Phase modulated optical data storage

Abstract

A method of writing data to a transparent substrate includes forming a first voxel by focusing a first laser pulse on a first location in the transparent substrate; and forming a second voxel by focusing a second laser pulse on a second location in the transparent substrate. The first laser pulse and the second laser pulse have different amplitudes, resulting in the first voxel and the second voxel having different intensities. The invention further provides a system for implementing the method. An optical data storage medium obtainable by the method; and a method of reading data from an optical data storage medium.

Description

Background Technology

[0001] There is a significant demand for data storage. It is estimated that in the near future, cloud storage providers will require zettabyte-level data storage capacity, which is one trillion gigabytes (10^6 kilobytes). 21 (Bytes). Most of the data will need to be stored for extended periods of time.

[0002] Examples of widely used data storage technologies include hard drives, magnetic tape, flash memory, and optical discs. All of these technologies have the drawback of requiring data to be periodically copied to replacement media. This is expensive in terms of energy consumption and hardware requirements.

[0003] Magnetic storage media such as hard drives and magnetic tapes gradually demagnetize. Flash memory is susceptible to read interference, where repeated readings from a particular flash cell can cause surrounding flash cells to fail. Reflective materials used for data storage in optical media such as DVDs degrade over time.

[0004] Birefringent optical data storage media have been proposed as a solution to these drawbacks. 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] Voxels possess optical properties that differ from those of the surrounding bulk substrate. Depending on the polarization and / or direction of the incident light, birefringent voxels exhibit different refractive indices. Birefringence can be controlled during voxel writing onto the substrate and is used to encode data.

[0006] The substrate is transparent in the sense that it is transparent to light at (multiple) wavelengths used for reading and writing voxels.

[0007] Birefringent optical data storage media and their fabrication have been described in, for example, the following literature: Anderson et al. Glass: A New Media for a New Era? 10th USENIX HotStorage 18 Symposium on Hot Topics in Storage and File Systems, 2018, and US10,236,027B1. Summary of the Invention

[0008] In one aspect, a method for writing data to a transparent substrate is provided. The method includes: forming a first voxel having a first intensity by focusing a first laser pulse at a first location in the transparent substrate, the first laser pulse having a first amplitude; and forming a second voxel having a second intensity different from the first intensity by focusing a second laser pulse at a second location in the transparent substrate, the second laser pulse having a second amplitude different from the first amplitude. Encoding data by amplitude modulation allows voxels to be written using a single laser pulse, thereby increasing data throughput. Alternatively or additionally, the method may allow voxels to be reliably formed in inexpensive substrates, such as borosilicate glass.

[0009] In another aspect, an optical data storage medium is provided. This optical data storage medium includes: a transparent substrate comprising a material having a bulk refractive index; a first voxel embedded in the transparent substrate, the first voxel having a first intensity; and a second voxel embedded in the transparent substrate, the second voxel having a second intensity different from the first intensity, resulting in the first voxel and the second voxel encoding different data symbols. The transparent substrate can be obtained by the method described herein.

[0010] Another approach provides a system for writing phase voxels onto a transparent substrate. This system is used to implement the method described herein. The system includes: a controller; a pulsed laser source; and a first amplitude modulator disposed downstream of the pulsed laser source in an optical path. The amplitude modulator is operatively linked to the controller. The controller is configured to cause the system to perform the method provided herein.

[0011] On the other hand, a method for reading data from an optical data storage medium as defined herein is provided. This method includes: acquiring voxel images using a refractive index-sensitive microscope; and processing the images using a processor to recover the data. Parts of the image with different signal intensities are encoded with different data symbols.

[0012] This paper also provides a method for using amplitude modulation to encode multiple different data symbols into voxels in a transparent substrate.

[0013] The present invention is provided to introduce, in a simplified form, the selection of concepts further described below in the detailed description. The present invention is not intended to identify key or essential features of the subject matter of the claims, nor is it intended to limit the scope of the claims. The subject matter of the claims is also not limited to the implementation of any or all of the disadvantages pointed out herein. Attached Figure Description

[0014] To aid in understanding the embodiments of this disclosure and to illustrate how to implement these embodiments, reference is made to the accompanying drawings by way of example only, wherein: Figure 1This is a flowchart outlining an example method for writing data to a transparent substrate; Figure 2 Through Figure 1 A schematic cross-section of an optical data storage medium obtainable by the method; Figure 3 This is a schematic plan view of an optical data storage medium, showing the modulation of voxel positions; Figure 4 It is a graph showing the contribution of the phase shift to the position of the three stacked voxels; Figure 5 This is a block diagram of a system for writing data to a transparent substrate; Figure 6 It is aimed at Figure 5 A block diagram of the system's controller; Figure 7 It is an overview through Figure 1 A flowchart of a method for reading data from a transparent substrate; Figure 8A It is a phase contrast micrograph of the "positive" sublayer of a set of voxels; Figure 8B It is a phase contrast micrograph of the "negative" sublayer of a set of voxels; Figure 8C From Figure 8A and Figure 8B The difference image generated from the micrograph; Figure 9A It is a histogram of an image of a voxel written onto a fused silica glass substrate; Figure 9B It is a histogram of the voxel image written onto the borosilicate glass substrate. Detailed Implementation

[0015] For ease of description, this document uses directional terms such as “top,” “bottom,” “left,” “right,” “above,” “below,” “horizontal,” and “vertical,” and refers to the orientations shown in the relevant figures. To avoid any ambiguity, these terms are not intended to limit orientation in an external frame of reference.

[0016] Ordinal numbers (“first”, “second”, “third”, etc.) are used in this document as markers to distinguish different elements. For example, a description of forming a “first” voxel covers the formation of one or more identical voxels.

[0017] As used herein, the “intensity” of a voxel is the magnitude of the phase change that occurs when light of (multiple) predetermined wavelengths passes through the voxel. The (multiple) predetermined wavelengths are the wavelengths of light used to read the voxel. The magnitude of the phase change is a function of the voxel’s refractive index and the path length of the light through the voxel, the latter being determined by the voxel’s thickness.

