Method and apparatus for measuring depth-resolved tissue birefringence using single input state polarization sensitive optical coherence tomography

Abstract

To provide a method for determining a retardance of a layer of a sample.SOLUTION: A method includes: transmitting a first portion of a polarized light to a sample arm S of an optical system and a second portion of the polarized light to a reference arm R of the optical system; combining first return light returned from the sample arm S and second return light from the reference arm R; detecting, using a detector, the combined light along a first polarization state and a second polarization state to produce polarization data, the second polarization state being different from the first polarization state; determining, using a processor coupled to the detector, polarization states of light returning from upper and lower surfaces of a layer of the sample based on detecting the combined light; and determining, using the processor, a retardance of the layer of the sample based on the determined polarization states.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 744,917, entitled "Method and Apparatus for Measuring Depth-Resolved Tissue Birefringence Using Single-Input State High Polarization Sensitive Optical Coherence Tomography," filed on October 12, 2018, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Near-infrared spectral band broadband or wavelength-swept laser sources are commonly used as imaging sources in optical coherence tomography ("OCT") systems. The light emitted from the source is split and directed into different paths, usually called the "reference arm" and the "sample arm", then redirected and reunited to interfere via a Michelson or Mach-Zehnder configuration, or similar hybrid configuration, before being received by a detector or spectrometer and processed by a computer. In the reference arm, the light is redirected back, often with a reflector, to provide the light with a specified optical path length. In the sample arm, the light is focused onto the sample and backscattered by structures within the sample. The optical path length difference of the light between the reference arm and the sample arm is encoded into an interference pattern recorded in the spectral domain. Using an extraction algorithm (usually an inverse Fourier transform), the depth-resolved intensity of the light backscattered from the tissue is extracted to form an image that represents the internal structure of the sample.

[0003] There is a need for improved OCT methods that provide additional information, such as birefringence information, while acquiring data at high imaging speeds. Summary of the Invention

[0004] Polarization-sensitive optical coherence tomography ("PS-OCT") uses a polarization-sensitive detection scheme to record the polarization state of output light backscattered from a sample. PS-OCT can therefore reveal the polarization properties of tissue in addition to traditional sample reflectance. Specifically, PS-OCT can detect tissue polarization properties including retardation and attenuation between the two principal polarization states, including the determination of the principal polarization states. can be.

[0005] Birefringence describes the difference in refractive index experienced by light polarized along the fast and slow principal polarization states, i.e., the optical axis, of a birefringent medium. Birefringence results in retardation, i.e., a delay of light polarized along the slow optical axis direction, resulting in a net retardance, i.e., a delay, after propagation along a specified distance. Retardance is usually expressed relative to the wavelength of light used, whereas birefringence is an absolute number. In this disclosure, retardation and birefringence may be used interchangeably.

[0006] One type of PS-OCT, known as "single-input-state PS-OCT", uses light with a single, usually circular, illumination polarization state to probe the tissue. Due to the presence of retardation at the sample, the polarization state of the light changes during its propagation through the tissue. The depth-resolved polarization state of the backscattered light can be extracted from the interference fringes recorded by the detector, and the cumulative change in polarization state compared to the input state, i.e., the depth-dependent retardance, can be calculated. Detectors in PS-OCT typically contain a polarizing beam splitter that separates the horizontal and vertical components of the light beam and records the interference fringes for each channel separately.

[0007] Since the probing light is focused into the tissue structure and scattered for collection, a direct interpretation of the depth-resolved output polarization state of the light corresponds to the influence of the tissue's cumulative polarization properties, including cumulative retardation and optic axis orientation. However, in most cases, the tissue under examination has various structures and is composed of layers with different birefringence properties, making it difficult to read the map of cumulative retardation. On the other hand, the map of the tissue's local retardation properties is inherently related to the birefringence at a specific location and provides direct information of the region of interest. Thus, by extracting the tissue's depth-resolved, i.e., local polarization properties including retardation, a more intuitive and accurate representation of the tissue's structure and function is provided. In this disclosure, local or depth-resolved retardation is used interchangeably with birefringence and local or depth-resolved birefringence.

