How to calibrate a system for reading birefringence data

Abstract

A method for calibrating a polarizing microscope includes placing an optical data storage medium on the sample stage of the polarizing microscope, wherein the optical data storage medium is a substrate having birefringent voxels embedded therein; measuring the background birefringence of the optical data storage medium; and adjusting the polarizing microscope to compensate for the background birefringence. By correcting the background birefringence, image contrast may be improved. After compensating for the background birefringence, further improvements may be achieved by optimizing the swing angle of the measurement state. A method for reading data from a birefringent optical data storage medium and a system useful for carrying out the method are also provided.

Description

[Background technology]

[0001] background There is a huge demand for data storage. Cloud storage providers are expected to need zettabyte-scale data storage capacity in the near future. One zettabyte is one trillion gigabytes (10⁶ bytes). 21 It is a byte. Much of the data needs to be stored for long periods of time.

[0002] Examples of widely used data storage technologies today include hard disk drives, magnetic tape, flash memory, and optical discs. All of these technologies have drawbacks, including the need to periodically copy data to replacement media. This is costly in terms of both 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, and repeated reading from a specific flash cell can cause failure of surrounding flash cells. Reflective materials used for data storage in optical media such as DVDs deteriorate over time.

[0004] To address these shortcomings, birefringent optical data storage media have been proposed. These media include a transparent 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 have optical properties that differ from those of the surrounding bulk substrate. In particular, voxels are birefringent, meaning they exhibit different refractive indices depending on the polarization and / or direction of the incident light. The optical properties of a voxel can be controlled at the time it is written to the substrate. These optical properties are used to encode data.

[0006] For more information on birefringent optical data storage media and their manufacture, see, for example, Anderson et al, Glass: A New Media for a New Era? 10th USENIX Workshop on Hot Topics in Storage and File Systems (HotStorage 18), 2018 and U.S. Patent No. 10,236,027B1. [Overview of the project] [Means for solving the problem]

[0007] overview One embodiment provides a method for calibrating a polarizing microscope. This method includes placing an optical data storage medium on the sample stage of a polarizing microscope, wherein the optical data storage medium is a substrate having birefringent voxels embedded therein; measuring the background birefringence of the optical data storage medium; and adjusting the polarizing microscope to compensate for the background birefringence.

[0008] Another embodiment provides a method for reading data from a birefringent optical data storage medium. This method includes positioning the birefringent optical data storage medium on a sample stage of a polarizing microscope, the birefringent optical data storage medium comprising a substrate having birefringent voxels embedded therein. The polarizing microscope is then calibrated by measuring the background birefringence of the optical data storage medium while the birefringent optical data storage medium is in the optical path of the polarizing microscope, the voxels being in the optical path of the polarizing microscope during the measurement, and by adjusting the polarizing microscope to compensate for the background birefringence. After the polarizing microscope is calibrated, it is then used to capture images of the voxels using elliptic polarization. The images are then processed using a processor to recover the data.

[0009] Another embodiment provides a system for reading data from a birefringent optical data storage medium. This system includes a light source for emitting light along an optical path; a polarization state generator located in the optical path, configured to receive light from the light source and output circularly polarized or elliptically polarized light in an input state; a sample stage configured to hold an optical data storage medium downstream of a polarization stage generator in the optical path; a polarization state analyzer located downstream of the sample stage in the optical path; a photodetector for detecting light in a detection state, located downstream of the polarization state analyzer in the optical path; and a controller including a processor and memory for storing computer executable instructions. When executed by the processor, the computer executable instructions cause the processor to control the system to measure the background birefringence of the optical data storage medium held by the sample stage and to adjust the polarization state generator and / or polarization state analyzer to compensate for the background birefringence.

[0010] This summary section is provided to briefly introduce some concepts that will be further detailed below in the detailed description section. This summary section is not intended to identify any major or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The claimed subject matter is not limited to any implementation that solves any or all of the defects described herein.

[0011] Brief explanation of the drawing To aid in understanding embodiments of this disclosure and to illustrate how such embodiments are carried out, the accompanying drawings are referenced as examples. [Brief explanation of the drawing]

[0012] [Figure 1] This is an illustrative cross-sectional view of a birefringent optical memory medium. [Figure 2] This is a plan view of a birefringent optical data storage medium. [Figure 3]A Poincaré sphere representing examples of five input states in the absence of background birefringence. [Figure 4] A Poincaré sphere representing the input states of FIG. 3 after interaction with background birefringence. [Figure 5] A flowchart outlining an exemplary method for calibrating a polarization microscope. [Figure 6] A Poincaré sphere representing an input state and annotated to explain the meaning of "swing". [Figure 7] A flowchart outlining an exemplary method for reading data from a birefringent optical data storage medium. [Figure 8] A block diagram showing the optical components of an exemplary system for reading data from a birefringent optical data storage medium. [Figure 9] A block diagram of a controller for controlling the system of FIG. 8.

Best Mode for Carrying Out the Invention

[0013] The drawings are schematic and not to scale. For ease of representation, the relative sizes of certain elements are exaggerated.

[0014] Detailed Description In this specification, for convenience of explanation, terms indicating directions such as "up", "down", "left", "right", "above", "below", "horizontal" and "vertical" are used to represent the directions shown in the related drawings. To avoid misunderstanding, this terminology is not intended to limit the orientation in an external reference frame.

[0015] This specification describes a method for calibrating a system for reading a birefringent optical data storage medium and a system for implementing the method.

[0016] First, referring to FIGS. 1 and 2, the structure of an exemplary birefringent optical data storage medium will be described. FIG. 1 is a schematic cross-sectional view of a multilayer optical data storage medium, and FIG. 2 is a schematic plan view of a multilayer optical data storage medium.

[0017] The data storage medium 100 includes a substrate 110. The substrate 110 may include glass, particularly fused silica. Fused silica is also called silica glass. Glass has excellent chemical stability, and data storage media based on glass substrates have an expected lifespan of several hundred years. Therefore, optical data storage media are useful for long-term data storage.

[0018] The substrate can have a thickness of up to 10 mm, optionally up to 5 mm, and optionally between 200 μm and 2 mm.

[0019] The shape of the substrate 110 is not particularly limited. Figures 1 and 2 show a rectangular parallelepiped substrate. Multiple voxels 120 are arranged on the substrate 110. A voxel is a discrete volume of the substrate that has been modified to have birefringence that it does not inherently possess. Voxels may be acquired by focusing a high-intensity laser beam, such as a femtosecond laser, on a location within the substrate 110. The laser causes a physical change in the structure of the substrate 110 at that location.

[0020] A voxel has a position that can be described by the coordinate set x, y, z. A voxel can exhibit different refractive indices for different polarizations. A voxel can cause a change in the polarization angle of light. A voxel has a linear retardance. Linear retardance is a measure of the magnitude of the shift between the orthogonal linear polarization components of light when light interacts with a voxel.

[0021] The position, retardation, and polarization angle changes can be controlled by modulating the polarization, energy, timing, and / or pulse count of the laser light used to write the voxel. Any combination of these properties of the voxel can be used to encode digital data. For example, a given voxel can store 1 to 4 bits of binary data.

[0022] If the data contains binary zeros, the zeros are encoded in voxels. In other words, zeros are not represented by the absence of voxels, but are explicitly written to the storage medium. The group of voxels storing the array of zeros is distinguishable from the blank substrate material. In the illustrated example, voxel 120 is arranged as a stack of layers. The stack includes an upper layer 130, a middle layer 132, and a bottom layer 134. In relation to the method discussed below, the “upper” layer is the layer closest to the illumination objective lens of the measurement system, and the “bottom” layer is the layer closest to the detection objective lens of the measurement system.

[0023] Voxels within a layer are separated from each other horizontally. Voxels can be arranged at equal intervals, for example, in a rectangular grid. Other arrangements are also possible, and the spacing between voxels is not necessarily equal.