[0018] Two voxels are considered to have “different” intensities when they cause phase changes that are sufficiently different to encode different data symbols.

[0019] The difference is higher than the noise floor of the writing system, ensuring that random fluctuations (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 phase-sensitive microscopy. In the context of this application, minor fluctuations in intensity due to random noise are not considered "differences".

[0020] As will become apparent from the following discussion, the magnitude of the intensity difference can be controlled by varying the amplitude of the laser pulse used to form the voxel. The difference can be appropriately selected based on the characteristics of the writing and reading systems. For instance, the refractive index difference is at least 10. -4 A pair of voxels of equal size can be considered to have different strengths. Future developments in writing and reading systems could allow for a reduction in this limitation.

[0021] Birefringent optical data storage allows for the long-term archiving of large amounts of data. However, birefringent voxels have some limitations. Forming a birefringent voxel requires at least two laser pulses, which limits the data throughput (in other words, the rate at which data can be written). High-purity glass substrates, such as fused silica, are typically required to allow for reliable formation of birefringent voxels.

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

[0023] This paper presents a method for writing data onto a transparent substrate, which allows for the writing of data symbols using a single laser pulse and / or allows for the use of a wider range of substrate materials. According to this method, data symbols are encoded by amplitude modulation, opposite to the orientation of the birefringent voxels.

[0024] The relevant parties have provided an optical data storage medium obtainable by the method; a system for implementing the method; and a method for reading data from the optical data storage medium.

[0025] Now refer to Figure 1 Explain an example method for writing data to a transparent substrate. Figure 1 This is a flowchart outlining the method.

[0026] At frame 101, a first voxel is formed by focusing a first laser pulse onto a first position in a transparent substrate.

[0027] Focusing a laser at a first location causes a permanent alteration to the structure of the substrate material at that location. The interaction between the laser and the transparent material can lead to a variety of structural changes. Examples include the formation of dense regions, the formation of sparse regions, localized changes in the glass matrix, crystallization, amorphization, or the formation of nanoscale voids. The nature of the change can vary depending on the laser parameters (e.g., energy, wavelength, pulse duration) and the properties of the transparent substrate material.

[0028] The change in material structure modifies the refractive index of the material at a first location relative to the unmodified bulk material. This modified region at the first location is called a voxel. A voxel can be more specifically called a "phase voxel" because electromagnetic waves (i.e., light) passing through it experience a phase shift. The amount of phase shift caused by the phase voxel can be referred to as its intensity.

[0029] In many materials, laser light induces modifications to the material's structure that are non-uniform along the direction of laser propagation. These non-uniform modifications include regions of increased refractive index and regions of decreased refractive index. These regions may be referred to as "sublayers."

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

[0031] The first laser pulse can be a single laser pulse. Using a single laser pulse to form each voxel allows voxel writing at a rate close to the repetition rate of the laser source used for writing the voxel. In principle, two or more laser pulses can be used. For example, using a GHz burst laser to write voxels can allow for improved voxel quality.

[0032] After the first voxel is formed, the second voxel is formed at frame 102.

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

[0034] The second position differs from the first position. The point on which the laser is focused can be changed by moving the transparent substrate relative to the laser and / or by adjusting the focusing position of the laser.

[0035] The extent to which the structure of the transparent substrate can be modified depends on the intensity of the laser pulse. Intensity can also be referred to as flux, energy, amplitude, or peak power. Therefore, the intensity of the phase voxel can be controlled by modulating the energy of the laser pulse.

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

[0037] Phase voxels of different intensities can encode different data symbols. Another data symbol can be represented by a voxel that is absent at a given position. Multi-bit encoding is possible by writing phase voxels with two or more different intensities. This allows for storing data at higher densities per unit volume.

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

[0039] The example involves the formation of two voxels of different intensities. Increasing the number of discrete voxel intensity levels allows for higher-density data storage, provided the reader used to recover the data can resolve voxels of different intensities. For example, it is expected that three to five voxels of different intensities, or even more, will be written.

[0040] As explained above, the shape of the voxel follows the contour of the laser beam used to write the voxel. The method can include modulating the beam shape. This can allow for higher bit encoding schemes, thereby increasing data density. For example, the method can include writing two voxels with equal intensity but different shapes (e.g., circular and elliptical). Alternatively, multiple laser pulses can be used to achieve shape variation. Shape modulation can allow for the acquisition of birefringent voxels.

[0041] In this example, the intensity of the voxel is modulated by changing the intensity of the laser pulse. The laser wavelength and pulse duration remain constant. In principle, the voxel intensity could alternatively be modulated by changing either the laser wavelength or the pulse duration. However, since pulse sources typically operate at a fixed wavelength and a fixed repetition rate, more complex hardware might be required to achieve modulation of the wavelength or pulse duration.

[0042] This method can be parallelized, with two or more instances of the method being executed in parallel. For example, a beam splitter can be used to divide the source laser pulses, and the divided pulses can be used to simultaneously write the corresponding voxels.

[0043] Parallelization allows for improved data recovery by establishing a correlation between the intensities of voxels written using laser pulses from the same source. The laser's energy output can vary over time, causing variations in the intensity of the written voxels. Voxels written simultaneously using laser pulses from the same source can withstand equal noise (especially equal noise originating from variations in the laser source's output). The correlation between different voxels can be used during image processing / decoding (e.g., leveraging machine learning models) to compensate for noise introduced during writing.

[0044] Now refer to Figure 2 Description via Figure 1 Example optical data storage medium 200 that can be obtained by the method. Figure 2 This is a schematic cross-section of an optical data storage medium.

[0045] Data storage medium 200 includes a transparent substrate 210. “Transparent” means “transparent to light of the wavelength used to write voxels into the transparent substrate.”