[0008] The local retardation properties can be obtained recursively from the accumulated maps, but this requires knowledge of the polarization state illuminating the sample and the polarization transformations introduced by the optical components in the detection path, which is often impractical, especially for catheter-based imaging. In some methods, the local retardation properties of tissue can be resolved by performing two or more measurements on the same tissue with different input polarization states. Typically, this type of PS-OCT system uses a polarization modulator to sequentially change the polarization state of the illumination light between the acquisition of the depth profile. A depth-resolved retardation map can be obtained from the recorded interference fringes of a pair of consecutive depth profiles with different polarization states, assuming that the two profiles correspond to the same location in the tissue. Importantly, it is not necessary to know the exact polarization state of the illumination light that is incident on the tissue after passing through the system components. However, the use of a modulator limits the imaging speed and the bandwidth of the laser source, and also increases the complexity and cost of the system. On the other hand, the embodiments described herein provide a solution to obtain local retardation measurements of tissue without the limitation of using a modulator.

[0009] The methods and apparatus described herein include a single-input state single-shot spectral domain PS-OCT scheme and a local birefringence extraction algorithm based on Stokes vector inference.The above and other features and advantages are defined by the appended claims.The following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings, is merely exemplary and not limiting, the scope of which is defined by the appended claims and their equivalents.

[0010] Various embodiments disclose the structure of a single-input PS-OCT system. Other embodiments disclose methods and devices for analyzing the local birefringence properties of a sample. A recently observed and confirmed physical phenomenon called the mirror state constraint is described herein, followed by a processing method based on the Stokes domain geometric reasoning and mirror state constraint of local tissue birefringence properties that can be applied to data recorded using the above-mentioned device.

[0011] Thus, in one aspect, the present invention provides a method for determining the retardance of a layer of a sample, the method comprising: sending a first portion of polarized light into a sample arm of an optical system and sending a second portion of the polarized light into a reference arm of the optical system, combining a first return light from the sample arm and a second return light from the reference arm, detecting with a detector the combined light along a first polarization state and a second polarization state different from the first polarization state to generate polarization data, determining with a processor coupled to the detector polarization states of light returning from top and bottom surfaces of the layer of the sample based on detection of the combined light, and determining with the processor the retardance of the layer of the sample based on the determined polarization states.

[0012] The polarization state of light backscattered by a retarding sample and measured along the same path used to illuminate the sample tends to align with a defined polarization state related to the input state. This constrains the evolution of the detected polarization state and helps determine the retardation of the sample. This , which is useful for measuring the depth-resolved birefringence of a sample using a single illumination polarization state in applications such as polarization-sensitive optical coherence tomography (PS-OCT). Previous implementations of PS-OCT required either using two multiplexed polarization states to illuminate the sample or using a single known input polarization state in combination with complex recursive reconstruction algorithms to determine sample properties for general samples with layered architectures, but the disclosed method overcomes this limitation and enables depth-resolved birefringence imaging using a single input state PS-OCT system without the need for a specific known illumination polarization state, and the method is compatible with fiber-based or catheter-based imaging. Embodiments of the present invention provide single input state systems, such as PS-OCT systems, that are lower cost, faster (due to the absence of the need to provide an input light source with two multiplexed polarization states), and have improved resolution, as compared to known systems that use multiple input polarization states. Birefringence imaging has applications in fields such as ophthalmology, dermatology, intravascular imaging, or imaging of the gastrointestinal tract.

[0013] In an exemplary embodiment of the method, the method may further include determining a mirror state associated with the polarization data.

[0014] In another exemplary embodiment of the method, the mirror states may include points on the Poincaré sphere.

[0015] In yet another exemplary embodiment of the method, the mirror state may include an input polarization state having a reversed helicity.

[0016] In yet another exemplary embodiment of the method, the layer of the sample may include a sub-surface layer of the sample. The layer of the sample from which polarization information is obtained may include not only above the top surface (or bottom surface) of the sample, but also sub-surface layers within the sample, for example.

[0017] In another exemplary embodiment of the method, the optical system may include an optical coherence tomography system, and detecting the combined light along a first polarization state and a second polarization state further includes detecting the combined light along a first polarization state and a second polarization state with the optical coherence tomography system. The use of an optical coherence tomography (OCT) system facilitates obtaining data from multiple layers within a sample.