[0024] There are an average of N voxels per unit area. The average lateral spacing between voxels is called the pitch p. The pitch p is related to the average number of voxels per unit area N by p = sqrt(1 / N). For example, the pitch can be less than 1 μm. Provided that the measurement system used can resolve individual voxels, a smaller pitch results in a higher data density that can be stored in the optical data storage medium. The method discussed below in this specification can improve contrast, thereby enabling the use of optical data storage mediums with smaller pitches.

[0025] Adjacent layers are spaced apart by a distance Λ. For example, Λ can be in the range of 5 to 20 μm. A layer of voxels can have a thickness t in the range of 1 to 5 μm. The ratio of the thickness of each voxel layer to the spacing between layers is called the duty cycle and is expressed as follows:

number

[0026] The duty cycle is not particularly limited and could be, for example, in the range of 15-25%, or arbitrarily set to around 20%.

[0027] The exemplary optical data storage medium shown in Figure 1 has three layers of voxels. In practice, the number of voxel layers is not particularly limited. An optical data storage medium may contain tens or hundreds of layers of voxels. For example, an optical data storage medium may contain 50 or more layers of voxels.

[0028] In the illustrated example, the voxel layers are planar. Other geometric shapes are also possible.

[0029] Figure 2 shows a plan view of the multilayer optical data recording medium 100. As shown, the voxels within the layers may be arranged within sectors 140a to 140f. A sector is a group of voxels that can be simultaneously imaged by an imaging system. In the illustrated example, each sector is a substantially planar 2D array of voxels arranged on the xy plane. The sectors may be separated from each other by blank regions of the substrate. A blank region is a region where no voxels are present.

[0030] In the illustrated example, the sectors are rectangular arrays. Other shapes are also possible. Optical data storage media may include vertical stacks of sectors. These vertical stacks of sectors are referred to herein as “tracks.”

[0031] Figure 2 further illustrates the field of view 150 of the wide-field imaging system. The wide-field imaging system captures images of multiple voxels in parallel, rather than reading one voxel at a time. For example, at least 100 μm. 2 Area, optionally selected: 125 μm 2 ~4mm 2 A field of view with an area within a certain range is considered "wide."

[0032] The imaging system's field of view can cover an area larger than that of a single sector. This allows for some error in the lateral positioning of the optical data storage medium relative to the imaging system, as sectors may remain readable even if they are not centered within the field of view.

[0033] When reading an optical data storage medium, a track may be selected by moving the imaging system and / or the storage medium to align the imaging system with the track. A sector within the track may be selected by changing the focus of the imaging system.

[0034] Tracks are separated laterally by blank regions. The lateral spacing between tracks can be chosen so that each image contains only one sector voxel. Recovering data from a single-sector image can be computationally less expensive compared to an image containing voxels from two or more sectors.

[0035] The optical data storage medium shown is transmissive. Reflective media are also possible. Reflective multilayer optical data storage media further include a reflective layer beneath the voxels.

[0036] Reading data from the storage medium 100 involves imaging the voxel 120 and processing the resulting image to determine the voxel's birefringence characteristics (i.e., azimuth and / or retardance). The birefringence characteristics encode the data stored in the storage medium 100. Therefore, the ability to recover the stored data depends heavily on the image quality.

[0037] It is desirable to provide an improved imaging method. Improved imaging may reduce the need for error correction, and therefore allow for a smaller percentage of parity bits to be written to the storage medium, potentially enabling higher data density storage. Improved imaging may extend the lifespan of the storage medium and / or the hardware used to read the storage medium. Improved imaging may enable voxel reading and / or writing using lower-cost components manufactured with relatively looser tolerances.

[0038] High-quality images have good contrast and appropriate brightness.

[0039] Contrast is a measure of the difference in brightness (i.e., the difference in signal intensity). An image has overall contrast, which corresponds to the difference in brightness between the brightest pixels in the image and the darkest pixels in the image. Voxel images also have "voxel contrast," that is, the difference in brightness between voxels encoding different symbols. When reading data from multilayer optical data storage media, it is particularly desirable to maximize voxel contrast.

[0040] Appropriate brightness is achieved when the intensity of light reaching the detector is adjusted to match the detector's dynamic range. If the intensity approaches the detector's lower detection limit, the signal-to-noise ratio of the image deteriorates. If the intensity is too high, the detector becomes oversaturated, resulting in information loss.

[0041] The quality of images captured by measurement systems such as polarizing microscopes depends on the calibration of the system.

[0042] Measuring the birefringence properties of a voxel often involves collecting multiple images of the voxel using light under different input conditions. The light under the "input conditions" has a specific polarization, typically elliptic polarization.

[0043] A method for determining birefringence based on measurements using two input states is disclosed in International Publication No. 2021 / 225706A1. A method for determining birefringence of a biological sample using three input states is disclosed in Shribak and Oldenbourg, Applied Optics, vol. 42, No. 16, pp. 3009-3017. Methods using four and five different input states are also disclosed.

[0044] When using multiple images, it is desirable to balance the image intensity (e.g., total camera count) so that all images have appropriate brightness and contrast. A set of two or more images is "balanced" if all images have sufficient voxel contrast to resolve the voxels. Total camera count provides a proxy measurement. For example, a set of two or more images may be considered "balanced" if the total camera counts for all images are within 10%, 5%, 2%, or 1% of each other. Balancing can be particularly difficult when images are acquired simultaneously.

[0045] The quality of images captured by optical measurement systems such as polarizing microscopes is affected by the calibration of the system.

[0046] The path from the light source to the detector may have varying amounts of attenuation. Variations in attenuation can occur, for example, due to changes in the characteristics of the components over time and / or variations in the mechanical alignment of the components. Techniques to compensate for these attenuation variations include changing the output of the light source, changing the sensitivity of the detector, and providing mechanical alignment compensation.

[0047] Optical measurement systems are also subject to unattenuated fluctuations, which degrade image contrast or signal-to-noise ratio. These unattenuated fluctuations cannot be fully compensated for by changing the light source output and detector sensitivity.

[0048] The inventors have found that the data storage medium itself can contribute to non-attenuated fluctuations. The macroscopic optical properties of the data storage medium may differ from those of the blank substrate, especially when the data storage medium contains a large number of voxel layers. In this context, "macroscopic" optical properties refer to the overall optical properties of a wide field of view through the data storage medium (e.g., at least 100 μm). 2 These are the characteristics of a field of view with a certain area. The optical properties of individual voxels can be called "microscopic" optical properties.

[0049] Light scattering reduces image contrast. Scattering converts polarized light to unpolarized light. Some of this unpolarized light inevitably reaches the detector, resulting in an undesirable background signal that is independent of polarization. Voxels in the medium can cause light scattering. Optical components of the measurement system can introduce false birefringence or polarization. The effects of optical components can be compensated for by calibrating the measurement system in air (in other words, without data storage medium or other samples). Calibration in air can be performed before implementing any of the methods provided herein, if necessary.

[0050] Furthermore, data storage media may also exhibit background birefringence, which can negatively affect the signal-to-noise ratio of measurements. While we do not wish to be bound by theory, stress birefringence caused by voxels is thought to contribute to background birefringence.

[0051] The effects of background birefringence will now be explained with reference to Figures 3 and 4. Figure 3 shows Poincaré spheres representing five exemplary input states before interaction with background birefringence. Figure 4 shows Poincaré spheres representing input states after interaction with background birefringence.

[0052] Voxels are read by detecting the change in polarization from elliptically polarized to elliptically polarized. Figure 3 shows the Stokes vectors for a right-circularly polarized input state P0 and four different elliptically polarized input states P1, P2, P3, and P4.

