices and / or different thicknesses, where thickness is the size of the voxel in the direction of laser pulse propagation.

[0038]

[0037] Phase voxels of different intensities can encode different data symbols. The absence of a voxel in a certain location can represent additional data symbols. By writing two or more phase voxels of different intensities, multi-bit encoding can be enabled. This can enable the storage of data at a higher density per unit volume.

[0039]

[0038] Various modifications may be made to the described method.

[0040]

[0039] The example refers to the formation of voxels of two different intensities. Assuming that the reader used to recover the data is capable of decomposing voxels of different intensities, higher density data storage can be achieved by increasing the number of discrete voxel intensity levels. For example, it is intended to write voxels of 3 to 5 different intensities, or more.

[0041]

[0040] As described above, the shape of the voxel follows the profile of the laser beam used to write the voxel. The method may also include modulating the beam shape. This can enable a higher bit encoding scheme, thereby increasing the data density. For example, the method may include writing two voxels having equal intensity but different shapes (e.g., circular and elliptical). Alternatively, variations in shape can be achieved using multiple laser pulses. Modulation of the shape can make it possible to obtain birefringent voxels.

[0042]

[0041] In this example, the voxel intensity is modulated by changing the intensity of the laser pulse. The laser wavelength and laser pulse duration are kept constant. In principle, the voxel intensity can be modulated by changing the laser wavelength or pulse duration instead. However, since pulse sources typically operate at a fixed wavelength and fixed repeat rate, more complex hardware may be required to implement modulation of wavelength or pulse duration.

[0043]

[0042] This method may be parallelized, and two or more instances of this method may be performed in parallel. For example, a beam splitter may be used to split the source laser pulse, and each voxel can be written simultaneously using the split pulse.

[0044]

[0043] Parallelization can improve data recovery by creating an intensity correlation between voxels written using the same source laser pulse. The energy output of the laser may fluctuate over time, which can lead to fluctuations in the intensity of the written voxels. Voxels written simultaneously using the same source laser pulse may be exposed to the same noise (specifically, the same noise resulting from fluctuations in the output of the laser source). The noise introduced during writing can be compensated for by using correlations between different voxels during image processing / decoding (e.g., using a machine learning model).

[0045]

[0044] Referring now to Figure 2, an exemplary optical data storage medium 200 obtained by the method of Figure 1 is described. Figure 2 is a schematic cross-sectional view of the optical data storage medium.

[0046]

[0045] The data storage medium 200 includes a transparent substrate 210. "Transparent" means "transparent to light of wavelengths used to write voxels onto the transparent substrate."

[0047]

[0046] The material of the substrate 210 is not particularly limited. Any transparent material can be used, provided that voxels can be formed on the substrate 210. For example, the substrate may include glass, glass ceramic, or organic polymer. In particular, the substrate may include glass. Suitable examples of glass include fused silica, borosilicate glass, and soda-lime glass. Borosilicate glass and soda-lime glass are less expensive than higher purity glasses such as fused silica. Glass has excellent chemical stability, and data stored as voxels on a glass substrate has an expected lifespan of several hundred years or longer under typical storage conditions. Optical data storage media based on borosilicate glass substrates have been shown to remain readable even after being exposed to temperatures of 380°C for more than 8 hours.

[0048]

[0047] The shape and dimensions of the substrate 210 are not particularly limited. Figure 2 shows a rectangular parallelepiped substrate, but other shapes (e.g., cylindrical) are also intended. The substrate may have a thickness of up to 10 mm, optionally up to 5 mm, and optionally 200 μm to 2 mm.

[0049]

[0048] Multiple phase voxels 220, 222 are arranged on the substrate 210. The first phase voxel 220 has a first intensity, and the second phase voxel 222 has a second intensity different from the first intensity. In this way, the first phase voxel 220 and the second phase voxel 222 encode different data symbols.

[0050]

[0049] The strength of a voxel is determined by its shape and refractive index. Voxels of different strengths may have different shapes and / or different refractive indices.

[0051]

[0050] As an example, if we consider a homogeneous voxel, the light of wavelength λ passing through the voxel undergoes a phase change of δ radians, and δ is

number

[0052]

[0051] The absence of a phase voxel at location 224 may be interpreted as a data symbol. Alternatively, all data symbols may be written to the substrate as voxels.

[0053]

[0052] In the illustrated example, the voxels are arranged as a stack of layers. The stack comprises an uppermost layer 230, an intermediate layer 232, and a lowermost layer 234. Generally, an optical data storage medium may comprise one or more voxel layers.

[0054]

[0053] Assuming that the data remains reconstructible, there is no particular upper limit on the number of layers. Phase voxels cause significantly less light scattering compared to birefringent voxels. Hundreds of voxel layers, for example, 200 to 1000 voxel layers, may exist. Assuming that voxels in different layers are separated from each other, the spacing between voxel layers is not particularly limited. The spacing can be appropriately selected depending on the size of the voxels. For example, adjacent layers may be spaced apart by a distance of 2 to 20 μm from each other.

[0055]

[0054] Voxels may include reference marks, also known as preambles.

[0056]

[0055] The reference mark is a group of voxels arranged in a predetermined pattern. The pattern may be a two-dimensional pattern. The size of the group is not particularly limited, provided that the number of voxels is large enough to allow clear identification of the pattern when reading and decoding the optical data storage medium. For example, a 2D reference mark may be at least 4 voxel width × 4 voxel height. The pattern may be repeated once or more times.

[0057]

[0056] Reading data from an optical data storage medium includes capturing an image of voxels embedded in the optical data storage medium. The position of a reference mark may be determined when the image is captured. The imager's aiming can be adjusted based on the determined position.

[0058]

[0057] When processing the image to decode the data, a reference mark can be identified, and the position of the voxels forming the 2D reference mark can be determined. Based on the determined position, the decoding process can compensate for positional fluctuations that occurred when the voxels were written and / or when the voxels were captured in the image.

