Patent Description:
According to aspects of the present invention there is provided a system as defined in the accompanying claims.

The examples disclosed herein relate to an optical data-storage system, comprising a laser, an imaging optic, and associated computer logic. The laser is configured to emit a pulsed wavefront having uniform phase and polarization. The imaging optic is configured to modulate the phase and polarization of different portions of the wavefront by different amounts, and to diffract light from the different portions to a substrate with writeable optical properties. The logic is configured to receive data and to control modulation of the phase and polarization such that the light diffracted from the imaging optic writes the data to the substrate.

This disclosure is presented by way of example, and with reference to the drawing figures listed above. Components, process steps, and other elements that may be substantially the same in one or more of the figures are identified coordinately and described with minimal repetition. It will be noted, however, that elements identified coordinately may also differ to some degree. It will be further noted that the figures are schematic and generally not drawn to scale. Rather, the various drawing scales, aspect ratios, and numbers of components shown in the figures may be purposely distorted to make certain features or relationships easier to see.

As described above, data can be written to a glass or other solid substrate using high-power, coherent irradiance. The term 'voxel' is used herein to refer to any discrete locus of a substrate where an individual data value (i.e., symbol) may be stored. The data stored in a voxel may take various forms. In principle, any of the Muller-matrix coefficients of the substrate lattice can be manipulated to encode data. In examples using silica glass substrates, the lattice perturbation from focused, polarized irradiance takes the form of a non-native birefringence localized at the focus. Accordingly, each voxel of the substrate may be modeled as a very small waveplate of a retardance δd and slow-axis orientation ϕ. These model parameters may be manipulated independently to write a desired symbol to a given voxel. Here, the polarization angle of the beam determines the slow-axis orientation ϕ, while the amplitude of the beam determines the strength of the waveplate grating, and therefore the retardance δd.

By dividing the continuous space of achievable slow-axis orientations and retardances into discrete intervals, multi-bit data values may be encoded into each voxel-viz. , by independently coercing the birefringence of that voxel to be within one of the discrete intervals. In this manner, each voxel may encode one of R ≥ <NUM> different retardance states at each of Q ≥ <NUM> different polarization angles. In some examples, many parallel layers of voxel structures may be written to the same substrate by focusing the laser irradiance to specified depths below the irradiated surface of the substrate. This mode of data storage is referred to as '5D optical storage'.

In order to write data at an acceptably high throughput, numerous voxels may be written in parallel. Although it is possible to write each voxel serially, the required overhead of high-speed, high-precision mechanical movement may make such an approach impractical. To write data in parallel, the output of a high-power laser may be split into a plurality of independently modulated, voxel-sized child beams, so that a corresponding plurality of voxels may be written simultaneously. Each child beam, however, must be rotated to the particular polarization state appropriate for the symbol it writes.

<FIG> illustrates the 5D optical approach in schematic detail. <FIG> shows aspects of an example optical data storage and retrieval system <NUM>. System <NUM> is configured to write and store data to substrate <NUM>. The substrate may differ from one example to the next, but generally includes a transparent substrate. In some examples, the substrate may be a polymer. In some examples, the substrate may be an inorganic glass, such as silica glass. In some examples, the substrate may take the form of a relatively thin optical layer (e.g., <NUM> to <NUM> microns thick), coupled to a mechanically stable supporting layer. In <FIG> and subsequent drawings, the substrate is shown in the form of rotating disk. In other examples, the substrate may be shaped differently-as a translating slab or rotating cylinder, for instance. In some examples, the substrate may be stationary with respect to the optical componentry of system <NUM>. Buffer <NUM> of system <NUM> is configured to buffer an input data stream <NUM> to be written to the substrate. Encoder <NUM> parses the data from the buffer and provides appropriate control signal to write head <NUM>, such that the data is written according to the desired encoding. Additional aspects of the encoding and write process are controlled by write controller <NUM>, which may take the form of a computer system (vide infra).

<FIG> shows aspects of an example write head 20A of optical data storage and retrieval system <NUM>. To enable simultaneous, parallel writing, write head 20A includes a high-power, laser <NUM>, an imaging optic in the form of an electronically addressable liquid-crystal spatial light modulator (LCSLM) 26A, and an electronically addressable polarization modulator (PM) <NUM>.