[0053] When polarization in the input state passes through a voxel, its polarization changes deterministically to produce transmitted light in the output state. This change in polarization can be represented as a rotation of the Stokes vector with respect to the output state compared to the input state. When reading the voxel, the measurement concerns the intensity of transmitted light with the selected polarization. Light with this polarization is said to be in the detected state. In this example, the detected state is left-circularly polarized L.

[0054] As mentioned earlier, the polarization changes caused by voxels are deterministic, and the properties of a voxel are controlled when it is written to it. Therefore, in an ideal case, by applying the assumption that no light scattering occurs, the optimal input and detection states can be analytically determined in advance based on the retardation and azimuthal angle values ​​selected when writing to the voxel. In reality, background birefringence shifts the observed azimuthal angle and retardation, and light scattering has been found to cause the optimal swing angles for the input and detection states to deviate from the ideal case.

[0055] Figure 4 shows the effect of background birefringence on the input state shown in Figure 3. (Litterdance δ) BG and azimuth angle θ BG Background birefringence with causes an undesirable rotation of the input state. This is visualized in Figure 4 as a rotation of the Stokes vector with respect to the input state in direction 402.

[0056] This effect is most clearly visible with respect to input state P0. State P0 was originally right-circularly polarized. Figure 4 shows that state P0 is elliptically polarized due to background birefringence. This is shown by the rotation of the Stokes vector of input state P0 toward direction 402. The Stokes vectors for input states P1, P2, P3, and P4 are also all similarly rotated toward direction 402 due to background birefringence.

[0057] Because the input states are shifted from their original polarization, the output states obtained after the interaction between the input states and the voxels also change. As a result, the amount of light in the detected state, and therefore receivable by the detector, differs from that in an ideal case. This can alter the brightness and / or contrast of the image. Contrast may be lost.

[0058] When using multiple detectors, a signal that would have reached the target detector in the absence of background birefringence may instead reach another detector.

[0059] This specification provides a method for calibrating a measurement system (e.g., a polarizing microscope) for reading birefringent optical data storage media. An exemplary implementation of this method is described here with reference to the flowchart shown in Figure 5.

[0060] In block 501, a birefringent optical data storage medium of the type described with reference to Figures 1 and 2 is placed on the sample stage of a polarizing microscope. The polarizing microscope may be a system as described below with reference to Figures 7 and 8. The polarizing microscope may be configured as or integrated with the read head of a system for reading the optical data storage medium.

[0061] The birefringent optical data storage medium is positioned so that light from the light source of the polarizing microscope passes through the voxels of the birefringent optical data storage medium.

[0062] In implementations of birefringent optical data storage media that include multiple voxel layers, light passes through all layers. In this case, the calibration of the reading system can benefit from incorporating the macroscopic effects resulting from propagation through the entire stack of layers.

[0063] In the comparative method, a blank substrate is used for calibration, or the system is focused on a blank area of ​​the data storage medium so that the light used for calibration does not pass through any voxels. The inventors have found that using a blank substrate or blank area for calibration makes it impossible to account for the effect of voxels on macroscopic optical properties. In block 502, the background birefringence of the optical data storage medium is measured.

[0064] The measurement of background birefringence involves focusing a polarizing microscope on a selected voxel layer. In an implementation where the optical data storage medium includes multiple voxel layers, the polarizing microscope may be focused on an intermediate layer 132. Light from the light source passes through several preceding layers of defocused voxels before reaching the selected layer, and after reaching the selected layer, passes through several subsequent layers before reaching the detector.

[0065] A polarizing microscope can be focused particularly on intermediate voxel layers. When an intermediate layer is selected, the number of preceding layers and the number of succeeding layers are either equal (if the stack has an odd number of layers) or exactly one different (if the stack has an even number of layers).

[0066] Selecting a mid-layer in the middle or near the middle of the stack may have the effect of averaging the contributions to background birefringence from preceding and succeeding layers. This may allow for better background correction.

[0067] A polarizing microscope has a field of view of 150, as shown in Figure 1. When measuring background birefringence, a wide field of view may be used so that the field of view encompasses multiple voxels within a selected layer.

[0068] By measuring background birefringence across the entire field of view encompassing multiple voxels, the effects of individual voxels are averaged. Voxels in the data storage medium are arranged substantially pseudo-randomly, encoding arbitrary data and therefore voxels encoding different symbols. Field of view 150 may have a region selected to encompass at least 10 voxels in a selected layer, at least 100 voxels (optionally), and at least 1000 voxels (optionally). As an example, the field of view of a polarizing microscope is at least 100 μm. 2 It may have an area of ​​.

[0069] The incident light used to measure background birefringence includes polarization in one or more different input states. The number of input states may be at least three, optionally at least four, and optionally at least five.

[0070] Background birefringence, i.e., background retardation and azimuthal angle, can be measured using a polarization state analyzer employing polarization measurement methods.

[0071] In implementations using multiple input states, background birefringence measurement may involve measuring the intensity of each light in the detected state received by the detector and calculating the background birefringence based on the difference between the measured intensities. Instead of changing the input state, the detected state may be changed.

[0072] When multiple input states are used, the data storage medium is illuminated sequentially with those input states. Multiple input states can be generated by a variable polarization state generator. In implementations where a single input state is used, a polarizing optical microscope includes multiple detectors, each configured to detect light in a different detection state. This can allow for the simultaneous detection of light in multiple different detection states. Alternatively, a single detector may be used with a variable polarization state analyzer, allowing different detection states to be examined sequentially.

[0073] After measuring background birefringence, in block 503, the optical components of the polarizing microscope are adjusted to compensate for background birefringence. In other words, the input and / or detection states of the polarizing microscope are adjusted to cancel out the effect of background birefringence.

[0074] Adjusting optical components may include correcting the polarization of the input state. Correcting the polarization of the input state may be called "pre-compensation."

[0075] As explained with reference to Figures 3 and 4, background birefringence shifts the polarization of incident light. Pre-compensation involves applying a correction to the input state to cancel out the effect of background birefringence.

[0076] In other words, the correction shifts the Stokes vector of the input state in the opposite direction to perturbation 402 shown in Figure 4. This causes background birefringence to shift the input state back toward the state desired for voxel reading.

[0077] The direction and magnitude of the correction can be selected to compensate for the effects of background birefringence on both the input state and the output state produced after the input state interacts with the voxel.

[0078] Optical components that are adjusted when performing pre-compensation may include polarization state generators. Alternatively or additionally, any other components capable of modifying the input state may also be used. Components capable of modifying the polarization state may generally be referred to as “adaptive optics.”

[0079] Adjusting the optical components may include correcting the detection state. Correcting the detection state may be called "ex post-compensation."

[0080] The output state generated when polarization passes through a voxel is shifted from the expected output state due to background birefringence. Post-compensation corrects this shift by modifying the detection state to more precisely correspond to the shifted output state.

[0081] The optical components adjusted when performing post-compensation may be polarization state analyzers. Alternative or additional adaptive optics may be used.

[0082] Adjustment may involve adjusting two or more optical components of a polarizing microscope. In some implementations, both pre- and post-compensation may be performed together.

[0083] In a measurement system implementation that includes a single illumination arm and multiple detection arms, using only pre-compensation can reduce hardware complexity. Instead of providing a compensatory optical system for each of the multiple detection arms, the compensatory optical system can be provided only for the illumination arm.

[0084] Similarly, if the measurement system has a single detection arm, the use of post-compensation only may be advantageous.

[0085] This method may further include capturing an image of the voxels in block 504 and determining whether the image satisfies one or more constraints. The operation in block 504 is optional; that is, the method may terminate after block 503 or proceed directly to block 505.

[0086] One or more constraints may include a predetermined minimum level of contrast.

[0087] Determining whether an image satisfies one or more constraints may include determining whether the pixels in the image have a brightness within a given range.