[0018] In yet another exemplary embodiment of the method, determining the polarization state of light returning from the top and bottom surfaces of the layer of the sample may further include determining a rotation angle and a rotation axis of a rotation circle associated with the polarization state from the top and bottom surfaces of the layer of the sample, and determining a retardance level and an apparent optic axis, respectively, based on determining the rotation angle and the rotation axis. The disclosed method facilitates determining information such as the rotation angle, the rotation axis, the retardance level, and the apparent optic axis.

[0019] In yet another exemplary embodiment of the method, the sample may include a plurality of layers, and determining the retardance of the layers of the sample based on the determined polarization state includes determining a retardance of each of the plurality of layers of the sample based on the determined polarization state. and wherein the method further comprises generating a reconstruction of the sample based on the retardance of each of the plurality of layers of the sample. The disclosed method facilitates generating a reconstruction based on retardance information obtained for each of a plurality of layers of a sample.

[0020] In another exemplary embodiment of the method, determining the retardance of the layer of the sample may further include determining the retardance of the layer of the sample using a wavelength dependence of the polarization state to reduce artifacts.

[0021] In another further exemplary embodiment of the method, determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts may further comprise determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts based on spectral binning.

[0022] In yet another exemplary embodiment of the method, the polarized light may include circularly polarized light.

[0023] In yet another exemplary embodiment of the method, the detector may comprise a first detector and a second detector, and detecting the combined light along a first polarization state and a second polarization state to generate polarization data further comprises sending the combined light to a polarizing beam splitter configured to send light having the first polarization state to the first detector and light having the second polarization state to the second detector. Two detectors may be used to collect data from two different polarization states simultaneously. The use of two separate detectors provides the technical effect of being able to collect data from two different polarization states simultaneously, thereby increasing the speed of data collection and the overall throughput of the system.

[0024] In another exemplary embodiment of the method, the sample may include at least one of an ophthalmological sample, a dermatological sample, an intravascular sample, or a gastrointestinal sample.

[0025] In a further exemplary embodiment of the method, the optical system may include a polarization-sensitive optical coherence tomography (PS-OCT) system.

[0026] In another exemplary embodiment of the method, the optical system may be implemented using a free space optical system or a fiber optic system.

[0027] In another aspect, the present invention provides an apparatus for determining the retardance of a layer of a sample, the apparatus comprising: an interferometer optical system having a sample arm and a reference arm, a light source coupled to the optical system, a detector, and a processor coupled to the detector, the light source configured to send a first portion of polarized light to the sample arm and a second portion of the polarized light to the reference arm, the optical system configured to combine a first return light from the sample arm and a second return light from the reference arm, the detector configured to detect the combined light along a first polarization state and a second polarization state different from the first polarization state to generate polarization data, the processor configured to determine polarization states of light returning from a top surface and a bottom surface of the layer of the sample based on detection of the combined light by the detector, and determine a retardance of the layer of the sample based on the determined polarization states.

[0028] In an exemplary embodiment of the apparatus, the processor determines a mirror state associated with the polarization data. The device may be further configured to determine

[0029] In another exemplary embodiment of the apparatus, the mirror states may include points on the Poincaré sphere.

[0030] In yet another exemplary embodiment of the apparatus, the mirror state may include an input polarization state with reversed helicity.

[0031] In yet another exemplary embodiment of the apparatus, the layer of the sample may include a subsurface layer of the sample. The layer of the sample from which polarization information is obtained may include layers on the top (or bottom) surface of the sample as well as subsurface layers, e.g., within the sample.

[0032] In another exemplary embodiment of the apparatus, the optical system may include an optical coherence tomography system, and the processor is further configured to detect the combined light along a first polarization state and a second polarization state using the optical coherence tomography system when detecting the combined light along a first polarization state and a second polarization state. The use of an optical coherence tomography (OCT) system facilitates obtaining data from multiple layers within a sample.