[0046] The material of substrate 210 is not specifically limited. Any transparent material can be used, as long as voxels can be formed in substrate 210. For example, the substrate can include glass, glass ceramics, or organic polymers. In particular, the substrate can include glass. Examples of suitable glasses include fused silica, borosilicate glass, and soda-lime glass. Borosilicate glass and soda-lime glass are less expensive than high-purity glasses such as fused silica. Glass has excellent chemical stability, and data stored as voxels on glass substrates has an expected lifespan of hundreds of years or even longer under typical storage conditions. Even after exposure to temperatures of 380°C for more than 8 hours, optical data storage media based on borosilicate glass substrates have been found to be readable.

[0047] There are no specific limitations on the shape and dimensions of the substrate 210. Figure 2 A cuboid substrate is shown, but other shapes (e.g., cylindrical) are conceivable. The substrate thickness can be up to 10 mm, optionally up to 5 mm, and optionally from 200 μm to 2 mm.

[0048] Multiple phase voxels 220, 222 are arranged in 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.

[0049] The intensity of a voxel is determined by its shape and refractive index. Voxels of different intensities can have different shapes and / or different refractive indices.

[0050] Taking a homogeneous voxel as an example, light of wavelength λ passing through the voxel undergoes a phase change of δ radians, where δ is given by the following equation: Where t is the thickness of the voxel (i.e., the path length of light through the voxel), and n voxel It is the refractive index of the voxel, and n bulk It is the refractive index of the surrounding bulk medium. In this example, the intensity of the voxel is the product of the voxel's thickness and the difference in refractive index between the voxel and the surrounding bulk material.

[0051] No phase voxel at position 224 can be interpreted as a data symbol. Alternatively, all data symbols can be written as voxels onto the substrate.

[0052] In the illustrated example, voxels are arranged as a stack of layers. The stack includes a top layer 230, a middle layer 232, and a bottom layer 234. Typically, optical data storage media may include one or more voxels.

[0053] There is no fixed upper limit to the number of layers, as long as the data remains recoverable. Even compared to birefringent voxels, phase voxels cause very little light scattering. Hundreds of voxel layers can exist, for example, 200 to 1000 voxel layers. There is no specific limitation on the spacing between voxel layers, as long as voxels in different layers are separated from each other. The spacing can be appropriately selected according to the size of the voxels. As illustrated, the distance between adjacent layers can range from 2 to 20 μm.

[0054] Voxels can include reference markers, also known as preambles.

[0055] A reference marker is a set of voxels arranged in a predetermined pattern. This pattern can be two-dimensional. There are no specific limitations on the size of the set, as long as the number of voxels is large enough to allow for clear identification of the pattern when reading and decoding optical data storage media. Illustrated, a 2D reference marker can be at least 4 voxels wide by 4 voxels high. The pattern can be repeated once or multiple times.

[0056] Reading data from an optical data storage medium involves acquiring images of voxels embedded in the optical data storage medium. The position of a reference marker can be determined during image acquisition. The aiming of the imager can be adjusted based on the determined position.

[0057] When processing images to decode data, reference markers can be identified, and the positions of voxels forming the 2D reference markers can be determined. Based on the determined positions, the decoding process can compensate for positional changes that occur during the writing and / or acquisition of voxel images.

[0058] For example, location information can be used as input to processing / decoding methods for recovering data from optical data storage media. These processing / decoding methods may include the use of machine learning models.

[0059] The reference marker can have a predetermined position relative to the sector of the data-encoded voxel. The reference marker can, for example, be arranged along one or more edges of the sector of the voxel. This allows for easier differentiation of different sectors from one another during image acquisition and / or data decoding. Other arrangements are conceivable. For example, alternatively or additionally, the 2D reference marker can be arranged at the center of the sector.

[0060] A sector is a two-dimensional group of voxels. In many implementations, a given layer of voxels in an optical data storage medium comprises two or more sectors, and is read using a reader with a field of view whose size is set to acquire an image including a complete sector as well as up to eight edge portions of adjacent sectors.

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

[0062] The symbols used to form the baseline markers can be selected to maximize the contrast between the symbols. This allows for easier marker detection.

[0063] Two or more distinct reference markers can exist within a single optical data storage medium. Adjacent sectors can be associated with different reference markers. This allows for easier differentiation between sectors.

[0064] For example, two different reference marks can be used, where the associated sectors are arranged in a checkerboard pattern so that no sector shares an edge with another sector that has the same reference mark.

[0065] When using high-throughput methods (such as reference) Figure 1 The method described is particularly useful when writing reference marks to the optical data storage medium. High-throughput methods may be sensitive to component drift. For example, operating the sample stage at high speed may involve a trade-off between positioning accuracy and performance.

[0066] A voxel has a position that can be described by a set of coordinates x, y, z. The position of a voxel can vary to encode data. This is in... Figure 3 As shown in the figure, Figure 3 It is a schematic plan view of an optical data storage medium 300 including a transparent substrate 310 in which voxels 320 are embedded.

[0067] Figure 3 A row of three voxels 320a, 320b, and 320c is shown. An imaginary scan axis 340 corresponding to the average position of voxel 320 in the y-direction is also shown.

[0068] Figure 3 The voxels 320 shown have equal intensity. The center of one voxel in voxel 320a is spaced from the scan axis 340 by a distance of +δy. The center of another voxel in voxel 320b is spaced from the scan axis 340 by a distance of -δy. The final voxel 320c is centered on the scan axis 340.

[0069] Based on the positions of voxels 320a, 320b, and 320c relative to the scan axis 340, each of voxels 320a, 320b, and 320c can be used to encode different data symbols. In other words, position modulation can be used to encode data.

[0070] Combining phase modulation with position modulation allows for higher bit encoding schemes, and thus higher data density per unit volume.

[0071] In the 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 is the easiest to achieve. In principle, alternatively or additionally, the position in the z-direction (i.e., parallel to the propagation direction of the laser pulse) can be modulated.