[0088] A group of images may be captured using different input or output states. In such an implementation, block 504 may include determining whether the group of images satisfies one or more constraints. One or more constraints may include a constraint on the difference in overall measured intensity between the images.

[0089] In response to a determination that one or more constraints are not met, further calibration rounds may be performed by repeating the operations in blocks 503 and 504.

[0090] In response to a determination that one or more constraints are met, the method may terminate or proceed to an optional block 505.

[0091] In block 505, the optical components of the polarizing microscope are adjusted to optimize the measurement state in order to compensate for light scattering. The measurement state can be either an input state or a detection state.

[0092] In implementations where multiple different input states are used, adjustments may be applied to each different input state. Similarly, in implementations where multiple different detection states are used, adjustments may be applied to each different detection state.

[0093] Optimization of the measurement state may include optimizing the swing angle χ of the measurement state. Optimization of the swing angle includes selecting a swing value to maximize the measurement sensitivity. Sensitivity is maximized when the contrast in the image is at a maximum.

[0094] To explain what is meant by "swing angle", an annotated Poincaré sphere 600 is shown in FIG. 6.

[0095] The Poincaré sphere represents the polarization state of light by mapping the last three components of the 4D Stokes vector to a 3D orthogonal coordinate system.

[0096] In the case of fully polarized light, a particular measurement state is represented by a particular position on the surface of the sphere that represents the particular polarization state of the light. Partially polarized light is represented by a point inside the surface of the Poincaré sphere.

[0097] The north pole represents right circular polarization (RCP). The south pole represents left circular polarization (LCP). The states on the equator are linearly polarized light with an angle defined by the azimuth angle on the sphere.

[0098] A general point on the sphere is elliptically polarized, and the ellipticity is determined by the angle between the state and the pole, and the azimuth angle is defined by the angle around the sphere.

[0099] Exemplary polarization states are shown at 602 on the sphere. The dashed curves are circles of constant latitude and correspond to polarization states having the same ellipticity.

[0100] The swing χ represents the angle 604 of the measurement state with respect to the pole of the sphere. The horizontal angle is the azimuth angle 606.

[0101] After the light in the input state passes through the voxel, the intensity I of the signal received by the detector is given by the following equation. I = 1 - cos χ cos δ - sin χ sin δ sin(2(θ - ν)) + I min Here, χ and ν represent the polar angle and longitude angle of the swing state, respectively, and δ and θ represent the voxel retardance and azimuth angle, respectively. min This is the depolarization component. In a high-quality polarizing microscope, the degree of polarization of the light produced by the microscope approaches 1, and the depolarization component is dominated by scattering-induced depolarization light from the sample.

[0102] Contrast C is defined as follows:

number

[0103] The maximum relative intensity is given by the following formula: max(I)=1-cosχcosδ+sinχsinδ+I min

[0104] The minimum relative intensity is as follows: min(I)=1-cosχcosδ-sinχsinδ+I min

[0105] Substituting these definitions into the equation for C and simplifying it, we get the following:

number

[0106] To find the maximum contrast C, the derivative

number

number

number

number

[0107] Therefore, the optimal swing value for the measurement conditions is given by the inverse cosine of the degree of polarization (DOP).

[0108] Optimizing the swing angle of the measurement state in this way is particularly useful when the data storage medium contains a large number of voxel layers, for example, 50 or more. As the number of voxel layers increases, scattering becomes more pronounced.

[0109] The above derivation was explained using the input state as an example. It should be understood that, due to symmetry, the same result is obtained even if the detected state is considered instead. The optimal swing angle with respect to the detected state is the inverse cosine of the polarization degree.

[0110] Therefore, optimizing the measurement state may involve determining the degree of polarization of the light transmitted through the optical data storage medium and adjusting the optical components of the optical microscope to set the swing angle of the measurement state to the inverse cosine of the degree of polarization. When the measurement state is an input state, the optical component to be adjusted may be a polarization state generator. When the measurement state is a detection state, the optical component to be adjusted may be a polarization state analyzer.

[0111] The above example discusses a single measurement state. As is understood, voxel imaging often involves the use of multiple measurement states. Optimization can be applied to multiple measurement states.

[0112] For example, multiple input states may be used. The swing angle of each input state in the multiple input states may be set to the inverse cosine of the DOP. The multiple input states may include, for example, two to five different input states.

[0113] Multiple detection states may be used. In such an implementation, the swing angle of each detection state may be set to the inverse cosine of the DOP. The multiple detection states may include, for example, two to five different detection states.

[0114] Multiple input states can be used in combination with a single detection state. Alternatively, a single input state can be used in combination with multiple detection states. A further possibility is to use multiple input states and multiple detection states.

[0115] Optimization can be applied to the input state only or to the detection state only. A further possibility is to apply optimization to both the input and detection states. If the swing angle of the measurement state is first attempted without correcting for background birefringence, the contrast across the set of images may decrease rather than increase (in other words, the image balance may worsen). In the worst case, the contrast of one of the images may be lost.

[0116] The calibration method discussed with reference to Figure 5 is useful in relation to methods for reading data from a birefringent optical data storage medium. Here, an exemplary method for reading data is described with reference to Figure 7, which is a flowchart outlining the method. In block 701, the polarizing microscope is calibrated according to the method in Figure 5. Subsequently, in block 702, images of voxels are captured using the polarizing microscope. This operation may include capturing multiple images of voxels using light under different input conditions. For example, two, three, four, five, or six or more images of a voxel may be captured.

[0117] Typically, images of multiple voxels are captured. For example, images of one or more sectors 140, as shown in Figure 2, may be captured. Voxels can be imaged as a group using a wide-field detector, or individually using a point detector.

[0118] Next, in block 703, the image is processed to recover the data encoded by voxels. Various computer vision techniques may be used to recover the stored binary data. Image processing may include the use of supplementary data, such as predetermined scale and offset values, as discussed, for example, in International Publication No. 2021 / 225706A1. The use of supplementary data may allow for the use of fewer different input states.

[0119] Data recovery may include performing error checking and / or using parity data to recover unreadable portions of the data.

[0120] When reading data from two or more data storage media, the calibration operation 701 may be repeated as appropriate. For example, a polarizing microscope may be recalibrated after a predetermined time interval.

[0121] If the second data storage medium is different from the first optical data storage medium used for initial calibration, calibration may be repeated on the second data storage medium. The first and second data storage mediums may differ in terms of years of use, substrate material, and the method used to write the voxels.

[0122] The improvements in measurement accuracy achievable by the calibration methods described herein may enable more reliable recovery of data from data storage media.

[0123] The comparison method computationally compensates for the effects of background birefringence. More specifically, the comparison method corrects background retardation by subtracting an auxiliary image. This technique requires capturing the auxiliary image while the birefringent voxel holding the data is outside the detector's field of view.

[0124] Capturing auxiliary images, in other words, performing additional sampling on an optical data storage medium, limits the maximum rate at which data can be read from the medium. The method provided herein can eliminate the need to capture auxiliary images and thus can enable improved data throughput.

[0125] Furthermore, the comparison method cannot compensate for background birefringence caused by the presence of voxel layers in the medium. The inventors have found that the effect of voxels is particularly pronounced when many voxel layers are present.

[0126] An exemplary system for reading data from a birefringent optical data storage medium is described with reference to Figures 8 and 9. Figure 8 is a schematic block diagram of the optical components of the system. Figure 9 is a schematic block diagram of a controller useful for controlling the operation of the optical components.

[0127] The system 800 includes, in order, a light source 810, a polarization state generator ("PSG") 820, a capacitor 830, a sample stage (occupied in this example by a birefringent optical data storage medium 840), a detection objective lens 850, a polarization state analyzer ("PSA") 860, a lens 870, and a detector 880.

[0128] The components located upstream of the sample stage on the optical path 812, namely the light source 810, PSG 820, and condenser 830, may collectively be called the illumination arm 802. The components located downstream of the sample stage, namely the detection objective lens 850, PSA 860, lens 870, and detector 880, may collectively be called the detection arm 704.