[0059]

[0058] For example, location information can be used as input to a processing / decoding method for recovering data from an optical data storage medium. The processing / decoding method may include using a machine learning model.

[0060]

[0059] The reference mark may have a predetermined position relative to the sector of the data encoding voxel. The reference mark may be positioned, for example, along one or more edges of the sector of the voxel. This can make it easier to distinguish different sectors from one another during image capture and / or data decoding. Other arrangements are also contemplated. For example, alternatively or additionally, the 2D reference mark may be positioned in the center of the sector.

[0061]

[0060] A sector is a two-dimensional group of voxels. In many implementations, a given voxel layer in an optical data storage medium comprises two or more sectors and is read using a reader with a field of view large enough to capture an image that includes one entire sector and the edges of up to eight directly adjacent sectors together.

[0062]

[0061] The predetermined pattern may be a Barker sequence or a Frank-Zadoff-Chu sequence. Other patterns may also be used.

[0063]

[0062] The symbols forming the reference mark may be selected to maximize the contrast between the symbols. This can allow for easier detection of the mark.

[0064]

[0063] A single optical data storage medium may have two or more different reference marks. Adjacent sectors may be associated with different reference marks. This can make it easier to distinguish sectors from one another.

[0065]

[0064] For example, two different reference marks may be used, such that there are no sectors that share an edge with another sector having the same reference mark, and the related sectors are arranged in a checkerboard pattern.

[0066]

[0065] Writing reference marks to the optical data storage medium is particularly useful when the optical data storage medium is written to using a high-throughput method, such as the method described with reference to Figure 1. High-throughput methods can be sensitive to component drift. For example, operating the sample stage at high speed may involve a trade-off with positioning accuracy.

[0067]

[0066] A voxel has a position that can be represented by a set of coordinates x, y, and z. The positioning of the voxels may vary in order to encode data. This is illustrated in Figure 3, a schematic plan view of an optical data storage medium 300 having a transparent substrate 310 on which voxels 320 are embedded.

[0068]

[0067] Figure 3 shows a row of three voxels 320a, 320b, and 320c. A virtual scan axis 340 is shown that corresponds to the average position of the voxels 320 in the y direction.

[0069]

[0068] The voxels 320 shown in Figure 3 have equal intensity. One center of voxel 320a is located at a distance of +δy from the scan axis 340. Another center of voxel 320b is located at a distance of -δy from the scan axis 340. The last voxel 320c is located at the center of the scan axis 340.

[0070]

[0069] Each of the voxels 320a, 320b, and 320c can be thought of as encoding a different data symbol based on their position relative to the scan axis 340. In other words, data can be encoded using position modulation.

[0071]

[0070] By combining phase modulation and position modulation, a higher bit encoding scheme and, consequently, a higher data density per unit volume can be enabled.

[0072]

[0071] In this example, the position is modulated perpendicular to the propagation direction of the laser pulse used to write the voxel. Position modulation in the xy plane may be the easiest to implement. In principle, alternatively or additionally, the position in the z direction (i.e., parallel to the propagation direction of the laser pulse) may be modulated.

[0073]

[0072] Figures 2 and 3 illustrate voxels separated laterally from each other. As shown with reference to Figure 4, by arranging voxels to overlap (shingling), the number of voxels per unit volume and, consequently, the data storage capacity per unit volume can be increased.

[0074]

[0073] Figure 4 shows a plot of the magnitude of the phase shift of the three voxels 420, 422, and 426 as a position function along the scan axis x. Each of the three voxels has a different peak intensity.

[0075]

[0074] In this example, the intensity of each voxel as a position function perpendicular to the propagation direction of the laser pulse is heterogeneous but radially symmetric. More specifically, the intensity follows a Gaussian distribution centered on the focal point of the beam used to write the voxel. This is typical of voxels written using a typical laser source, as the intensity of the laser light perpendicular to the beam follows a Gaussian distribution.

[0076]

[0075] When two voxels partially overlap, that is, the pitch p between the centers of adjacent voxels x When the gap is not zero but is smaller than the width of the voxel, these voxels are said to be "shingled."

[0077]

[0076] Despite partial overlap, the overlapping voxels can be decomposed by the reader by detecting changes in the strength of the phase shift as a position function.

[0078]

[0077] Arranging them to overlap can enable a higher voxel density per unit volume by eliminating the margins between voxels. As a result, this can increase data storage capacity.

[0079]

[0078] An exemplary system for writing data to a transparent substrate is described here with reference to Figures 5 and 6. Figure 5 is a block diagram of system 500, and Figure 6 is a block diagram of an exemplary controller 600 for controlling system 500. This system is useful for implementing a method for writing data to a transparent substrate as described herein.

[0080]

[0079] The system 500 includes a laser source 510, a writing path 520A, and a sample stage 530 that holds the optical data storage medium 200 when in use.

[0081]

[0080] The laser source 510 generates pulses of laser light. Typically, the laser pulses have a period t p It is generated at a constant repetition rate. For example, the repetition rate may be on the order of 10 MHz. The laser source 510 may be a femtosecond laser.

[0082]

[0081] As previously stated, the methods provided herein may be parallelized. For this purpose, the exemplary system 500 includes a beam splitter 512. The beam splitter 512 receives a laser pulse from a laser source and splits the laser pulse into two or more beams. Each beam is directed to its respective write path 520A, 520B. Each write path may implement its respective instance of the method shown in Figure 1.

[0083]

[0082] The writing path, such as the writing path 520A, includes an amplitude modulator 522, a beam scanner 524, and an objective lens 526.