[0033] In yet another exemplary embodiment of the apparatus, the processor may be further configured to, upon determining the polarization states of light returning from the top and bottom surfaces of the layer of the sample, determine a rotation angle and a rotation axis of a circle of rotation associated with the polarization states from the top and bottom surfaces of the layer of the sample, and determine a retardance level and an apparent optic axis, respectively, based on determining the rotation angle and the rotation axis. The disclosed apparatus facilitates determining information such as the rotation angle, rotation axis, retardance level, and apparent optic axis.

[0034] In yet another exemplary embodiment of the apparatus, the sample may have multiple layers, and the processor is further configured to determine the retardance of each of the multiple layers of the sample based on the determined polarization state when determining the retardance of the layers of the sample based on the determined polarization state, and the processor is further configured to generate a reconstruction of the sample based on the retardance of each of the multiple layers of the sample. The disclosed method facilitates generating a reconstruction based on retardance information obtained for each of multiple layers of a sample.

[0035] In yet another exemplary embodiment of the apparatus, the processor may be further configured to determine the retardance of the sample layer using the wavelength dependence of the polarization state to reduce artifacts in determining the retardance of the sample layer.

[0036] In another exemplary embodiment of the apparatus, the processor may be further configured to, when determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts, determine the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts based on spectral binning.

[0037] In yet another exemplary embodiment of the apparatus, the polarized light may include circularly polarized light.

[0038] In yet another exemplary embodiment of the apparatus, the detector may include a first detector and a second detector, and the apparatus is configured to send the combined light to a polarizing beam splitter, the polarizing beam splitter configured to send light having the first polarization state to the first detector and to send light having the second polarization state to the second detector. Using one detector, data can be collected from two different polarization states simultaneously. The use of two separate detectors has the technical effect of allowing data from two different polarization states to be collected simultaneously, which increases the speed of data collection and the overall throughput of the system.

[0039] Further exemplary embodiments of the device may include at least one of an ophthalmological sample, a dermatological sample, an intravascular sample, or a gastrointestinal sample.

[0040] In yet another exemplary embodiment of the apparatus, the optical system may include a polarization-sensitive optical coherence tomography (PS-OCT) system.

[0041] In another exemplary embodiment of the apparatus, the optical system may be implemented using a free space optical system or a fiber optic system.

[0042] These and other aspects and advantages of the present invention will become apparent from the following description. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which preferred embodiments of the present invention are illustrated. However, such embodiments do not necessarily represent the full scope of the invention, which should be interpreted with reference to the claims herein.

[0043] Further objects, features and advantages of the present disclosure will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings illustrating exemplary embodiments of the present disclosure. [Brief description of the drawings]