[0129] The light source 810 is configured to emit light along the optical path 812. The properties of the light source 810 are not particularly limited. The light source may produce unpolarized light. The light source may be, for example, an LED light source or a laser light source.

[0130] The polarization state generator 820 is positioned on the optical path 812. The PSG 820 receives light from the light source 810 and is configured to provide polarization in one or more input states. In the illustrated example, the PSG 820 includes a linear polarizer 822 and a quarter-wave plate 824 arranged in series.

[0131] The PSG820 may be configured to allow modification of the input state when performing pre-compensation as described, for example, with reference to block 503 in Figure 5. For example, the linear polarizer 822 and the quarter-wave plate 824 may be configured to rotate relative to each other. For this purpose, one or both of the linear polarizer 822 and the quarter-wave plate 824 may be mounted on a rotating mount.

[0132] The polarization state generator may, as an alternative, include a liquid crystal variable retarder. The polarization state generated by the liquid crystal variable retarder can be modified by applying an electrical signal to the liquid crystal.

[0133] Capacitor 830 is positioned downstream of polarization state generator 820 on the optical path 812. Capacitor 830 is configured to focus the polarization received from the polarization state generator onto the optical data storage medium 840, providing uniform illumination within the detector's field of view.

[0134] The system further includes a sample stage configured to hold an optical data storage medium 840 downstream of the illumination objective lens 830 in the optical path 812. Any suitable support structure may be used. The sample stage may be configured to allow the optical data storage medium 840 to move relative to the detection objective lens 850.

[0135] The sample stage may be configured to move the optical data storage medium in the z-direction to focus on voxels in different layers.

[0136] The detection objective lens 850 is positioned downstream of the sample stage on the optical path 813. The detection objective lens receives light that has passed through the data storage medium 840 and focuses that light on the polarization state analyzer 860.

[0137] The polarization state analyzer 860 in this example is structurally similar to the polarization state generator 820 and includes a quarter-wave plate 862 and a linear polarizer 864 arranged in series. The polarization state analyzer 860 receives light that has passed through the data storage medium 840. The polarization state analyzer 860 selectively transmits light in a specific polarization state, referred to herein as the detected state, to the detector 880 via the lens 870.

[0138] The polarization state analyzer 860 may be configured to allow the detection state to be changed. For example, the quarter-wave plate 862 and the linear polarizer 864 may be mounted such that the quarter-wave plate 862 and the linear polarizer 864 are rotatable relative to each other.

[0139] Alternative polarization state analyzers may also be used. For example, a polarization state analyzer may include a liquid crystal variable retarder.

[0140] System 800 further includes a detector 880 and a lens 870. The lens 870 receives light in a detected state from a polarization state analyzer and focuses the light onto the detector 880. The detector 880 may include, for example, a CMOS image sensor or other suitable photodetector array capable of imaging an entire field of view on a focal plane positioned in an optical data storage medium, thereby imaging multiple arranged voxels in the same image. Alternatively, the detector 880 may be a point detector that can be used to construct an image point by point. Examples of point detectors include photodiodes, phototransistors, and single-photon avalanche diodes ("SPADs").

[0141] Various modifications can be made to the optical components of the example system.

[0142] The exemplary system includes both PSG and PSA, and is therefore configured to enable both pre- and post-compensation. To provide a system configured to enable either pre- or post-compensation, one of the PSG or PSA may be replaced by a fixed polarizing filter.

[0143] The exemplary system may be modified, for example, to include additional optical components to correct the optical path.

[0144] The detection objective lens 850 and / or lens 870 may be omitted in some implementations.

[0145] The system shown in Figure 8 includes one detector 880 and one polarization state analyzer 860. Variations of the system may include multiple detectors; for example, there may be two to five detectors.

[0146] In such a modified configuration, the system may further include a beam splitter configured to split the light received from the detection objective lens 850 into multiple beams and direct each beam to its respective detector. Each detector may be associated with its own PSA or polarizing filter. Each PSA or polarizing filter may be configured to transmit light in different detection states to the associated detector.

[0147] Several techniques for recovering data from voxels utilize multiple images (e.g., 2-5 images) of the voxel measured using different detection states. By providing a system with multiple corresponding detectors, it may be possible to acquire images in parallel, thereby improving throughput.

[0148] The system, which includes multiple detectors, can utilize pre-compensation, and the polarization state is adjusted by the illumination arm 802. This allows each detector to be associated with a fixed polarization filter rather than a variable PSA, thus reducing the hardware cost related to the detection arm 804.

[0149] Instead of providing multiple detectors, or in addition to them, the illumination arm 702 includes multiple PSGs to provide their respective input states. Each PSG may be associated with a different light source, or a single light source may provide light to multiple PSGs by using one or more beam splitters.

[0150] System 800 further includes controllers for controlling the operation of various optical components in order to carry out the method described herein. A block diagram of an exemplary controller 900 is provided in Figure 9.

[0151] The controller 900 includes a processing unit 910 and a data storage device 920.

[0152] The processing unit 910 includes one or more processing units mounted within one or more dies, IC (integrated circuit) packages and / or housings located in one or more geographical locations.

[0153] Each of the processing units may take any appropriate form, such as a dedicated form of a coprocessor or accelerator processor, including a general-purpose central processing unit (CPU), a graphics processing unit (GPU), or a digital signal processor (DSP). Each of the processing units may include one or more cores.

[0154] The processing unit may include one or more digital processing units in addition to one or more analog processing units. One or more analog processing units may be configured to perform analog signal processing, such as electronic filtering or signal conditioning. The analog processing units may be useful for processing signals from an autofocus system.

[0155] When we say that a computer program is executed on a processing unit, this may mean that it is executed by any one or more processing units that make up the processing unit 910.

[0156] The processing unit 910 may further include working memory such as random access memory and / or one or more memory caches within one or more processing units.

[0157] The data storage device 920 includes one or more memory units implemented in one or more memory media within one or more housings located in one or more geographical locations. The data storage device 920 stores computer executable instructions 922, which, when executed by the processing unit 910, cause the system to perform operations, such as those described with reference to Figure 5 or Figure 7.

[0158] The data storage device 920 may further store representations of one or more characteristics of the optical data storage medium to be read using the system. Such storage of characteristics may enable reading data from voxels using fewer measurements, as described in International Publication No. 2021 / 225706A1.

[0159] Each of the memory units may employ any suitable storage medium known in the art, such as a magnetic storage medium like a hard disk drive or a magnetic tape drive, or an electronic storage medium like a solid-state drive, flash memory, or electrically erasable programmable read-only memory.

[0160] The controller 900 is operably connected to the optical components of the system 800, particularly the detector 980, and at least one of the PSG820 and PSA860. [Examples]

[0161] Examples The polarization state analyzer of the measurement system shown in Figure 8 was initially calibrated in air to detect left-hand circular polarization. The polarization state analyzer included a fixed waveplate and a linear polarizer in a rotating mount, and the linear polarizer could be rotated to change the detection to any desired state.

[0162] To detect left-circular polarization, the linear polarizer and quarter-wave plate were configured as follows.

number

[0163] The measurement system included a wide-field detector for simultaneously imaging multiple voxels. More specifically, in this example, the measurement system had a square field of view with sides of 170 μm in length.

[0164] The birefringent optical data storage medium shown in Figures 1 and 2 was placed in the measurement system. The substrate of the optical data storage medium was a glass substrate.

[0165] The intensity of five different input polarization states was measured after passing through the region of interest of the optical data storage medium and then through a polarization state analyzer.

[0166] Input polarization state 0 was defined as right-circular polarization. Input polarization states 1 to 4 were defined as various different elliptically polarized states. The input states were assumed to have equal intensity.