[0084]

[0083] The amplitude modulator 522 is sometimes also called a high-speed intensity attenuator or high-speed energy modulator. The amplitude modulator 522 receives an input laser pulse from the laser source 510 and attenuates the laser pulse, thereby producing an attenuated laser pulse with a desired amplitude.

[0085]

[0084] The amplitude modulator 522 may include, for example, an acousto-optic deflector (optionally in combination with a polarizer). Alternatively, the amplitude modulator may include a combination of an electro-optic modulator (e.g., a Pockels cell) and a deflector.

[0086]

[0085] The attenuated laser pulse from the amplitude modulator 522 then arrives at the scanner 524. The scanner 524 may include, for example, a spinning polygon mirror, a MEMS mirror, a galvanometer scanner, an electro-optical scanner, or an acousto-optical scanner. The scanner 524 deflects the attenuated laser pulse, thereby allowing control of the position within the transparent substrate 200 to which the attenuated laser pulse arrives.

[0087]

[0086] For example, the scanner 524 may sweep in alternating directions at a constant speed such that a series of laser pulses form a row of voxels in the x-direction.

[0088]

[0087] The writing path 520A further includes an objective lens 526 located downstream of the scanner. The objective lens focuses the laser pulse received from the scanner to a location within the transparent substrate 200.

[0089]

[0088] The objective lens 526 may include a variable focus lens. By changing the focus of the objective lens 526, the position of the voxels in the z direction can be changed.

[0090]

[0089] The writing path 520A may optionally include one or more additional optical components. For example, the writing path may include a relay optical system positioned between the scanner 524 and the objective lens 526. The relay optical system may comprise a scan lens, one or more spherical lenses, and one or more tube lenses. The relay optical system is sometimes alternatively referred to as a delivery optical system.

[0091]

[0090] The writing path 520A delivers attenuated laser pulses to the optical data storage medium 200, thereby causing voxels to form within the optical data storage medium 200.

[0092]

[0091] An exemplary system 500 includes a sample stage 530 that holds an optical data storage medium. The sample stage is configured to move in parallel with respect to the objective lens 526 of the write path, thereby allowing the laser pulse to be directed to different parts of the optical data storage medium.

[0093]

[0092] For example, the z position of the voxel may be controlled by adjusting the focus of the objective lens 526, the y position of the voxel may be controlled by the scanner 524, and the x position of the voxel may be controlled by moving the sample stage in the x direction.

[0094]

[0093] The system 500 is configured to perform the operation as described with reference to Figure 1. For this purpose, the system includes at least a controller 600 that is operablely linked to an amplitude modulator 522. In many implementations, the controller 600 is also operablely linked to at least one, most typically all, of the laser source 510, scanner 524, and sample stage 530.

[0095]

[0094] The nature of the controller 600 is not particularly limited. The controller 600 may include dedicated hardware circuitry, programmable computer hardware, or any combination thereof.

[0096]

[0095] Here, an example implementation of the controller 600 will be described with reference to the block diagram in Figure 6.

[0097]

[0096] The controller 600 includes a data storage 620 and a processing unit 610 operably linked to an optional user terminal 630. The data storage 620 stores a computer program 622 that is executed by the processing unit 610. When executed by the processing unit 610, the computer program 622 causes the controller 600 to control the system 500 to perform the actions described herein.

[0098]

[0097] The user terminal 630 may include a user input device and a display device.

[0099]

[0098] The user input device may include any one or more suitable input devices known in the art for receiving input from the user. Examples of input devices include pointing devices such as a mouse, stylus, joystick, touchscreen, trackpad, and / or trackball. Other examples of input devices include a keyboard, a microphone when used with a speech recognition algorithm, and / or a video camera when used with a gesture recognition algorithm.

[0100]

[0099] When this specification refers to receiving input from a user via a user input device, this may mean via any one or more user input devices that make up the user input device.

[0101]

[0100] User input devices may be useful to enable a user to specify data to be encoded and written to an optical data storage medium, and / or to enable a user to specify parameters describing the optical data storage medium, such as identifying the properties of the transparent substrate material.

[0102]

[0101] The display device may take any suitable form for outputting an image, such as a light-emitting diode (LED) screen, a liquid crystal display (LCD), a plasma screen, or a cathode ray tube (CRT). The display device may also include a touchscreen and thus may form at least part of a user input device. The touchscreen may allow input by touching with the user's finger and / or using a stylus.

[0103]

[0102] Including a display device is optional. A display device is useful in cases where it is desirable to display human-readable output to the user.

[0104]

[0103] The processing apparatus 610 includes one or more processing units mounted on one or more dies, IC (integrated circuit) packages, and / or housings, at one or more geographical sites.

[0105]

[0104] Each of the one or more processing units may take any suitable form known in the art, such as a general-purpose central processing unit (CPU), or a dedicated form of coprocessor or accelerator processor such as a graphics processing unit (GPU), digital signal processor (DSP). Each of the one or more processing units may include one or more cores.

[0106]

[0105] When it is said that a computer program is executed on a processing unit, this may mean that it is executed by any one or more processing units that make up the processing unit 610.

[0107]

[0106] The processing unit 610 typically further comprises working memory such as random access memory and / or one or more memory caches within one or more processing units.

[0108]

[0107] The data storage 620 includes one or more memory units implemented in one or more memory media within one or more housings at one or more geographic sites.

[0109]

[0108] Each of the one or more memory units may employ any suitable storage medium known in the art, such as a magnetic storage medium like a hard disk drive or a magnetic tape drive; or an electronic storage medium like a solid-state drive (SSD), flash memory, or electrically erasable programmable read-only memory (EEPROM); or an optical storage medium like an optical disk drive or glass or memory crystal-based storage.

[0110]

[0109] In this specification, when a data item is said to be stored in or in the data storage 610, this may mean that it is stored in any part of any one or more memory devices that make up the data storage 620.