[0072] Figure 2 and Figure 3 The voxels are shown to be laterally separated from each other. (Refer to...) Figure 4 As described, the number of voxels per unit volume and therefore the data storage capacity per unit volume can be increased by stacking voxels.

[0073] Figure 4 The diagram illustrates the magnitude of the phase shift for three voxels 420, 422, and 426 at positions along the scan axis x. Each of the three voxels has a different peak intensity.

[0074] In this example, the intensity of each voxel at a position perpendicular to the propagation direction of the laser pulse is non-uniform but radially symmetrical. More specifically, the intensity follows a Gaussian distribution centered on the focal point of the beam used to write the voxel. This represents a voxel written using a typical laser source, since the laser intensity perpendicular to the beam follows a Gaussian distribution.

[0075] When two voxels partially overlap, in other words, when the distance p between the centers of adjacent voxels... x When the two voxels are non-zero but smaller than the width of the voxel, they are called "overlapping".

[0076] Despite partial overlap, stacked voxels can be distinguished by a reader by detecting intensity changes in phase shifts based on position.

[0077] By eliminating the margins between voxels, stacking allows for a higher density of voxels per unit volume. This, in turn, can increase data storage capacity.

[0078] Now refer to Figure 5 and Figure 6 Describe an example system for writing data to a transparent substrate. Figure 5 It is a block diagram of system 500, and Figure 6This is a block diagram of an example controller 600 used in the control system 500. This system is used to implement a method for writing data onto a transparent substrate as described herein.

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

[0080] Laser source 510 generates laser pulses. Typically, the laser pulses have a period t. p A constant repetition rate is generated. This repetition rate can be on the order of 10 MHz. The laser source 510 can be a femtosecond laser.

[0081] As previously stated, the method presented herein can be parallelized. For this purpose, example system 500 includes a beam splitter 512. The beam splitter 512 receives laser pulses from a laser source and splits the laser pulses into two or more beams. Each beam is guided to a corresponding write path 520A, 520B. Each write path can implement... Figure 1 The corresponding instance of the method.

[0082] The write path (e.g., write path 520A) includes an amplitude modulator 522, a beam scanner 524, and an objective lens 526.

[0083] The amplitude modulator 522 can also be referred to as a fast intensity attenuator or a fast energy modulator. The amplitude modulator 522 receives an input laser pulse from the laser source 510 and attenuates the laser pulse to produce an attenuated laser pulse with a desired amplitude.

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

[0085] The attenuated laser pulse from amplitude modulator 522 then reaches scanner 524. Scanner 524 may include, for example, a rotating faceted mirror, a MEMS mirror, a Galvano scanner, an electro-optic scanner, or an acousto-optic scanner. Scanner 524 deflects the attenuated laser pulse, thereby allowing control over the position in the transparent substrate 200 where the attenuated laser pulse will arrive.

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

[0087] The write path 520A also includes an objective lens 526 disposed downstream of the scanner. The objective lens focuses the laser pulses received from the scanner onto a location in the transparent substrate 200.

[0088] Objective lens 526 may include a variable focus lens. By changing the focal point of objective lens 526, the position of the voxel in the z-direction can be changed.

[0089] The write path 520A may optionally include one or more additional optical components. For example, the write path may include relay optics disposed between the scanner 524 and the objective lens 526. The relay optics may include a scanning lens, one or more spherical lenses, and one or more tube lenses. Alternatively, the relay optics may be referred to as delivery optics.

[0090] Write path 520A delivers attenuated laser pulses to optical data storage medium 200, resulting in the formation of voxels in optical data storage medium 200.

[0091] Example system 500 includes a sample stage 530 for holding an optical data storage medium. The sample stage can be configured to translate relative to an objective lens 526 along a write path, thereby allowing laser pulses to be directed to different portions of the optical data storage medium.

[0092] By way of explanation, the z-position of the voxel can be controlled by adjusting the focus of the objective lens 526; the y-position of the voxel can be controlled by the scanner 524; and the x-position of the voxel can be controlled by moving the sample stage in the x-direction.

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

[0094] There are no specific limitations on the nature of the controller 600. The controller 600 may include dedicated hardware circuitry, programmable computer hardware, or any combination thereof.

[0095] Now refer to Figure 6 The block diagram describes an example implementation of controller 600.

[0096] The controller 600 includes a processing unit 610 operatively linked to a data storage device 620, and an optional user terminal 630. The data storage device 620 stores a computer program 622 for execution 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 methods described herein.

[0097] User terminal 630 may include user input devices and display devices.

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

[0099] When this article refers to receiving input from a user via a user input device, this can mean via any one or more user input devices that constitute a user input device.

[0100] User input devices can be used to allow users to specify data to be encoded and written to the optical data storage medium, and / or allow users to specify parameters describing the optical data storage medium, such as identifying the properties of the transparent substrate material.

[0101] The display device can take any suitable form for outputting images, 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 include a touchscreen and thus also form at least part of a user input device. Input can be enabled by the user's fingers and / or by using a stylus.

[0102] The inclusion of a display device is optional. A display device is useful in examples where it is desirable to display human-readable output to a user.

[0103] The processing device 610 includes one or more processing units implemented in one or more dies, IC (integrated circuit) packages and / or housings at one or more geographical locations.

[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), a digital signal processor (DSP), etc. Each of the one or more processing units may include one or more cores.

[0105] In the case where it is said that a computer program is executed on a processing device, this could mean that it is executed by any one or more processing units constituting the processing device 610.

[0106] The processing device 610 typically also includes working memory, such as random access memory and / or one or more memory caches within one or more processing units.

[0107] The data storage device 620 includes one or more memory cells implemented in one or more memory media in one or more housings at one or more geographical locations.

[0108] Each memory cell in one or more memory cells may employ any suitable storage medium known in the art, such as magnetic storage media (e.g., hard disk drives, magnetic tape drives, etc.); or electronic storage media (e.g., solid-state drives (SSDs), flash memory, or electrically erasable programmable read-only memory (EEPROM); or optical storage media (e.g., optical disc drives or glass- or memory crystal-based storage devices, etc.).