[0167] The measurement results are shown in Table 1.

[0168] [Table 1]

[0169] A significant signal was received when using input polarization state 0. The intensity of right-circular polarization reaching the polarization state analyzer, configured to allow detection of left-circular polarization, is ideally 0. Furthermore, the intensity of the received signal varied significantly for input states 1-4. For example, the measurement for state 2 yielded approximately five times the intensity of state 1. Therefore, Table 1 indicates that the measurement system was not calibrated.

[0170] Using the measured intensity, the background retardation δ was calculated using the algorithm described by Shribak and Oldenbourg (Appl. Opt. 42, 3009-3017). BG and azimuth angle θ BG The following calculations were performed. The sampled region was identified to have a background retardation of 29 nm and an azimuth angle of 145°.

[0171] The detection state was adjusted based on the background retardation. In this example, the polarization state analyzer was adjusted so that the detection state was the opposite pole of perturbation input state 0 on the Poincaré sphere. More specifically, the linear polarizer and quarter-wave plate were adjusted as follows:

number

number

[0172] If a PSA including a liquid crystal variable retarder is used instead, the voltage is -π+δ BG The latitude angle, and

number

[0173] Intensity measurements were repeated using the adjusted detection state. The results are listed in Table 2.

[0174] [Table 2]

[0175] The measured intensity for input state 0 decreased significantly, thus approaching the ideal case. After adjustment, the signal intensity for states 1-4 changed by a maximum of only 2%.

[0176] It is expected that further improvements can be made to the signal balancing between detectors by calculating residual background birefringence based on the intensity values ​​measured after one optimization round.

[0177] It should be understood that the embodiments described above are for illustrative purposes only. More generally, according to one aspect disclosed herein, a method for calibrating a polarizing microscope is provided. This method includes placing an optical data storage medium on the sample stage of a polarizing microscope, wherein the optical data storage medium is a substrate having birefringent voxels embedded therein; measuring the background birefringence of the optical data storage medium; and adjusting the polarizing microscope to compensate for the background birefringence. The background birefringence can be measured using a polarizing microscope. It has been found that the presence of the optical data storage medium results in background birefringence. By compensating for background birefringence, it may be possible to capture images with improved contrast.

[0178] This method may further include measuring the residual background birefringence of the optical data storage medium and adjusting the polarizing microscope to compensate for the residual birefringence. Repeating this calibration may allow for more effective compensation of the background birefringence.

[0179] This method may further involve adjusting the polarizing microscope to compensate for background birefringence, and then optimizing the swing angle χ of the polarizing microscope's measurement state to compensate for light scattering. Optimizing the swing angle after compensating for background birefringence has been shown to allow for improved voxel contrast.

[0180] Optimization can be applied to multiple measurement states. The swing angle of each of the multiple input states can be optimized. Alternatively or additionally, the swing angle of each of the multiple detection states can be optimized.

[0181] Adjusting the swing angle χ of the measurement state may involve determining the degree of polarization of the light transmitted through the voxels of the optical data storage medium and adjusting the polarizing microscope so that the swing angle of the measurement state is equal to the inverse cosine of the degree of polarization.

[0182] Calibrating a polarizing microscope to compensate for background birefringence may include performing pre-compensation. Pre-compensation involves calibrating the polarizing microscope to adjust the input states generated by the polarizing microscope so as to counteract the polarization shift of the input states caused by background birefringence. Pre-compensation may also include calibrating the polarization state generator of the polarizing microscope.

[0183] Calibrating a polarizing microscope to compensate for background birefringence may include performing post-compensation, which involves calibrating the polarizing microscope to adjust the detection state of the polarizing microscope so as to offset the polarization shift of the input state caused by background birefringence. Post-compensation may also include calibrating the polarization state analyzer of the polarizing microscope.

[0184] Background birefringence can be measured across the field of view of a polarizing microscope, which encompasses multiple voxels. In this way, the effects of individual voxels can be averaged out.

[0185] Determining background birefringence may involve illuminating the field of view in an optical data storage medium with light in the input state, transmitting the light in the input state through the optical data storage medium, measuring the intensity of each light in two or more different detection states, and calculating the background birefringence using a data processor based on the difference in these intensities. Calibrating a polarizing microscope to compensate for background birefringence may involve modifying two or more different detection states and / or input states to balance the respective measured intensities.

[0186] Alternatively, measuring background birefringence may involve illuminating the field of view in an optical data storage medium with light under two or more different input conditions, measuring the intensity of each light in the detected state transmitted through the optical data storage medium for two or more different input conditions, and calculating the background birefringence using a data processor based on the difference in these intensities. Calibrating a polarizing microscope to compensate for background birefringence may involve modifying two or more different input conditions and / or the detected state to balance the respective measured intensities.

[0187] Birefringent voxels may be arranged as a stack of layers, the stack including an upper layer, a middle layer, and a bottom layer. A polarizing microscope may be focused on the middle layer during background birefringence measurement. The stack may contain at least 50 birefringent voxels. Focusing on the middle layer of voxels may allow for more effective correction of background birefringence by averaging the effects of the upper and lower voxel layers of the selected layer.

[0188] In another embodiment, a method is provided for reading data from a birefringent optical data storage medium. This method includes positioning the birefringent optical data storage medium on a sample stage of a polarizing microscope, the birefringent optical data storage medium comprising a substrate having birefringent voxels embedded therein. The polarizing microscope is then calibrated by measuring the background birefringence of the optical data storage medium while the birefringent optical data storage medium is in the optical path of the polarizing microscope, the voxels being in the optical path of the polarizing microscope during the measurement, and by adjusting the polarizing microscope to compensate for the background birefringence. After the polarizing microscope is calibrated, it is then used to capture an image of the voxels using elliptic polarization. The image is then processed using a processor to recover the data. By applying the calibration method provided herein, it may be possible to obtain an image with improved contrast. This may enable more reliable data recovery.

[0189] As can be understood, calibrating a polarizing microscope may include performing the methods described with reference to the preceding embodiments.

[0190] For example, calibrating a polarizing microscope may further involve adjusting the polarizing microscope to compensate for background birefringence, and then optimizing the swing angle χ of the polarizing microscope's measurement state.

[0191] Optimizing the swing angle may involve determining the degree of polarization of light transmitted through the voxels of the optical data storage medium and adjusting the polarizing microscope so that the swing angle of the measurement state is equal to the inverse cosine of the degree of polarization. The measurement state may be a detection state. The measurement state may be an input state.

[0192] Optical data storage media may have multiple birefringent voxel layers. These layers may be present in the optical path during background birefringence measurements. When multiple voxel layers are present, the effects of light scattering and background birefringence can become more pronounced.

[0193] Calibrating a polarizing microscope involves adjusting the polarizing microscope to compensate for background birefringence, and then measuring the residual background birefringence, wherein the voxels are within the optical path of the polarizing microscope during the measurement, and may further include measuring and further adjusting the polarizing microscope to compensate for residual background birefringence.

[0194] Image capture may involve capturing multiple images using light under multiple, each different measurement conditions. Each image may be captured using a different input condition. Each image may be captured using a different detection condition.

[0195] Another embodiment provides a system for reading data from a birefringent optical data storage medium. This system includes a light source for emitting light along an optical path, a polarization state generator on the optical path configured to receive light from the light source and output circularly polarized or elliptically polarized light in an input state, a sample stage configured to hold an optical data storage medium downstream of a polarization stage generator on the optical path, a polarization state analyzer downstream of the sample stage on the optical path, a photodetector for detecting light in a detection state, located downstream of the polarization state analyzer on the optical path, and a controller including a processor and memory for storing computer executable instructions. When executed by the processor, the computer executable instructions cause the processor to control the system to measure the background birefringence of the optical data storage medium held by the sample stage and to adjust the polarization state generator and / or polarization state analyzer to compensate for the background birefringence.