Laser <NUM> is configured to emit a pulsed wavefront having uniform phase and polarization. The laser may be a femtosecond laser emitting in a narrow wavelength band of fixed (e.g., plane) polarization. Irradiance from the laser may comprise a repeating pulsetrain of sub-picosecond photon pulses-e.g., tens to hundreds of femtoseconds in duration, for example. In some implementations, laser <NUM> may be one or more of Q-switched and mode-locked, to provide very brief pulses of very high energy. In some cases, shorter wavelengths of a light maybe formed by using optical harmonic generators utilizing non-linear processes. Other forms of laser irradiance are also envisaged.

LCSLM 26A is configured as a dynamic digital hologram. The LCSLM includes an array of pixel elements that receive the wavefront from laser <NUM>. The liquid crystal (LC) within each pixel element imparts a variable phase delay to the irradiance passing through that element. In a state-of-the-art LCSLM, the phase delay is in a unique direction common to all pixel elements of the array. Because each pixel element is independently addressable, the magnitude of the variable phase delay may be controlled down to the pixel level. As with any grating, a phase delay imparted in the near field of the LCSLM creates an interference pattern in the far field, where substrate <NUM> is positioned. By controlling the near-field phase delay from each pixel element of the LCSLM, the far-field interference pattern may be controlled so as to irradiate each voxel of any layer of the substrate with the desired intensity.

In write head 20A of <FIG>, the holographic projection from LCSLM 26A passes through PM <NUM>. The PM is a non-imaging active optic configured to rotate, by a controllably variable angle, the polarization state of the holographic projection. To the substrate, therefore, the holographic projection 'appears' as a parallel 2D array of write beams, each having controlled polarization and intensity, and each being mapped to a corresponding voxel of substrate <NUM>. It will be noted that the mapping of LCSLM pixels to write beams (i.e., voxels) is not necessarily a <NUM>:<NUM> mapping, but may be <NUM>:<NUM>, <NUM>:<NUM>, or <NUM>:<NUM>, among other suitable mappings. In some examples, the number of write beams achievable practically is about one-fourth the number of pixels on the LCSLM. For example, with about <NUM> million LCSLM pixels, one-million or more child beams may be formed. Moreover, the array of write beams may be reconfigured at the full refresh rate of the LCSLM. State-of-the-art LCSLMs employing nematic liquid crystals have refresh rates of the order of <NUM> frames per second.

In write head 20A, LCSLM 26A and PM <NUM> are each coupled operatively to encoder <NUM>. To the LCLSM, the encoder provides electronic signal that digitally defines the holographic projection; to the PM, the encoder provides electronic signal that defines the variable rotation applied to the polarization state of the holographic projection.

In some implementations, the array of pixel positions of LCSLM 26A may be grouped into a plurality of non-overlapping or marginally overlapping holographic zones, which are exposed sequentially to the wavefront of laser <NUM>. Each holographic zone may be a two-dimensional area of any desired shape-e.g., rectangular, wedge-shaped, ring-shaped, etc. Accordingly, LCSLM 26A of system <NUM> may be coupled mechanically to a scanning stage <NUM>, configured to change the relative positioning of the LCSLM versus the laser. In this manner, each of the holographic zones of the LCSLM may be irradiated in sequence. The scanning stage may be translational and/or rotational, and may be advanced a plurality of times (<NUM>, <NUM>, <NUM> times, etc.) for each time that the LCSLM is addressed. This approach effectively multiplies the temporal bandwidth of the LCSLM beyond its maximum refresh rate. Nevertheless, the laser, LCSLM, PM, and substrate may be fixed in position in some examples. In examples in which data is to be written to a plurality of depth layers of substrate <NUM>, write head 20A may include an adjustable objective lens system <NUM> configured to focus the irradiance of the write beams from the LCSLM to any selected depth layer of the substrate.