[0109] In the case that some data items mentioned herein are stored in data storage device 620 or its area, this can mean stored in any part of any one or more memory devices that constitute data storage device 620.

[0110] The processing device 610 and the data storage device 620 are operatively linked. The processing device and the data storage device are configured such that the processing device 610 can read data from at least a portion of the data storage device 620 and write data to at least a portion of the data storage device 620. The processing device 610 can communicate with the data storage device 620 via 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 can be wired or wireless.

[0111] Various modifications can be made to the sample system.

[0112] In the illustrated example, light propagates between components through free space. Variations of the system may include waveguides, fiber optic cables, etc., for propagating light.

[0113] The example system includes a beam splitter separate from the laser source. In variants, the laser source can be a multi-channel laser, and the beam splitter can be omitted.

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

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

[0116] The example system modulates the power of the laser pulse by attenuation. Alternatively, the pulse can be amplified.

[0117] Scanner 524 and objective lens 526 can be shared by two or more write paths.

[0118] In principle, the scanner 524 can be omitted. The position of the laser pulse arrival can be controlled by moving the optical data storage medium 200 using the sample stage 530.

[0119] In the example, the sample stage translates the transparent substrate. Alternatively or additionally, the sample stage can rotate the transparent substrate.

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

[0121] In the example, the sample stage is described as movable. More typically, the system can have any configuration that allows for relative movement of the write path and optical data storage medium. The sample stage can be fixed in place using other components of the system mounted on the actuator.

[0122] Methods for reading optical data storage media Figure 7 The flowchart is shown.

[0123] At box 701, an image of a voxel in an optical data storage medium is acquired using a refractive index-sensitive microscope.

[0124] Refractive index-sensitive microscopes can be phase-contrast microscopes or differential interference microscopes. When using phase-contrast microscopes or differential interference microscopes, it is possible to observe that voxels with different refractive indices have different brightness.

[0125] Image acquisition can include acquiring wide-field-of-view images. Wide-field imaging simultaneously images multiple voxels. Alternatively, a point scanner can be used to acquire images by probing one voxel at a time.

[0126] It can acquire a single image of a given voxel layer.

[0127] In many implementations, a voxel can include both regions with increasing refractive index and regions with decreasing refractive index. These regions can be referred to as "sublayers".

[0128] Image acquisition can include acquiring images of each sublayer. The sublayer to be imaged can be selected by adjusting the focal position of the refractive index-sensitive microscope.

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

[0130] More generally, it has been found that adjusting the focusing parameters of a microscope (e.g., focal position, and / or spherical aberration correction of the microscope objectives) can modify image contrast. By acquiring two or more images of a given voxel using different focusing parameters, and optionally identifying the differences between the images, data recovery can be made more reliable. This method is particularly useful for non-uniform voxels, but is also applicable to uniform voxels.

[0131] After the image is acquired, it is processed at frame 702 to recover the stored data. Image processing is a computer-implemented process.

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

[0133] Data recovery can be achieved in various ways. For example, machine learning models can be used to process (multiple) images. Machine learning models can include artificial neural networks, such as convolutional neural networks. Training data can be used to train machine learning models, which includes images of voxels labeled with symbols encoded by voxels. Example 1

[0134] Usage Reference Figure 1 The described method writes voxels onto a borosilicate glass substrate. Four symbols in a 2-bit encoding scheme are represented using laser pulses with four different amplitudes. A single laser pulse is used for each voxel.

[0135] A voxel is an inhomogeneous voxel. An inhomogeneous voxel consists of a sublayer with an increased refractive index and a sublayer with a decreased refractive index, arranged on either side of the focal point of the laser pulse used to form the voxel in the propagation direction of the laser pulse.

[0136] Two phase-contrast microscopic images of a voxel layer were acquired. The images are in... Figure 8A and Figure 8B As shown in the image.

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

[0138] A difference image is generated by subtracting a negative image from a positive image. The difference image is... Figure 8CAs shown in the figure, the difference image has a greater dynamic range. Subtracting one image from another also removes artifacts and background effects without requiring further post-processing steps. The distance between the two sub-layers of a voxel is approximately a few micrometers, and the background does not change significantly at this distance scale. Example 2

[0139] Usage Reference Figure 1 The described method writes voxels into fused silica glass substrates and borosilicate glass substrates.

[0140] Voxels are imaged using phase-contrast microscopy. Figure 9A and Figure 9B Histograms of the images are shown below.

[0141] The histogram shows the correlation between the brightness measured at the voxel location in fused silica and borosilicate and the expected value of the symbol written to the substrate. This indicates that the symbols can be sufficiently distinguished from each other, and that this method can support multi-bit data storage per voxel.

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

[0143] It should be understood that the above embodiments have been described by way of example only.

[0144] More generally, according to one aspect disclosed herein, a method for writing data onto a transparent substrate is provided. The method includes: forming a first voxel having a first intensity by focusing a first laser pulse at 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 at a second location in the transparent substrate. The second laser pulse has a second amplitude different from the first amplitude. Encoding data by amplitude modulation allows voxels to be written using a single laser pulse, thereby increasing data throughput. Furthermore, voxels can be reliably formed in inexpensive substrates such as borosilicate glass.

[0145] Voxels can be referred to as "phase voxels".

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

[0147] The first position can be spaced apart from the second position by a distance chosen such that the first and second voxels partially overlap. In other words, the first and second voxels can be stacked. Stacking allows for an increase in the number of voxels per unit area, thereby increasing data density.

[0148] The method may further include forming a third voxel by focusing a third laser pulse at a third position in a transparent substrate, the third voxel having an intensity equal to that of the first voxel. The first and third voxels may have different shapes. Alternatively or additionally, the first voxel may be spaced apart from the scan axis by a first lateral spacing, and the third voxel may be spaced apart from the scan axis by a third lateral spacing, the first and third lateral spacings being different. Modulating the shape and / or position of voxels with equal intensity can allow a greater number of bits per voxel to be encoded.