[0111]

[0110] The processing unit 610 and the data storage 620 are operationally linked. The processing unit and the data storage are configured such that the processing unit 610 can read data from at least a portion of the data storage 620 and write data to at least a portion of the data storage 620. The processing unit 610 can communicate with the data storage 620 by a local connection (e.g., a physical data bus) and / or via a network such as a local area network or the Internet. In the latter case, the network connection may be wired or wireless.

[0112]

[0111] Various modifications may be made to the example system.

[0113]

[0112] In the illustrated example, light propagates between components through free space. Modified forms of the system may include waveguides, optical fiber cables, or similar for propagating light.

[0114]

[0113] The exemplary system includes a beam splitter separate from the laser source. In a modified form, the laser source may be a multi-channel laser, and the beam splitter may be omitted.

[0115]

[0114] In yet another variant, the beam may be split downstream of the amplitude modulator. A polarization modulator, such as a polarizer and a Pockels cell, may be located downstream of the amplitude modulator 522. A polarization-sensitive beam splitter, such as a polarizing grating or a polarizing lens, may be located downstream of the polarization modulator. The polarization modulator and the polarization-sensitive beam splitter can split the laser pulse into two pulses having different amplitudes. The two pulses may pass through the same component downstream of the polarization-sensitive beam splitter. The two pulses may be focused at different positions in the medium 200 to simultaneously form two phase voxels of different amplitudes.

[0116]

[0115] Parallelization of this method is optional. The system may include a single write path 520A, and the beam splitter 512 may be omitted.

[0117]

[0116] An exemplary system modulates the power of the laser pulse by attenuation. Alternatively, the pulse may be amplified.

[0118]

[0117] The scanner 524 and the objective lens 526 may be shared by two or more writing paths.

[0119]

[0118] The scanner 524 can be omitted in principle. Control of the position where the laser pulse arrives may be achieved by moving the optical data storage medium 200 using the sample stage 530.

[0120]

[0119] In this example, the sample stage moves the transparent substrate in a parallel direction. Alternatively or additionally, the sample stage may rotate the transparent substrate.

[0121]

[0120] Similarly, the objective lens 526 may have a fixed focus, in which case the z position of the voxel may be controlled by moving the sample stage.

[0122]

[0121] In this example, the sample stage is described as movable. More generally, the system may have any configuration that allows relative movement of the write path and the optical data storage medium. The sample stage may be fixed by other components of the system attached to an actuator.

[0123]

[0122] The method for reading an optical data storage medium is illustrated in the flowchart of Figure 7.

[0124]

[0123] In block 701, a refractive index-sensitive microscope is used to capture images of voxels in an optical data storage medium.

[0125]

[0124] The refractive index-sensitive microscope may be a phase-contrast microscope or a differential interference microscope. When using phase-contrast microscopy or differential interference microscope techniques, it is observed that voxels with different refractive indices have different brightness levels.

[0126]

[0125] Image acquisition may include acquiring wide-field images. Wide-field imaging images multiple voxels simultaneously. Alternatively, images may be acquired using a point scanner to probe voxels one at a time.

[0127]

[0126] A single image of a given voxel layer may be captured.

[0128]

[0127] In many implementations, a voxel may contain both a region with a high refractive index and a region with a low refractive index. These regions are sometimes called "sublayers".

[0129]

[0128] Image acquisition may include capturing an image of each sublayer. The sublayer to be imaged may be selected by adjusting the focal position of the refractive index-sensitive microscope.

[0130]

[0129] By capturing images from both sublayers and then subtracting one image from the other, background noise can be reduced and / or contrast can be improved. This can enable more reliable data recovery.

[0131]

[0130] More generally, it has been found that the contrast of an image can be changed by adjusting the focusing parameters of the microscope (e.g., the focal position and / or the spherical aberration correction of the microscope's objective lens). By capturing two or more images of a given voxel using different focusing parameters and optionally determining the difference between the images, data recovery can be made more reliable. This technique is particularly useful for heterogeneous voxels, but it can also be applied to homogeneous voxels.

[0132]

[0131] After capturing the image, in block 702 the image is processed in order to restore the saved data. Image processing is a computer implementation process.

[0133]

[0132] Voxels with different intensities encode different data symbols and have different brightness levels in the image. The data symbols are reconstructed based on the brightness of the pixels in the image.

[0134]

[0133] Data recovery can be implemented in various different ways. For example, one or more images may be processed using a machine learning model. The machine learning model may include an artificial neural network (e.g., a convolutional neural network). The machine learning model may be trained using training data that includes images of voxels labeled with symbols encoded by voxels.

[0135] Example 1

[0134] Voxels were written onto a borosilicate glass substrate using the method described with reference to Figure 1. Four laser pulses with different amplitudes were used to represent four symbols in a 2-bit encoding scheme. One laser pulse was used per voxel.

[0136]

[0135] The voxel was a heterogeneous voxel. The heterogeneous voxel comprises a sublayer with a high refractive index and a sublayer with a low refractive index, positioned on either side of the focal point of the laser pulse used to form the voxel in the propagation direction of the laser pulse.

[0137]

[0136] Two phase-contrast microscope images of the voxel layer were captured. These images are shown in Figures 8A and 8B.

[0138]

[0137] Figure 8A is a "positive" image captured at the first focus position corresponding to the first sublayer of the voxel. Figure 8B is a "negative" image of the same voxel captured at the second focus position corresponding to the second sublayer of the voxel.

[0139]

[0138] A difference image was generated by subtracting a "negative" image from a "positive" image. The difference image is shown in Figure 8C. As can be seen in the figure, the difference image has a wider dynamic range. By subtracting one image from the other, artifacts and background effects are also removed without requiring any further post-processing steps. The distance between the two sublayers of voxels is on the order of a few micrometers, and the background does not change significantly at this distance scale.