[0196] This system may also be called a polarizing microscope. This system may be used to carry out the methods described herein.

[0197] The computer executable instructions may optionally cause the processor to perform the method for calibrating the polarizing microscope described above as part of the method for reading data from the birefringent optical data storage medium described above. The system may implement any combination of any of the features of these methods.

[0198] The memory may store computer-executable instructions, which, when executed by the processor, cause the processor to optimize the swing angle χ of the input or detected state after adjusting the polarization state generator and / or polarization state analyzer to compensate for background birefringence.

[0199] Optimizing the swing angle may involve determining the degree of polarization (DOP) of light transmitted through the voxels of an optical data storage medium, and adjusting the polarization state generator so that the swing angle of the input state is equal to the inverse cosine of the degree of polarization, or adjusting the polarization state analyzer so that the swing angle of the detected state is equal to the inverse cosine of the degree of polarization.

[0200] The memory may store computer-executable instructions, which, when executed by the processor, cause the processor to control the system to measure residual background birefringence and to adjust the polarization state generator and / or polarization state analyzer to compensate for background birefringence.

[0201] The system may further include a beam splitter downstream of the sample stage for dividing the optical path into two or more branches. Each of the two or more branches may include its own polarization state analyzer and its own detector for detecting light in its respective detection state.

[0202] This disclosure provides the following terms:

[0203] Clause 1. A method for calibrating a polarizing microscope, comprising: placing an optical data storage medium on the sample stage of the polarizing microscope, wherein the optical data storage medium is a substrate having birefringent voxels embedded therein; measuring the background birefringence of the optical data storage medium; and adjusting the polarizing microscope to compensate for the background birefringence.

[0204] Clause 2. The method according to Clause 1, further comprising performing calibration of the polarizing microscope in air before placing the optical data storage medium on the sample stage.

[0205] Clause 3. The method according to Clause 1 or 2, further comprising optimizing the swing angle χ of the polarizing microscope's measurement state to compensate for light scattering after adjusting the polarizing microscope to compensate for background birefringence.

[0206] Clause 4. Adjusting the swing angle χ in the measurement state is, Determining the degree of polarization (DOP) of light transmitted through the voxels of an optical data storage medium, Adjust the polarizing microscope so that the swing angle of the measurement state is equal to the inverse cosine of the degree of polarization. The method described in Article 3, including the method described in Article 3.

[0207] Clause 5. The measurement state is the detection state, as described in Clause 3 or 4.

[0208] Clause 6. The method according to any one of Clauses 3 to 5, including optimizing the swing angle χ of multiple measurement states of a polarizing microscope to compensate for light scattering.

[0209] Clause 7. Multiple measurement states include multiple input states as described in Clause 6.

[0210] Clause 8. Multiple input states include 2 to 5 different input states as described in Clause 7.

[0211] Clause 9. Multiple measurement states include multiple detection states as described in any one of Clauses 6 to 8.

[0212] Clause 8. Multiple measurement states include 2 to 5 different measurement states as described in Clause 7.

[0213] Clause 9. Calibrating a polarizing microscope to compensate for background birefringence includes performing pre-compensation, the method of any one of Clauses 1 to 8, which includes calibrating the polarizing microscope to adjust the input state produced by the polarizing microscope to offset the polarization shift of the input state caused by background birefringence.

[0214] Clause 10. Prior compensation is the method described in Clause 9, including adjusting the polarization state generator of a polarizing microscope.

[0215] Clause 11. Calibrating a polarizing microscope to compensate for background birefringence includes performing post-compensation, the method according to any one of Clauses 1 to 10, wherein post-compensation includes calibrating the polarizing microscope to adjust the detection state of the polarizing microscope so as to offset the polarization shift of the input state caused by background birefringence.

[0216] Clause 12. Post-incident compensation is the method described in Clause 11, including adjusting the polarization state analyzer of a polarizing microscope.

[0217] Clause 13. Background birefringence is measured across the field of view of a polarizing microscope, the field of view encompassing multiple voxels, as described in any one of Clauses 1 to 12.

[0218] Article 14. Determining background birefringence is: Illuminating the field of view within an optical data storage medium with light in the input state, After light in the input state has passed through an optical data storage medium, the intensity of the light in two or more different detection states is measured. Calculate background birefringence using a data processor based on the difference in intensity. The method described in any one of the clauses 1 to 13, including the method described in any one of the clauses 1 to 13.

[0219] Clause 15. Calibrating a polarizing microscope to compensate for background birefringence, the method of Clause 14, which includes correcting two or more different detection and / or input conditions to balance the respective measured intensities.

[0220] Article 16. Determining background birefringence is: Illuminating the field of view within an optical data storage medium with light under two or more input conditions. For two or more different input states, the intensity of light transmitted through an optical data storage medium in each detection state is measured. Calculate background birefringence using a data processor based on the difference in intensity. The method described in any one of the clauses 1 to 15, including the method described in any one of the clauses 1 to 15.

[0221] Clause 17. Calibrating a polarizing microscope to compensate for background birefringence, the method of Clause 16, which includes correcting two or more different input and / or detection states to balance the respective measured intensities.

[0222] Clause 18. The method according to any one of Clauses 1 to 17, wherein the birefringent voxels are arranged in a stack of layers, the stack comprising an upper layer, an intermediate layer and a bottom layer.

[0223] Clause 19. During background birefringence measurement, the polarizing microscope is focused on the intermediate layer, as described in Clause 18.

[0224] Clause 20. The stack comprises at least 50 layers of birefringent voxels, as described in Clause 18 or 19.

[0225] Clause 21. A polarizing microscope is a system as defined in any one of Clauses 30 to 34, as described in any one of Clauses 1 to 20.

[0226] Article 22. A method for reading data from a birefringent optical data storage medium, The process involves placing or positioning a birefringent optical data storage medium on the sample stage of a polarizing microscope, wherein the birefringent optical data storage medium is a substrate, including a substrate having birefringent voxels embedded therein, and positioning. With the birefringent optical data storage medium located within the optical path of a polarizing microscope, The measurement involves measuring the background birefringence of an optical data storage medium, wherein the voxel is within the optical path of a polarizing microscope during the measurement. Adjusting the polarizing microscope to compensate for background birefringence and Calibrating the polarizing microscope by, Next, using a polarizing microscope, capture at least one image of the voxel using elliptic polarization, Using a processor, process at least one image to recover the data. A method that includes this.

[0227] Clause 23. Calibrating a polarizing microscope is the method described in Clause 22, which includes performing the method defined in any one of Clauses 1 to 20.

[0228] Clause 24. A polarizing microscope is a system as defined in any one of Clauses 30 to 34, as described in Clause 22 or 23.

[0229] Clause 25. Calibrating a polarizing microscope is Determining the degree of polarization (DOP) of light transmitted through the voxels of an optical data storage medium, Adjust the polarizing microscope so that the swing angle of the measurement state is equal to the inverse cosine of the degree of polarization. The method according to any one of the clauses 22 to 24, further comprising adjusting the polarizing microscope to compensate for background birefringence, and then optimizing the swing angle χ of the measurement state generated by the polarizing microscope.

[0230] Clause 26. The optical data storage medium has multiple layers of birefringent voxels, the multiple layers are in the optical path during background birefringence measurement, as described in any one of Clauses 22 to 25.

[0231] Clause 27. Calibrating a polarizing microscope involves adjusting the polarizing microscope to compensate for background birefringence. The measurement involves measuring residual background birefringence, where the voxel is within the optical path of the polarizing microscope during the measurement. Further adjustment of the polarizing microscope to compensate for residual background birefringence and The method described in any one of the clauses 22 to 26, further including the method described in any one of the clauses 22 to 26.

[0232] Clause 28. A computer program product that, when executed by a processor operably coupled to a polarizing microscope, includes instructions that cause the processor to control the polarizing microscope and perform the actions described in any one of Clauses 1 to 27.