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

[0150] The transparent substrate can include glass. The glass can be borosilicate glass or soda-lime glass. It has been found that the method presented herein can be successfully implemented using glass substrates of relatively low purity. High-purity glass such as fused silica can be used, but this is not necessary. Other substrate materials, such as organic polymers, are also considered.

[0151] This method may include forming at least two layers of voxels in a transparent substrate, optionally at least 100 layers of voxels in a transparent substrate. Phase voxels cause very small light scattering. Therefore, many layers of voxels can be written into a given substrate, thereby increasing the data density per unit volume.

[0152] This method can be parallelized. Two or more instances of the method can be executed simultaneously. Parallelized implementations may include forming at least two first voxels simultaneously at corresponding first positions. Parallelization can increase data throughput. Parallelization can create a strong correlation between voxels written simultaneously, which can aid in data recovery.

[0153] This method may include forming multiple voxels arranged as reference markers. The reference markers may be two-dimensional. Reference markers can allow for easier recovery of data written using this method from optical data storage media. The aiming of the imaging system can be adjusted based on the detected reference markers. Error correction can be applied based on the reference markers during the decoding process.

[0154] A related aspect provides an optical data storage medium obtainable by this method. The optical data storage medium includes: a transparent substrate comprising a material having a bulk refractive index; a first voxel embedded in the transparent substrate, the first voxel having a first intensity; and a second voxel embedded in the transparent substrate, the second voxel having a second intensity different from the first intensity, resulting in the first voxel and the second voxel encoding different data symbols. A voxel is a region in the transparent substrate having a refractive index different from the bulk refractive index.

[0155] The first and second voxels can be non-uniform voxels. Non-uniform voxels include positive regions with a refractive index greater than the bulk refractive index and negative regions with a refractive index less than the bulk refractive index. The positive and negative regions are arranged in series along an axis corresponding to the propagation direction of the laser used to form the voxel. When reading voxels, the dynamic range of the reader and / or background effects can be increased and / or reduced by acquiring images of the positive and negative regions, and by subtracting one image from another.

[0156] The first and second voxels can be stacked. In other words, the first and second voxels can partially overlap. This can increase voxel density, leading to an increase in data capacity.

[0157] It should be understood that the optical data storage medium may include features produced by any of the various optional method steps described in the reference method aspect. For example, the optical data storage medium may include shape modulation voxels and / or position modulation voxels.

[0158] Another approach provides a system for writing phase voxels onto a transparent substrate. This system is used to implement the methods described herein. The system includes: a controller; a pulsed laser source; and a first amplitude modulator disposed downstream of the pulsed laser source in an optical path. The amplitude modulator is operatively linked to the controller. The controller is configured to cause the system to perform the methods provided herein.

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

[0160] The system may include a write 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 at a location in a transparent substrate.

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

[0162] The system may include at least two write paths. In this implementation, the system may also include a beam splitter for receiving laser pulses from a laser source and outputting correspondingly split laser pulses to each write path.

[0163] The sample stage can be operatively linked to the controller and configured to translate relative to at least one write path. This allows for further control over the positioning of the voxels within the transparent substrate.

[0164] Another approach provides a method for reading data from an optical data storage medium as defined herein. The method includes: acquiring voxel images using a refractive index-sensitive microscope; and processing the images using a processor to recover data, wherein portions of the image with different signal intensities encode different data symbols.

[0165] Refractive index sensitive microscopes can be phase contrast microscopes or differential interference contrast microscopes.

[0166] Acquiring voxel images may include acquiring a first image using a first focusing parameter (e.g., focus position) and a second image using a second focusing parameter (e.g., focus position) different from the first focusing parameter. Acquiring multiple images allows for more reliable data recovery. For example, the contrast between voxels can be varied between images.

[0167] Voxels can be non-uniform voxels. Non-uniform voxels include positive sublayers with a refractive index greater than the bulk refractive index of the substrate and negative sublayers with a refractive index less than the bulk refractive index. In a voxel non-uniform implementation, one image in the image can be an image of a positive sublayer, and the other image in the image can be an image of a negative sublayer.

[0168] When acquiring two voxel images, image processing can also include subtracting the first image from the second image. This allows for more reliable data recovery, improves image contrast, and reduces background artifacts.

[0169] Image processing can include using machine learning models. Machine learning models include convolutional artificial neural networks. Machine learning models can be trained using images with labels representing data symbols encoded by voxels.

[0170] In an implementation where the optical data storage medium includes voxels arranged as reference markers, image processing may include determining the position of the voxels within the reference markers.

[0171] In this implementation, the method may further include compensating for variations in the position of voxels in the image based on the positions of voxels in the reference markers. Random variations in voxel positions can occur during the writing process, and / or the acquired image may be subject to distortion. The reference markers comprise voxels arranged in a predetermined pattern. Therefore, the decoding process can compensate for such variations and distortions based on the detected positions of the voxels in the reference markers.

[0172] Alternatively or additionally, the method may further include: identifying voxels as members of a sector based on their positions in a reference marker.

[0173] This paper also provides a method for encoding data as voxels in a transparent substrate using laser pulse energy modulation. The laser pulse creates a volumetric refractive index modification (i.e., a voxel) in the transparent substrate. Modulating the laser pulse energy can alter the volume of the voxel and / or the magnitude of the change in the refractive index of the substrate material.

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

[0175] This paper also provides a method for writing data onto a substrate. The method includes: A first laser pulse with a first energy is focused onto a first region in the substrate, resulting in a first volume refractive index modification; A second laser pulse with a second energy, different from the first energy, is focused onto a second region in the substrate, resulting in a second volume refractive index modification that has optical properties different from the first volume refractive index modification.