[0140] Example 2

[0139] Voxels were written onto the fused silica glass substrate and the borosilicate glass substrate using the method described with reference to Figure 1.

[0141]

[0140] The voxels were imaged using phase-contrast microscopy. The histograms of the images are shown in Figures 9A and 9B, respectively.

[0142]

[0141] The histogram shows that the brightness measured at the location of the voxel in fused silica and borosilicate correlates with the intended value of the symbol written on the substrate. This demonstrates that the symbols are sufficiently distinguishable from each other and that the method allows for the storage of multiple bits of data per voxel.

[0143]

[0142] Figure 9B demonstrates that symbols can be successfully written onto a borosilicate glass substrate and that symbols can be successfully restored from the borosilicate glass substrate. Borosilicate glass contains approximately 80% SiO2, which is significantly lower in purity than fused silica (>99% SiO2). Unlike conventional methods using birefringent voxels, this method does not necessarily require a high-purity substrate.

[0144]

[0143] It will be understood that the above embodiments are described merely as examples.

[0145]

[0144] More generally, a method for writing data to a transparent substrate is provided according to one embodiment disclosed herein. This method includes forming a first voxel having a first intensity by focusing a first laser pulse to a first location in the transparent substrate, and forming a second voxel having a second intensity different from the first intensity by focusing a second laser pulse to a second location in the transparent substrate. The second laser pulse has a second amplitude different from the first amplitude. By encoding the data by amplitude modulation, it is possible to write voxels using a single laser pulse, thereby increasing data throughput. Furthermore, voxels can be reliably formed on inexpensive substrates such as borosilicate glass.

[0146]

[0145] Voxels are sometimes called "phase voxels".

[0147]

[0146] Light passing through a phase voxel undergoes a phase change. Voxels of different intensities produce phase changes of different magnitudes. Voxels of different intensities may have different refractive indices and / or different sizes.

[0148]

[0147] The first location may be spaced apart from the second location by a distance selected such that the first voxel and the second voxel partially overlap. In other words, the first voxel and the second voxel may be placed so that they overlap. By placing them so that they overlap, it is possible to increase the number of voxels per unit area, thereby increasing the data density.

[0149]

[0148] The method may further include forming a third voxel by focusing a third laser pulse at a third location in a transparent substrate, wherein the third voxel has the same intensity as the first voxel. The first and third voxels may have different shapes. Alternatively or additionally, the first voxel may be spaced a first lateral distance from the scan axis, and the third voxel may be spaced a third lateral distance from the scan axis, wherein the first and third lateral distances are different. By modulating the shape and / or position of voxels of equal intensity, it is possible to increase the number of bits encoded per voxel.

[0150]

[0149] Each voxel can be formed by a single laser pulse. Single-pulse writing can enable increased data throughput. Single-pulse writing can enable more efficient use of the laser light source.

[0151]

[0150] The transparent substrate may include glass. The glass may be borosilicate glass or soda-lime glass. The method provided herein has been found to be successfully implemented using glass substrates of relatively low purity. High-purity glass such as fused silica may be used, but is not required. The use of other substrate materials (e.g., organic polymers) is also intended.

[0152]

[0151] The method may include forming at least two layers of voxels, or optionally, at least 100 layers of voxels, within a transparent substrate. Phase voxels cause very little light scattering. Therefore, many voxel layers can be written to a given substrate, thereby increasing the data density per unit volume.

[0153]

[0152] The method may be parallelized. Two or more instances of the method may be performed simultaneously. A parallelized implementation may include simultaneously forming at least two first voxels at their respective first locations. Parallelization can increase data throughput. Parallelization can create a strong correlation between voxels written simultaneously, which can aid in data recovery.

[0154]

[0153] The method may include forming a plurality of voxels positioned as reference marks. The reference marks may be two-dimensional reference marks. The reference marks can enable easier data recovery from an optical data storage medium written using the method. The aiming of the imaging system may be adjusted based on the detected reference marks. Error correction may be applied based on the reference marks during the decoding process.

[0155]

[0154] Related embodiments provide an optical data storage medium obtained by the present method. The optical data storage medium comprises a transparent substrate containing a material having a bulk refractive index, a first voxel embedded in the transparent substrate having a first intensity, and a second voxel embedded in the transparent substrate having a second intensity different from the first intensity, thereby resulting in a first voxel and a second voxel encoding different data symbols. The voxel is a region of the transparent substrate having a refractive index different from the bulk refractive index.

[0156]

[0155] The first and second voxels may be heterogeneous voxels. A heterogeneous voxel includes a positive region where the refractive index is greater than the bulk refractive index and a negative region where the refractive index is less than the bulk refractive index. The positive and negative regions are arranged in series in the axial direction corresponding to the propagation direction of the laser light used to form the voxel. When reading the voxels, the dynamic range of the reader can be expanded and / or background effects can be reduced by capturing an image of the positive region, capturing an image of the negative region, and subtracting one image from the other.

[0157]

[0156] The first voxel and the second voxel may be arranged to overlap. That is, the first voxel and the second voxel may partially overlap. This can increase the voxel density, which in turn increases the data capacity.

[0158]

[0157] As can be understood, the optical data storage medium may include features resulting from any of the various optional method steps described in relation to the aspects of the method. For example, the optical data storage medium may comprise shape-modulated voxels and / or position-modulated voxels.

[0159]

[0158] Another embodiment provides a system for writing phase voxels onto a transparent substrate. This system is useful for implementing a method such as that described herein. This system comprises a controller, a pulsed laser source, and a first amplitude modulator located downstream of the pulsed laser source in the optical path. The amplitude modulator is operably linked to the controller. The controller is configured to cause the system to perform a method such as that provided herein.

[0160]

[0159] The system may further include means for changing the focus position of the intensity-modulated laser pulse. These means may include a beam scanner, a movable sample stage, a variable focus optical system, or any combination thereof.