In the configuration described above, write head 20A includes a LCSLM 26A in combination with a non-imaging PM <NUM>. As noted above, this LCSLM is used primarily to divide the laser wavefront into the required number of child beams, while the PM sets the rotation of the far-field polarization based on the data to be written. This enables a symbol Si to be written to each voxel i of the substrate, encoded by the slow-axis angle and retardance of that voxel. The symbol may be expressed as a digital value or bit sequence-e.g., <NUM>, <NUM>, <NUM>, <NUM>. This can be achieved, for example by two different polarization angles and two possible retardance values for each angle. Other ways include the use of four polarization angles and a single retardance value. Naturally, a larger menu of polarization angles and/or retardance values would correspond to a longer bit sequence.

One disadvantage of write head 20A is that for each exposure of substrate <NUM>, all of the voxels written to the substrate must have the same symbol Si. This is shown by example in <FIG>. Accordingly, if each voxel were to store two bits of information, four different write operations would be required. <FIG> illustrates and example sequence of exposures to achieve this, where an <NUM> x <NUM> voxel sector of data is written to the substrate in the likely case where the number of voxels assigned the same symbol is different for each exposure.

To enact the write sequence of <FIG>, write head 20A would need to compensate, in terms of laser power, for the variable number of voxels written during each exposure. This could be done, for instance, by splitting each of the indicated exposures into two or more smaller exposures, which would reduce throughput. Alternatively, it could be done by over-budgeting the laser power, requiring significant power from every exposure to be wasted. In sum, the unpredictability of the symbol distribution required for each exposure renders write head 20A inefficient in terms of bandwidth and/or optical power utilization.

Thus, described in further detail below are examples related to a modified write head for optical data storage and retrieval system <NUM> and associated modes of parallel data writing. Briefly, these examples use an LCSLM to divide the laser irradiance into numerous voxel-sized child beams and simultaneously control the polarization of each of the child beams. This is achieved without using multiple active modulators, which would increase cost and complexity and reduce throughput or efficiency, as described above. The modified write head operates based on the principle that shifting a grating or a hologram spatially causes its far field to change phase.

Conceptually, the present approach is akin to the writing mode illustrated in <FIG> shows a first grating 34X and an adjacent second grating 34Y of the same frequency, or pitch. The two gratings each image the incident wavefront onto a single spot <NUM>, which may correspond to a single substrate voxel. Significantly, if gratings 34X and 34Y are irradiated by light of different polarizations-e.g., the vertical and horizontal plane polarizations indicated by the boxed arrows of <FIG>-then, by controlling the phase and intensity of the two superimposed beams, the polarization as well as the amplitude of the resultant irradiance of spot <NUM> may be controlled.

The configuration of <FIG> may have certain practical limitations. First, although the beams from gratings 34X and 34Y are coincident at the imaged spot <NUM>, they differ from each other in angle space. Accordingly, as the beam expands again, downstream of <NUM>, the polarization content will again separate. For some data-writing applications, this effect may have undesirable resolution-limiting consequences related to reduction in the effective aperture size and numerical aperture.

One way to address this limitation is to divide grating 34X into numerous component gratings with the same diffraction properties, divide grating 34Y in the same way, and comingle the component gratings over a suitably small length scale. This could be done, for instance, using an LCSLM optoelectronically 'tiled' with alternating portions of gratings 34X and 34Y. Tiles of grating 34X would be irradiated with one polarization state, and tiles of grating 34Y would be illuminated with another polarization state. In this approach, it would be necessary to irradiate different regions of the LCSLM with different polarized beams. In some examples, this may be achieved using a polarization-modulating sheet arranged between the laser and the LCSLM. The polarization-modulating sheet may comprise an alternating tiling of cells arranged in registry with the pixel elements of the array, each cell configured to rotate the polarization of a portion of the wavefront passing through that cell.