[0176] This method may include any of the various optional features described with reference to the previously discussed methods. A “voxel” may be referred to as a “volume refractive index modification”.

[0177] A related aspect provides a transparent optical data storage medium, comprising: a transparent substrate; a first volume refractive index modification at a first location in the transparent substrate, the first volume refractive index modification having a first volume and a first refractive index distribution; and a second volume refractive index modification at a second location in the transparent substrate, the second volume refractive index modification having a second volume and a second refractive index distribution, wherein the first refractive index distribution is different from the second refractive index distribution and / or the first volume is different from the second volume.

[0178] This disclosure provides the following terms: Clause 1. A method comprising: By focusing a first laser pulse at a first position in a transparent substrate, a first voxel with a first intensity is formed, and the first laser pulse has a first amplitude; and By focusing a second laser pulse at a second position in a transparent substrate, a second voxel is formed having a second intensity different from the first intensity, and the second laser pulse has a second amplitude different from the first amplitude. Clause 2. The method according to Clause 1, wherein the first position and the second position are spaced apart by a distance, the distance being selected such that the first voxel and the second voxel partially overlap. Clause 3. The method according to Clause 1 or Clause 2 further includes: forming a third voxel by focusing a third laser pulse at a third position in a transparent substrate, the third voxel having an intensity equal to that of the 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 Clause 4, wherein the first voxel is spaced from the scan axis by a first lateral spacing, the third voxel is spaced from the scan axis by a third lateral spacing, and the first lateral spacing and the third lateral spacing are different. Clause 6. The method according to any of the preceding clauses, wherein each voxel is formed by a single corresponding laser pulse. Clause 7. The method according to any of the preceding clauses, wherein the transparent substrate comprises glass. Clause 8. The method according to Clause 7, wherein the glass is borosilicate glass or soda-lime glass. Clause 9. The method according to any of the preceding clauses includes: forming at least two layers of voxels in a transparent substrate. Clause 10. The method according to Clause 9, comprising: forming at least 100 layers of voxels in a transparent substrate. Clause 11. The method according to any of the preceding clauses, comprising: simultaneously forming at least two first voxels at corresponding first locations. Clause 12. The method according to any of the preceding clauses includes: forming a plurality of voxels arranged as reference marks. Clause 13. An optical data storage medium comprising: A transparent substrate comprising a material having a bulk refractive index; A first voxel, embedded in a transparent substrate, having a first strength; and A second voxel, embedded in a transparent substrate, has a second intensity that is different from the first intensity, causing the first voxel and the second voxel to encode different data symbols. Clause 14. The optical data storage medium pursuant to Clause 13, wherein the first voxel and the second voxel are non-uniform voxels, and each comprises: A positron layer having a refractive index greater than that of the bulk; and A negative sublayer with a refractive index less than that of the bulk. Clause 15. An optical data storage medium pursuant to Clause 13 or Clause 14, wherein a first voxel and a second voxel are stacked. Clause 16. An optical data storage medium pursuant to any of Clauses 13 to 15 is obtainable by any of the methods described in Clauses 1 to 12. Clause 17. A system comprising: Controller; Pulsed laser source; and The first amplitude modulator is positioned downstream of the pulsed laser source in the optical path; The amplitude modulator is operatively linked to the controller and, in use, enables the system to perform methods including: By focusing a first laser pulse at a first position in a transparent substrate, a first voxel with a first intensity and a first amplitude is formed; and By focusing a second laser pulse at a second position in a transparent substrate, a second voxel is formed having a second intensity different from the first intensity, and the second laser pulse has a second amplitude different from the first amplitude. Clause 18. The system pursuant to Clause 17 also includes a scanner, which is arranged downstream of the amplitude modulator in the optical path. Clause 19. In the system described in Clause 17 or Clause 18, the beam splitter is positioned between the pulsed laser source and the first amplitude modulator; and The second amplitude modulator is arranged parallel to the first amplitude modulator; The second amplitude modulator is operatively linked to the controller, and in use, the controller controls the system to perform a method that also includes forming a third voxel by focusing a third laser pulse at a third position in a transparent substrate while forming the first voxel. Clause 20. The system pursuant to any one of Clauses 17 to 19 further includes a movable sample stage to hold the transparent substrate. Clause 21. A method for reading data from an optical data storage medium, the optical data storage medium being defined as in any one of Clauses 13 to 16, the method comprising: Voxel images are acquired using a refractive index-sensitive microscope; The image is processed by a processor to recover data, where portions of the image with different signal intensities are encoded with different data symbols. 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. Acquiring a voxel image according to the method of Clause 21 or Clause 22 includes: acquiring a first image using a first focusing parameter, and acquiring a second image using a second focusing parameter different from the first focusing parameter. Clause 24. The method of Clause 23, wherein the voxels are non-uniform voxels, each voxel comprising a positive sublayer having a refractive index greater than the bulk refractive index of the transparent substrate and a negative sublayer having a refractive index less than the bulk refractive index, and image acquisition comprises acquiring an image of the positive sublayer and acquiring an image of the negative sublayer. Clause 25. The method of Clause 23 or Clause 24, wherein image processing further comprises subtracting the first image from the second image. Clause 26. The method of any one of Clauses 21 to 25, wherein processing an image includes processing the image using a machine learning model. Clause 27. The method of Clause 26, wherein the machine learning model includes a convolutional artificial neural network. Clause 28. The method of any one of Clauses 21 to 27, wherein the optical data storage medium includes voxels arranged as reference markers, and wherein processing the image includes determining the positions of the voxels arranged as reference markers. Clause 29. The method according to Clause 28 further includes: compensating for changes in the position of voxels in the image based on the position of voxels in the reference markers. Clause 30. Use laser pulse energy modulation to encode data as voxels in a transparent substrate. Clause 31. The use of Clause 30 also includes encoding data using voxel shape modulation. Clause 32. The use of Clause 30 or Clause 31 also includes encoding data using modulation of voxel positions relative to the scan axis. Clause 33. A method comprising: A first voxel is formed by focusing a first laser pulse at a first location in a transparent substrate; and A second voxel is formed by focusing a second laser pulse at a second position in a transparent substrate; The first laser pulse and the second laser pulse have different beam shapes, which results in the first voxel and the second voxel having different shapes. Clause 34. The method according to Clause 33, wherein the first position and the second position are spaced apart by a distance, the distance being selected such that the first voxel and the second voxel partially overlap. Clause 35. The method according to Clause 33 or Clause 34 further includes: forming a third voxel by focusing a third laser pulse at a third position in a transparent substrate, the third voxel having 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 intensities. Clause 37. The method according to Clause 35 or Clause 36, wherein the first voxel is spaced from the scan axis by a first lateral spacing, the third voxel is spaced from the scan axis by a third lateral spacing, and the first lateral spacing and the third lateral spacing are different. Clause 38. The method according to any one of Clauses 33 to 37, wherein each voxel is formed by a single corresponding laser pulse. Clause 39. The method of any one of Clauses 33 to 38, wherein the transparent substrate comprises glass. Clause 40. The method of 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 in the transparent substrate. Clause 42. The method of Clause 41, comprising: forming at least 100 layers of voxels in a transparent substrate. Clause 43. The method of any one of Clauses 33 to 42, comprising: simultaneously forming at least two first voxels at corresponding first locations. Clause 44. The method according to any one of Clauses 33 to 43 includes: forming a plurality of voxels arranged as reference markers. Clause 45. An optical data storage medium comprising: A transparent substrate comprising a material having a bulk refractive index; The first voxel, embedded in a transparent substrate; and The second voxel is embedded in a transparent substrate; Voxels are regions of a transparent substrate whose refractive index differs from the bulk refractive index; and The first voxel has a different shape than the second voxel, which results in the first voxel and the second voxel encoding different data symbols. Clause 46. The optical data storage medium pursuant to Clause 45, wherein the first voxel and the second voxel are non-uniform voxels, and each comprises: A positron layer having a refractive index greater than that of the bulk; and A negative sublayer with a refractive index less than that of the bulk. Clause 47. An optical data storage medium pursuant to Clause 45 or Clause 46, wherein a first voxel and a second voxel are stacked. Clause 48. An optical data storage medium pursuant to any of Clauses 45 to 47 is available by any of the methods described in Clauses 33 to 44. Once the disclosure herein is given, other variations or use cases of the disclosed technology will become apparent to those skilled in the art. The scope of this disclosure is not limited to the described embodiments, but only to the appended claims.