[0161]

[0160] The system may include a writing path comprising an amplitude modulator, a scanner for changing the propagation direction of an intensity-modulated laser pulse, and an objective lens for receiving the intensity-modulated laser pulse from the scanner and focusing the intensity-modulated laser pulse to a location within a transparent substrate.

[0162]

[0161] The controller may comprise one or more hardware circuits (e.g., application-specific integrated circuits) configured to implement one or more operations of the method. The controller may comprise one or more processors operably linked to a computer-readable medium, which, when executed by the one or more processors, stores instructions that cause the one or more processors to control the system to implement one or more operations of the method. The controller may comprise a combination of one or more hardware circuits, one or more processors, and a computer-readable medium.

[0163]

[0162] The system may include at least two write paths. In such an implementation, the system may further include a beam splitter for receiving laser pulses from a laser source and outputting the respective divided laser pulses to each of the write paths.

[0164]

[0163] The sample stage may be operablely linked to the controller and configured to move in parallel with respect to at least one write path. This allows for further control over the positioning of voxels within the transparent substrate.

[0165]

[0164] Another embodiment provides a method for reading data from an optical data storage medium, the optical data storage medium being as defined herein. This method involves capturing an image of voxels using a refractive index-sensitive microscope, Processing an image using a processor in order to restore data, including processing parts of the image having different signal strengths to encode different data symbols.

[0166]

[0165] The refractive index-sensitive microscope may be a phase-contrast microscope or a differential interference contrast microscope.

[0167]

[0166] Capturing images of voxels may include capturing a first image using a first focusing parameter (e.g., focusing position) and capturing a second image using a second focusing parameter (e.g., focusing position) different from the first focusing parameter. Capturing multiple images can enable more reliable data recovery. For example, the contrast between voxels may differ between images.

[0168]

[0167] The voxels may be heterogeneous voxels. A heterogeneous voxel comprises a positive sublayer having a refractive index greater than the bulk refractive index of the substrate and a negative sublayer having a refractive index smaller than the bulk refractive index. In a mounting configuration in which the voxels are heterogeneous, one side of the image may be an image of the positive sublayer, and the other side of the image may be an image of the negative sublayer.

[0169]

[0168] If two images of voxels are captured, image processing may further include subtracting the first image from the second image. This can enable more reliable data recovery. Image contrast can be improved. Background effects can be reduced.

[0170]

[0169] Image processing may include processing images using machine learning models. Machine learning models include convolutional artificial neural networks. Machine learning models may be trained using images of voxels, which have labels representing data symbols encoded by voxels.

[0171]

[0170] In an implementation in which an optical data storage medium includes voxels positioned as reference marks, processing an image may include determining the position of the voxels within the reference marks.

[0172]

[0171] In such an implementation, the method may further include compensating for variations in the position of voxels in the image based on the position of voxels within a reference mark. Random variations in the position of voxels may occur during the writing process, and / or the captured image may be subjected to distortion. The reference mark includes voxels arranged in a predetermined pattern. Therefore, the decoding process can compensate for the above variations and distortions based on the detected position of voxels within the reference mark.

[0173]

[0172] Alternatively or additionally, the method may further include identifying voxels as components of a sector based on their position within a reference mark.

[0174]

[0173] The Specification also provides the use of laser pulse energy modulation to encode data as voxels within a transparent substrate. A laser pulse forms a volumetric change in refractive index (i.e., a voxel) within the transparent substrate. Modulating the laser pulse energy can change the magnitude of the change in volume and / or refractive index of the substrate material of the voxel.

[0175]

[0174] Laser pulse energy modulation can be used in combination with other types of modulation (e.g., voxel shape modulation and / or modulation of voxel position relative to the scan axis) to enable higher bit encoding schemes.

[0176]

[0175] This specification also provides a method for writing data to a circuit board. This method is A first laser pulse having a first energy is focused onto a first region in the substrate, thereby causing a first volumetric change in refractive index. The method of focusing a second laser pulse having a second energy onto a second region within a substrate, wherein the second energy is different from the first energy, thereby resulting in a second volumetric refractive index change having different optical properties from the first volumetric refractive index change. Includes.

[0177]

[0176] This method may include any of the various optional features described in relation to the previously mentioned method. A "voxel" is sometimes referred to as a "volumetric refractive index change."

[0178]

[0177] Related embodiments provide a transparent optical data storage medium comprising a transparent substrate, a first volumetric refractive index change at a first position in the transparent substrate, wherein the first volumetric refractive index change has a first volume and a first refractive index profile, and a second volumetric refractive index change at a second position in the transparent substrate, wherein the second volumetric refractive index change has a second volume and a second refractive index profile, wherein the first refractive index profile is different from the second refractive index profile and / or the first volume is different from the second volume.

[0179]