[0233] Clause 29. Computer program products as described in Clause 28, embodied in a non-temporary computer-readable medium.

[0234] Article 30. A system for reading data from a birefringent optical data storage medium, A light source for emitting light along the optical path, A polarization state generator on an optical path, configured to receive light from a light source and output elliptic polarization in the input state, A sample stage configured to hold an optical data storage medium downstream of a polarization stage generator in the optical path, A polarization state analyzer downstream of the sample stage on the optical path, A photodetector for detecting light in a detection state, comprising a photodetector located downstream of a polarization state analyzer in the optical path, A controller including a processor and memory for storing computer executable instructions. Including, computer executable instructions, when executed by the processor, the processor, The system is controlled to measure the background birefringence of the optical data storage medium held by the sample stage, Adjust the polarization state generator and / or polarization state analyzer to compensate for background birefringence. A system that enables this to happen.

[0235] Clause 31. A system as described in Clause 30, in which, when a computer executable instruction is executed by the processor, causes the processor to control the system to carry out the method described in any one of Clauses 1 to 27.

[0236] Clause 32. Memory further stores computer executable instructions, and when computer executable instructions are executed by the processor, the processor... After adjusting the polarization state generator and / or polarization state analyzer to compensate for background birefringence, optimize the swing angle χ of the input state or detected state. By performing this action and optimizing the swing angle, Determining the degree of polarization (DOP) of light transmitted through the voxels of an optical data storage medium, Adjust the polarization state generator so that the swing angle of the input state is equal to the inverse cosine of the polarization degree, or adjust the polarization state analyzer so that the swing angle of the detected state is equal to the inverse cosine of the polarization degree. The system described in Clause 30 or 31, including the system described in Clause 30 or 31.

[0237] Clause 33. Memory further stores computer executable instructions, and when computer executable instructions are executed by the processor, the processor... Controlling the system to measure residual background birefringence, Adjust the polarization state generator and / or polarization state analyzer to compensate for background birefringence. A system that enables the execution of any one of the provisions 30 to 32.

[0238] Clause 34. The system further includes a beam splitter downstream of the sample stage for splitting the optical path into two or more branches. Each of the two or more branches is, Each polarization state analyzer and Each detector is used to detect light in each detection state. A system including any one of clauses 30 to 33.

[0239] Other variations or uses of the disclosed techniques will become apparent to those skilled in the art by reading the disclosure herein. The scope of this disclosure is not limited by the embodiments described herein, but is limited only by the appended claims.

Claims

1. A method for calibrating a polarizing microscope, The optical data storage medium is placed on the sample stage of the polarizing microscope, wherein the optical data storage medium is a substrate, and the substrate includes a substrate having birefringent voxels embedded therein. To measure the background birefringence of the optical data storage medium, Adjusting the polarizing microscope to compensate for the background birefringence. A method that includes this.

2. The method according to claim 1, further comprising adjusting the polarizing microscope to compensate for the background birefringence, and then optimizing the swing angle χ of the measurement state of the polarizing microscope to compensate for light scattering, wherein the measurement state is optionally a detection state.

3. Adjusting the swing angle χ in the measurement state means Determining the degree of polarization of light transmitted through the voxels of the aforementioned optical data storage medium, Adjusting the polarizing microscope so that the swing angle in the measurement state is equal to the inverse cosine of the degree of polarization. The method according to claim 2, including the method described in claim 2.

4. The method according to any one of claims 1 to 3, wherein adjusting the polarizing microscope to compensate for the background birefringence includes performing pre-compensation, the pre-compensation includes adjusting the polarizing microscope to adjust the input state generated by the polarizing microscope to offset the polarization shift of the input state caused by the background birefringence, and the pre-compensation includes adjusting the polarization state generator of the polarizing microscope.

5. The method according to any one of claims 1 to 4, wherein adjusting the polarizing microscope to compensate for the background birefringence includes performing post-compensation, the post-compensation includes adjusting the polarizing microscope to adjust the detection state of the polarizing microscope so as to cancel out the polarization shift of the input state caused by the background birefringence, and the post-compensation includes adjusting the polarization state analyzer of the polarizing microscope.

6. The method according to any one of claims 1 to 5, wherein the background birefringence is measured over the field of view of the polarizing microscope, and the field of view comprises a plurality of voxels.

7. Determining the background birefringence means that The field of view within the optical data storage medium is illuminated with light in the input state, After the light in the input state has passed through the optical data storage medium, the intensity of the light in two or more different detection states is measured. Based on the differences in intensity of each of the above, the background birefringence is calculated using a data processor. The method according to any one of claims 1 to 6, wherein adjusting the polarizing microscope to compensate for the background birefringence includes correcting the two or more different detection states and / or the input states to adjust the balance of the respective measured intensities.

8. Determining the background birefringence means that The field of view within the optical data storage medium is illuminated with light under two or more different input conditions, For the two or more different input states, the intensity of the light transmitted through the optical data storage medium in each detection state is measured, Based on the differences in intensity of each of the above, the background birefringence is calculated using a data processor. The method according to any one of claims 1 to 7, wherein adjusting the polarizing microscope to compensate for the background birefringence includes correcting the two or more different input states and / or the detection state to adjust the balance of the respective measured intensities.

9. The birefringent voxels are arranged in a stack of layers, and the stack includes an upper layer, an intermediate layer and a bottom layer. The method according to any one of claims 1 to 8, wherein during the measurement of the background birefringence, the polarizing microscope is focused on the intermediate layer.

10. The method according to claim 9, wherein the stack comprises at least 50 birefringent voxels.

11. A method for reading data from a birefringent optical data storage medium, Calibrating a polarizing microscope by the method described in any one of claims 1 to 10, Next, using the polarizing microscope, images of voxels are captured using elliptical polarization, Using a processor to process the image and recover the data A method that includes this.

12. A system for reading data from a birefringent optical data storage medium, A light source for emitting light along the optical path, A polarization state generator on the optical path, configured to receive light from the light source and output light in a circularly or elliptically polarized input state, A sample stage, configured to hold an optical data storage medium downstream of the polarization stage generator on the optical path, A polarization state analyzer downstream of the sample stage in the optical path, A photodetector for detecting light in a detection state, comprising a photodetector located downstream of the polarization state analyzer in the optical path, A controller including a processor and memory for storing computer executable instructions. The computer executable instruction includes, when executed by the processor, the processor, The system is controlled to measure the background birefringence of the optical data storage medium held by the sample stage, Adjusting the polarization state generator and / or the polarization state analyzer to compensate for the background birefringence. A system that enables this to happen.

13. The memory further stores computer executable instructions, and when the computer executable instructions are executed by the processor, the processor receives After adjusting the polarization state generator and / or the polarization state analyzer to compensate for the background birefringence, optimize the swing angle χ of the input state or the detected state. Performing this action and optimizing the swing angle is Determining the degree of polarization (DOP) of light transmitted through the voxels of the aforementioned optical data storage medium, Adjust the polarization state generator so that the swing angle of the input state is equal to the inverse cosine of the polarization degree, or adjust the polarization state analyzer so that the swing angle of the detection state is equal to the inverse cosine of the polarization degree. The system according to claim 12, including the above.

14. The memory further stores computer executable instructions, and when the computer executable instructions are executed by the processor, the processor receives Controlling the system to measure residual background birefringence, Adjusting the polarization state generator and / or the polarization state analyzer to compensate for the background birefringence. The system according to claim 12 or 13, which causes the following to occur.

15. Downstream of the sample stage, the system further includes a beam splitter for dividing the optical path into two or more branches. Each of the two or more branches mentioned above is Each polarization state analyzer and Each detector for detecting light in each detection state and A system according to any one of claims 12 to 14, including the system described in any one of claims 12 to 14.

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