Claims

1. A method comprising: By focusing a first laser pulse at a first position in a transparent substrate (200), a first voxel (220) with a first intensity is formed (101), and the first laser pulse has a first amplitude; as well as By focusing a second laser pulse at a second position in the transparent substrate (200), a second voxel (222) with a second intensity different from the first intensity is formed (102), and the second laser pulse has a second amplitude different from the first amplitude.

2. The method of claim 1, wherein the first position is spaced apart from the second position by a distance, the distance being selected such that the first voxel and the second voxel partially overlap.

3. The method according to claim 1 or claim 2, further comprising: A third voxel (320c) is formed by focusing a third laser pulse at a third position in the transparent substrate, the third voxel (320c) having 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 claim 4, wherein the first voxel (320a) is spaced from the scan axis (340) by a first lateral interval, the third voxel (320c) is spaced from the scan axis (340) by a third lateral interval, and the first lateral interval and the third lateral interval are different.

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

7. The method according to any one of the preceding claims, wherein the transparent substrate (200) comprises 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 the preceding claims, comprising: At least two layers (230, 232) of voxels (220, 222) are formed in the transparent substrate (200).

10. The method of claim 9, comprising: At least 100 layers of voxels (220, 222) are formed in the transparent substrate (200).

11. The method according to any one of the preceding claims, comprising: At least two first voxels (220) are formed simultaneously at the corresponding first positions.

12. The method according to any one of the preceding claims, comprising: Multiple voxels are formed and arranged as reference markers.

13. An optical data storage medium (200), comprising: A transparent substrate (200) comprising a material having a bulk refractive index; A first voxel (220) is embedded in the transparent substrate (200) and has a first strength; as well as A second voxel (222) is embedded in the transparent substrate (200) and has a second intensity that is different from the first intensity, causing the first voxel (220) and the second voxel (222) to encode different data symbols.

14. The optical data storage medium according to claim 13, wherein the first voxel (220) and the second voxel (222) are non-uniform voxels, and each comprises: A positive sublayer having a refractive index greater than the bulk refractive index; as well as A negative sublayer having a refractive index less than the bulk refractive index.

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

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

17. A system (500), comprising: Controller (600); Pulsed laser source (510); as well as A first amplitude modulator (522) is arranged downstream of the pulsed laser source (510) in the optical path; The controller (600) is operatively linked to the amplitude modulator (522) and, in use, causes the system (500) to perform methods including: By focusing a first laser pulse at a first position in a transparent substrate (200), a first voxel (220) with a first intensity is formed (101), and the first laser pulse has a first amplitude; as well as By focusing a second laser pulse at a second position in the transparent substrate (200), a second voxel (222) with a second intensity different from the first intensity is formed (102), and the second laser pulse has a second amplitude different from the first amplitude.

18. The system of claim 17 further includes a scanner (524) arranged downstream of the amplitude modulator (522) in the optical path.

19. The system according to claim 17 or claim 18, further comprising: Beam splitter (512), which is arranged between the pulsed laser source (510) and the first amplitude modulator (522); as well as A second amplitude modulator is arranged parallel to the first amplitude modulator; The second amplitude modulator is operably linked to the controller (600), and In use, the controller (600) controls the system (500) to perform the following method: while forming the first voxel, a third voxel is formed by focusing a third laser pulse at a third position in the transparent substrate (200).

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

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