[0178] This disclosure provides the following terms: Clause 1. Forming a first voxel having a first intensity by focusing a first laser pulse at a first location within a transparent substrate, wherein the first laser pulse has a first amplitude. The method involves focusing a second laser pulse at a second location within a transparent substrate to form a second voxel having a second intensity different from that of the first voxel, wherein the second laser pulse has a second amplitude different from that of the first voxel. Methods that include... Clause 2. The method according to Clause 1, wherein the first location is spaced apart from the second location by a distance selected such that the first and second voxels partially overlap. Clause 3. The method according to Clause 1 or 2, further comprising forming a third voxel by focusing a third laser pulse at a third location in a transparent substrate, wherein the third voxel has an intensity equal to that of a first voxel. Clause 4. The method according to Clause 3, wherein the first voxel and the third voxel have different shapes. Clause 5. The method according to Clause 3 or 4, wherein the first voxel is spaced a first lateral distance from the scan axis, the third voxel is spaced a third lateral distance from the scan axis, and the first and third lateral distances are different. Clause 6. The method described in any one of the preceding clauses, wherein each voxel is formed by a single laser pulse. Clause 7. The method described in any one of the preceding clauses, wherein the transparent substrate includes glass. Clause 8. The method according to Clause 7, wherein the glass is borosilicate glass or soda-lime glass. Clause 9. The method of any one of the preceding clauses, comprising forming at least two layers of voxels within a transparent substrate. Clause 10. The method according to Clause 9, comprising forming at least 100 layers of voxels within a transparent substrate. Clause 11. The method of any one of the preceding clauses, comprising simultaneously forming at least two first voxels in their respective first locations. Clause 12. The method of any one of the preceding clauses, including forming a plurality of voxels positioned as reference marks. Clause 13. A transparent substrate containing a material having a bulk refractive index, A first voxel embedded in a transparent substrate, wherein the first voxel has a first strength, A second voxel embedded in a transparent substrate, wherein the second voxel has a second intensity different from the first intensity, thereby resulting in a first voxel and a second voxel that encode different data symbols, An optical data storage medium equipped with [a specific feature]. Article 14. The first voxel and the second voxel are heterogeneous voxels, and each of them is A positive sublayer having a refractive index greater than that of the bulk refractive index, A negative sublayer having a refractive index smaller than that of the bulk refractive index, An optical data storage medium as described in Clause 13, comprising the features described above. Clause 15. An optical data storage medium according to Clause 13 or 14, wherein the first voxel and the second voxel are arranged to overlap. Clause 16. An optical data storage medium as described in any one of Clauses 13 to 15, obtained by the method described in any one of Clauses 1 to 12. Clause 17. Controller and, A pulsed laser source, A first amplitude modulator positioned downstream of a pulsed laser source on the optical path, A system equipped with, The amplitude modulator is linked to the controller in an operable manner, and when in use, The method involves focusing a first laser pulse at a first location within a transparent substrate to form a first voxel having a first intensity, wherein the first laser pulse has a first amplitude. The method involves focusing a second laser pulse at a second location within a transparent substrate to form a second voxel having a second intensity different from that of the first voxel, wherein the second laser pulse has a second amplitude different from that of the first voxel. A system that causes the system to perform a method that includes this. Clause 18. The system described in Clause 17, further comprising a scanner positioned downstream of the amplitude modulator in the optical path. Clause 19. A beam splitter positioned between the pulsed laser source and the first amplitude modulator, A second amplitude modulator is arranged in parallel with the first amplitude modulator, A second amplitude modulator is operably linked to the controller, and The system according to Clause 17 or 18, wherein, during use, the controller controls the system to perform a method that further includes forming a third voxel by focusing a third laser pulse to a third location in a transparent substrate at the same time as forming a first voxel. Clause 20. The system described in any one of Clauses 17 to 19, further comprising a movable sample stage for holding a transparent substrate. Article 21. A method for reading data from an optical data storage medium, wherein the optical data storage medium is Capturing images of voxels using a refractive index-sensitive microscope, To restore data, an image is processed using a processor, where parts of the image with different signal strengths encode different data symbols. A method that includes any of the methods defined in any one of the clauses 13 to 16. Clause 22. The method according to Clause 21, wherein the refractive index-sensitive microscope is a phase-contrast microscope or a differential interference contrast microscope. Clause 23. The method according to Clause 21 or 22, wherein capturing an image of voxels includes capturing a first image using a first focusing parameter and capturing a second image using a second focusing parameter different from the first focusing parameter. Clause 24. The method according to Clause 23, wherein the voxel is a heterogeneous voxel comprising a positive sublayer having a refractive index greater than that of the bulk refractive index of a transparent substrate and a negative sublayer having a refractive index less than that of the bulk refractive index, and the image capture includes capturing an image of the positive sublayer and capturing an image of the negative sublayer. Clause 25. The method of Clause 23 or 24, wherein the processing of the image further includes subtracting the first image from the second image. Clause 26. The method described in any one of Clauses 21-25, including processing images using a machine learning model. Clause 27. The method described in Clause 26, wherein the machine learning model includes a convolutional artificial neural network. Clause 28. The method according to any one of Clauses 21 to 27, wherein the optical data storage medium includes voxels positioned as reference marks, and processing the image includes determining the position of the voxels positioned as reference marks. Clause 29. The method of Clause 28, further comprising compensating for variations in the position of voxels in an image based on the position of voxels within a reference mark. Clause 30. Use of laser pulse energy modulation to encode data as voxels within a transparent substrate. Clause 31. Uses as described in Clause 30, further including the use of voxel shape modulation to encode data. Clause 32. Uses described in Clause 30 or 31, further including the use of modulation of voxel position relative to the scan axis to encode data. Clause 33. Forming a first voxel by focusing a first laser pulse on a first location within a transparent substrate, By focusing a second laser pulse on a second location within the transparent substrate, a second voxel is formed. A method including, A method wherein a first laser pulse and a second laser pulse have different beam shapes, thereby producing a first voxel and a second voxel having different shapes. Clause 34. The method according to Clause 33, wherein the first location is spaced from the second location by a distance selected such that the first voxel and the second voxel partially overlap. Clause 35. The method according to Clause 33 or 34, further comprising forming a third voxel by focusing a third laser pulse on a third location within a transparent substrate, wherein the third voxel has the same shape as the first voxel. Clause 36. The method according to Clause 35, wherein the first voxel and the third voxel have different strengths. Clause 37. The method according to Clause 35 or 36, wherein the first voxel is spaced a first lateral distance from the scan axis, and the third voxel is spaced a third lateral distance from the scan axis, and the first and third lateral distances are different. Clause 38. The method according to any one of Clauses 33 to 37, wherein each voxel is formed by a single laser pulse. Clause 39. The method described in any one of Clauses 33 to 38, wherein the transparent substrate includes glass. Clause 40. The method according to Clause 39, wherein the glass is borosilicate glass or soda-lime glass. Clause 41. The method of any one of the preceding clauses, comprising forming at least two layers of voxels within a transparent substrate. Clause 42. The method according to Clause 41, comprising forming at least 100 layers of voxels within a transparent substrate. Clause 43. The method according to any one of Clauses 33 to 42, comprising simultaneously forming at least two first voxels in their respective first locations. Clause 44. The method described in any one of Clauses 33 to 43, including forming a plurality of voxels positioned as reference marks. Clause 45. A transparent substrate containing a material having a bulk refractive index, A first voxel embedded in a transparent substrate, A second voxel embedded in a transparent substrate, An optical data storage medium comprising, A voxel is a region of a transparent substrate having a refractive index different from the bulk refractive index, and An optical data storage medium in which the shape of the first voxel differs from the shape of the second voxel, thereby resulting in the first and second voxels encoding different data symbols. Article 46. The first voxel and the second voxel are heterogeneous voxels, and each of them is A positive sublayer having a refractive index greater than that of the bulk refractive index, A negative sublayer having a refractive index smaller than that of the bulk refractive index, An optical data storage medium as described in Clause 45, comprising the features described above. Clause 47. An optical data storage medium according to Clause 45 or 46, wherein the first voxel and the second voxel are arranged to overlap. Clause 48. An optical data storage medium according to any one of Clauses 45 to 47, obtained by the method described in any one of Clauses 33 to 44. Other variations or uses of the disclosed techniques may become apparent to those skilled in the art if the disclosure herein is given. The scope of this disclosure is not limited by the embodiments described herein, but is limited only by the appended claims.