In examples according to the claimed invention, to avoid aperture-size reduction, but without requiring piece-wise illumination of the LCSLM, a single LCSLM is used to control both phase and polarization, down to the pixel level. This operation is within the ability of a suitably configured LCSLM, and is enacted by write head 20B of <FIG>. The array of pixel elements of LCSLM <NUM> of <FIG> is configured to modulate the phase and polarization of different portions of the wavefront by different amounts, and to diffract light from the different portions to a substrate with writeable optical properties. In particular, the LCSLM is configured to modulate the different portions of the wavefront to different near-field polarizations and to image the light to an array of substrate voxels at different far-field polarizations. To this end, the encoder logic is configured to receive data and to control modulation of the phase and polarization such that the light diffracted from the imaging optic writes the data to the substrate. Such data may include inequivalent first and second data values written simultaneously by the light diffracted from the imaging optic. In other words, the first data value may be written to a first voxel by light of a first far-field polarization while the second data value is written to a second voxel by light of a second, inequivalent far-field polarization.

Control of two different parameters may be effected independently or with correlation. Conceptually, the more straightforward mode of controlling both phase and polarization is to control each parameter independently. This may be achieved via an LCSLM in which the various pixel elements are addressable to modulate phase, and independently addressable to modulate polarization. In other words, the LCSLM is configured to provide two independent degrees of freedom in the nematic director. Rotation in one direction affects phase and the other polarization.

<FIG> shows aspects of a state-of-the-art, single-axis LCSLM 26A. The skilled reader will appreciate that a state-of-the-art LC LCSLM includes one independently addressable electrode <NUM> per pixel element. A controllable voltage Vij is applied to this electrode via a thin-layer transistor (TFT) arranged at the intersection of every row i and column j of pixel elements. Vij controls the magnitude of the external electric field to which the LC molecules <NUM> of the pixel element are subject. The electric field has a variable magnitude but a fixed direction, which is the same for all pixel elements of the LCSLM array. Accordingly, the electric field orients the LC molecules of a given pixel element in one direction only, to a greater or lesser degree depending on the voltage applied to the electrode. This provides a variable phase delay of the polarization component aligned parallel to the major axis of the oriented LC molecules, which, again, is the same for every pixel element of the array.

In examples according to the claimed invention, a modified LCSLM has two or more independently addressable electrodes per pixel element. An example of such an LCSLM is described in <CIT>. As shown schematically in <FIG>, the two independently addressable electrodes <NUM> and <NUM> of LCSLM 26B are biased independently of the other by a pair of TFTs arranged at each intersection. A first controllable voltage Vij is applied to electrode <NUM>, and a second controllable voltage Uij is applied to electrode <NUM>. In this configuration, the average of Vij and Uij, relative to the voltage V<NUM> of common electrode <NUM> determines the magnitude of the electric field in the transmission direction, while the absolute difference between Vij and Uij determines the magnitude of the electric field in a direction substantially orthogonal to the transmission direction. When voltages Vij and Uij are applied, a controllable portion of the LC molecules align in the transmission direction, and another controllable portion in the orthogonal direction. This condition enables different amounts of phase delay to be imparted independently in two, substantially orthogonal directions. Moreover, these different amounts may be varied from one pixel to the next. In other configurations, independently addressable electrodes may be arranged such that the electric field that orients the LC molecules may have a controllably variable direction as well as a controllably variable magnitude.

In effect, the pixel configuration of modified LCSLM 26B enables each pixel to modulate the phase of the wavefront for X and Y polarization components independently. Thus, if the incoming wavefront is plane polarized, the modified LCSLM can variably rotate as well as variably retard each portion of the wavefront independently, controlled by the voltages Vij and Uij applied to the independently addressable electrodes. In particular, the average voltage applied to the electrodes affects the phase delay of a first component of the portion of the wavefront passing through the associated element, while the differential voltage Vij - Uij affects the phase of a non-parallel (e.g., orthogonal) second component of the portion of the wavefront.

As noted above, LCSLM 26B, may be used to achieve independent (i.e., uncorrelated) pixel-wise control of phase and polarization, which lends itself to operationally straightforward, parallel data writing. However, correlated control of these parameters is the basis of another useful data-writing mode. Moreover, correlated control may be achieved using a simpler LCSLM 26A, which provides only one depth-of-field per pixel.

To this end, LCSLM 26A may be programmed to simultaneously project two different, but interrelated holograms: one representing phase retardance for horizontal polarization, the other representing phase retardance for vertical polarization. The required programming may be enacted by the logic of encoder <NUM>, which is configured to execute a holographic-design algorithm to control modulation of the phase and polarization. Generally, the modifications recognize (a) that two different holograms are to be projected from each area of the LCSLM, for modulation of the two polarization states; (b) that each pixel of the LCSLM simultaneously modulates both phase and polarization; and (c) that there may be an infinite number of combinations of spot phases suitable for achieving the desired polarization at a given spot.

Suitable input for the holographic-design algorithm includes the position, polarization, and amplitude for each voxel of an imaged depth layer of the substrate. The algorithm transforms this holographic content into a matrix of scalar electric field components for X and Y polarizations. In doing so, the algorithm optimizes the voltage V(x,y) applied to each pixel element of the LCSLM to minimize the errors in polarization and phase (as no one pixel can provide independent control of polarization and phase).

<FIG> illustrates an example holographic-design method <NUM> based on a modified Gerchberg-Saxton (GS) algorithm. At <NUM> of method <NUM>, a target output plane is created. The target output plane consists of the amplitude of the electric field in the horizontal (|Eh_target|) and vertical (|Ev_target|) polarization states and the phase difference ΔθVH between the two polarization states. At <NUM> the vertical electric-field amplitude Ev is set equal to the target amplitude for the vertical polarization state, for a randomly selected phase. At <NUM> the horizontal electric-field amplitude Eh is set equal to the target amplitude for the horizontal polarization, with a phase less than Ev + ΔθVH. At <NUM> a hologram for the two polarization states is found using an inverse Fourier-transform function-viz. , Hh = IFT(Eh), Hv = IFT(Ev) where IFT stands for Inverse Fourier Transform At <NUM>, for each pixel of Hh and Hv, a refined pixel voltage V is found that minimizes the pixel error. At <NUM> the found holograms are refined to Hh' and Hv' using the selected voltages. At <NUM> the actual electric-field reconstruction of the two holograms, Eh' and Ev', is found using a Fourier transform (FT). At <NUM> a new phase and amplitude for Ev and Eh are selected using Eh', Ev', and ΔθVH. Execution of method <NUM> then returns to <NUM>.

<FIG> illustrates an example holographic-design method <NUM> based on a modified Direct Binary Search (DBS) or 'simulated-annealing' algorithm. At <NUM> of method <NUM>, the voltage matrix V(x,y) is set to a randomly selected value. At <NUM> is computed Hv = f(V) and Hh = f(V). At <NUM> is computed Eh and Ev using an FT. At <NUM> a merit function is computed. At <NUM> a random change to V(x,y) is made. At <NUM> are computed Hv and Hh. At <NUM> are computed Ev and Eh. At <NUM>, a new merit function is computed. At <NUM>, the random change made to V(x,y) is accepted or rejected in a probabilistic manner, based on whether the merit function has improved. Execution of algorithm <NUM> then returns to <NUM>.

Returning briefly to <FIG>, read head <NUM> of optical data storage and retrieval system <NUM> reads the data that has been stored on substrate <NUM> according to parameters supplied by read controller <NUM>. The read data is then passed to decoder <NUM>, which decodes and outputs the data to read buffer <NUM>, from which output stream <NUM> is made available.

<FIG> shows aspects of an example read head <NUM>. The read head includes a polarized optical probe <NUM> and an analyzer camera <NUM>. The polarized optical probe may include a low-power diode laser or other polarized light source. Read controller <NUM> is coupled operatively to the polarized optical probe and configured to control the angle of the polarization plane of emission of the polarized optical probe.

Analyzer camera <NUM> may include a high-resolution / high frame-rate CMOS or other suitable photodetector array. The analyzer camera is configured to image light from polarized optical probe <NUM>, after such light has interacted with the voxels of substrate <NUM>. Although <FIG> shows transmission of polarized light rays through the medium and on to the camera, the light rays may, in alternative configurations, reach the camera by reflection from the medium.