Claims

1. (101) A first voxel (220) having a first intensity is formed by focusing a first laser pulse at a first location within a transparent substrate (200), wherein the first laser pulse has a first amplitude (101), (102) A second voxel (222) having a second intensity different from the first intensity is formed by focusing a second laser pulse at a second location within the transparent substrate (200), wherein the second laser pulse has a second amplitude different from the first amplitude (102), Methods that include...

2. The method according to claim 1, wherein the first location is spaced from the second location by a distance selected such that the first voxel and the second voxel partially overlap.

3. The method according to claim 1 or 2, further comprising focusing a third laser pulse on a third location within the transparent substrate to form a third voxel (320c), wherein the third voxel (320c) has an intensity equal to that of the first voxel (320a).

4. The method according to claim 3, wherein the first voxel (320a) and the third voxel (320c) have different shapes.

5. The method according to claim 3 or 4, wherein the first voxel (320a) is spaced from the scan axis (340) by a first lateral distance, and the third voxel (320c) is spaced from the scan axis (340) by a third lateral distance, and the first lateral distance and the third lateral distance are different.

6. The method according to any one of claims 1 to 5, wherein each voxel (220, 222) is formed by a single laser pulse.

7. The method according to any one of claims 1 to 6, wherein the transparent substrate (200) includes glass.

8. The method according to claim 7, wherein the glass is borosilicate glass or soda-lime glass.

9. The method according to any one of claims 1 to 8, comprising forming at least two layers (230, 232) of voxels (220, 222) within the transparent substrate (200).

10. The method according to claim 9, comprising forming at least 100 layers of voxels (220, 222) within the transparent substrate (200).

11. The method according to any one of claims 1 to 10, comprising simultaneously forming at least two first voxels (220) at their respective first locations.

12. The method according to any one of claims 1 to 11, comprising forming a plurality of voxels positioned as reference marks.

13. A transparent substrate (200) containing a material having a bulk refractive index, A first voxel (220) embedded in the transparent substrate (200), wherein the first voxel (220) has a first strength, A second voxel (222) embedded in the transparent substrate (200), wherein the second voxel (222) has a second intensity different from the first intensity, thereby resulting in the first and second voxels (220, 222) encoding different data symbols, An optical data storage medium (200) comprising the above.

14. The first and second voxels (220, 222) are heterogeneous voxels, and each of them is A positive sublayer having a refractive index greater than that of the bulk refractive index, A negative sublayer having a refractive index smaller than the bulk refractive index, An optical data storage medium according to claim 13, comprising the above.

15. The optical data storage medium according to claim 13 or 14, wherein the first and second voxels (220, 222) are arranged to overlap.

16. The optical data storage medium according to any one of claims 13 to 15, wherein the first and second voxels (220, 222) have different shapes.

17. Controller (600) and A pulsed laser source (510), A first amplitude modulator (522) is located downstream of the pulsed laser source (510) on the optical path, A system (500) comprising, The controller (600) is operably linked to the amplitude modulator (522), and when in use, (101) A first voxel (220) having a first intensity is formed by focusing a first laser pulse at a first location within a transparent substrate (200), wherein the first laser pulse has a first amplitude (101), (102) A second voxel (222) having a second intensity different from the first intensity is formed by focusing a second laser pulse at a second location within the transparent substrate (200), wherein the second laser pulse has a second amplitude different from the first amplitude (102), A system (500) that causes the system (500) to perform a method including the above.

18. The system according to claim 17, further comprising a scanner (524) located downstream of the amplitude modulator (522) on the optical path.

19. A beam splitter (512) is positioned between the pulse laser source (510) and the first amplitude modulator (522), A second amplitude modulator is arranged in parallel with the first amplitude modulator, Furthermore, The second amplitude modulator is operably linked to the controller (600), and The system according to claim 17 or 18, wherein, during use, the controller (600) controls the system (500) to perform a method that further includes forming a third voxel by focusing a third laser pulse to a third location in the transparent substrate (200) at the same time as forming the first voxel.

20. The system according to any one of claims 17 to 19, further comprising a movable sample stage (530) for holding the transparent substrate (200).

Images