Each image frame acquired by analyzer camera <NUM> may include a plurality of component images captured simultaneously or in rapid succession. The analyzer camera may resolve, in corresponding pixel arrays of the component images, localized intensity in different polarization planes. To this end, the analyzer camera may include switchable or tunable polarization control in the form of a liquid-crystal retarder or Pockels cell, for example. In one particular example, four images of each target portion of substrate <NUM> are acquired in sequence by the analyzer camera as the polarized optical probe <NUM> is rotated through four different polarization angles. This process is akin to measuring basis vectors of a multi-dimensional vector, where here the 'vector' captures the birefringent properties of the voxels of the imaged target portion. In some examples, a background image is also acquired, which captures the distribution of sample-independent polarization noise in the component images.

In examples in which data is to be read from a plurality of layers of substrate <NUM>, read head <NUM> may include an adjustable collection lens system <NUM>. The adjustable collection lens system may collect light rays diffracted from a selected depth layer of the optical storage medium, and reject other light rays. In other implementations, lensless imaging based on interferometry may be employed.

In <FIG>, data decoder <NUM> is configured to receive the component images from analyzer camera <NUM> and to enact the image processing necessary to retrieve the data stored in substrate <NUM>. Such data may be decoded according to a machine-learned method and/or a canonical method in which an observable physical property is connected through one or more intermediates to the data read from the substrate.

The foregoing description and drawings should not be considered in a limiting sense, because numerous variations, extensions, and omissions are contemplated as well. For instance, while the LCSLMs described above are indeed suitable for modulating the phase and polarization of different portions of a wavefront by different amounts, and diffracting light from the different portions to a substrate, imaging optics of other types may also be suitable. Further, while optical data storage has been described in the context of an integrated read-write system (optical data storage and retrieval system <NUM>), this disclosure is also consonant with various write-only systems in which data is written to a data-storage substrate that may be removed from the system and read elsewhere.

In some embodiments, the methods and processes described herein may be tied to a computer system of one or more computing devices.

<FIG> schematically shows a non-limiting embodiment of a computer system <NUM> that can enact one or more of the methods and processes described above. Computer system <NUM> is shown in simplified form. Computer system <NUM> may take the form of one or more bench-top or server computers and/or dedicated electronic controllers. Encoder <NUM>, controllers <NUM> and <NUM>, and decoder <NUM> are examples of a computer system <NUM>.

Computer system <NUM> includes a logic processor <NUM> volatile memory <NUM>, and a non-volatile storage device <NUM>. Computer system <NUM> may optionally include a display subsystem <NUM>, input subsystem <NUM>, communication subsystem <NUM>, and/or other components not shown in <FIG>.

When included, input subsystem <NUM> may comprise or interface with one or more user-input devices such as a keyboard, mouse, touch screen, etc. When included, communication subsystem <NUM> may be configured to communicatively couple various computing devices described herein with each other, and with other devices. In some embodiments, the communication subsystem may allow computer system <NUM> to send and/or receive messages to and/or from other devices via a network such as the Internet.

Claim 1:
An optical data-storage system (<NUM>), comprising:
a laser (<NUM>) configured to emit a sub-picosecond pulsed wavefront having uniform phase and polarization;
an imaging optic (<NUM>) configured to modulate the phase and polarization of different portions of the wavefront by different amounts, and to diffract light from the different portions to a substrate (<NUM>) with writeable optical properties, the imaging optic being configured to modulate the different portions of the wavefront to different near-field polarizations and to image the light to an array of substrate voxels at different far-field polarizations, wherein a substrate voxel comprises a discrete locus of a substrate where an individual data value and/or symbol may be stored, and operatively coupled to the imaging optic, logic (<NUM>, <NUM>) configured to receive data (<NUM>) and to control modulation of the phase and polarization such that the light diffracted from the imaging optic (<NUM>) writes the data to the substrate,
characterized in that the imaging optic is a single liquid-crystal spatial light modulator, LCSLM, having an array of pixel elements configured to modulate the phase and polarization of different portions of the wavefront by different amounts, and to diffract the light from the different portions to the substrate, wherein the LCSLM includes two independently addressable electrodes per pixel element and is configured to modulate the phase independent of the polarization based on a first controllable voltage applied to a first electrode and a second controllable voltage applied to a second electrode per pixel element.