[0044] [Figure 1] FIG. 1 is a schematic diagram of a single-input-state single-shot free-space spectral-domain polarization-sensitive optical coherence tomography system according to one embodiment. [Figure 2A] A sample consisting of multiple layers with different birefringent properties is shown. [Figure 2B] The principle of the observed constraints on the change in the detected polarization state is illustrated. [Diagram 3] Birefringence images of a phantom having layers with different birefringence are presented, demonstrating the improved detection capability of the proposed method compared to conventional reconstruction strategies. [Figure 4] A comparison of birefringence in a layered phantom measured with the proposed method and conventional two-input state PS-OCT is shown. [Diagram 5]Demonstration of the polarization mirror states. (a) is a schematic of the employed optical system. L1-L4 are lenses, M is a mirror, Pl is a polarizer, PC is a polarization controller, Q1 is a quarter-wave plate, and SMF is a single-mode fiber. (b)-(d) show the change in polarization states on the Poincaré sphere by moving the three paddles of the PC when using circular, elliptical, and linearly polarized input states, respectively, indicated by blue arrows. The polarization mirror states appear by the repeated crossing of the change in polarization states at specific points, highlighted by red arrows on each Poincaré sphere. [Figure 6] Diagram of the theoretical description of the polarization mirror states. (a) shows a model of round-trip propagation through a reciprocal sample with varying birefringence M(x) and a fixed element A. (b) shows that the rotation vectors of all linear retarders are localized within a circle of radius π in the QU plane of the Poincaré sphere. The green traces represent the simulated rotation vector evolution for synchronous motion of the polarization controller paddles. The blue curves represent the rotation vectors D·AT·MT(x)·D·M(x)·A for M(x) with linearly increasing retardation for three representative sets of different A and M(x). The red curves represent the end points of the rotation vectors that map a randomly selected input polarization state [x,y,z]T exactly to its mirror state. Each crossing of the green and blue lines with the red line corresponds to a change in polarization state through the mirror state. [Figure 7]Experimental change in coherence-gated polarization states in a three-layer birefringent phantom. (a) shows a schematic sketch of the phantom with color-coded layers, each layer featuring a different optical axis orientation. (b)-(d) show the polarization state change traces (color coding corresponds to (a)) of each layer on the Poincaré sphere for different input polarization states ((b) shows circularly polarized input light, (c) shows elliptically polarized input light, and (d) shows linearly polarized input light). Purple lines are fitting circles of the change in polarization state of each phantom layer. The mirror and input polarization states are indicated by red and blue arrows, respectively. It can be seen that the mirror and input states are symmetric with respect to the QU plane and correspond to light with inverted polarization helicity. [Figure 8] Intensity and birefringence images of the phantom and pig retina are shown. (a)-(c) show the schematic sketch, cumulative retardation image, and local retardation image of the two-layer phantom sample, respectively. (d)-(f) show the schematic sketch, cumulative retardation image, and local retardation image of the inverted phantom sample, respectively. The scale bars shown in (b) for (b)-(c) and (e)-(f) are vertical: 100 μm, horizontal: 400 μm. (g) shows the cross-sectional intensity image of the pig retina. (h) shows the cross-sectional local birefringence image of the pig retina reconstructed using the polarized mirror state. The scale bars shown in (h) for (g)-(h) are vertical: 100 μm, horizontal: 400 μm. (i) shows the en face intensity projection of the RNFL layer of the pig retina. The yellow dashed line indicates the location of the cross-sectional images shown in (g) and (h). (j) shows the average local birefringence within the RNFL layer. Scale bars shown in (j) for panels (i) to (j) are 400 μm vertically and 400 μm horizontally. [Figure 9] We demonstrate the application of mirror state constraints in fiber-based systems and the suppression of artifacts arising from the alignment of the apparent optic axis and depth-dependent polarization state using spectral binning. [Figure 10]1 shows a schematic diagram of the round trip transmission through the elements from A to B and back to A, described by the Jones and Mueller matrices, respectively. [Figure 11] We show the possible rotation vector traces that map an input state u to its mirror state u'. (a) shows the maximum and minimum rotation angles α, β on the Poincaré sphere and the corresponding rotation axes τAn, τBn. (b) shows that τAn and τBn define the orientation and curvature of the possible rotation vector traces (red lines). [Figure 12] 1 illustrates a setup that may be used in various embodiments disclosed herein. [Figure 13] 1 illustrates an example of a system for determining birefringence of a layer of a sample, according to some embodiments of the disclosed subject matter. [Figure 14] 1 illustrates example hardware that can be used to implement computing devices and servers in accordance with some embodiments of the disclosed subject matter. [Figure 15] 1 illustrates an example process for determining birefringence of a layer of a sample according to some embodiments of the disclosed subject matter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Therefore, the present application discloses embodiments of a system and method for obtaining birefringence information of a sample, which can provide local birefringence information rather than cumulative birefringence information. Fiber optic based implementations are more suitable for endoscopic and intravascular imaging. Free space bulk optics implementations are suitable for ophthalmological imaging.

[0046] Although modifications to the laser source device of the present application are illustrated using the example of a free-space optical device that is part of an apparatus for optical coherence tomography, those skilled in the art will understand that embodiments may also be implemented with the use of fiber optic elements, or may be used as a stand-alone device.

[0047] The polarization state of light backscattered by a sample with retardation and measured along the same path used to illuminate the sample tends to align with a defined polarization state related to the input state. This constrains the changes in the detected polarization state, which helps to determine the retardation of the sample. This is useful for polarization-sensitive optical coherence tomography (PS-OCT) to measure the depth-resolved birefringence of a sample using a single illumination polarization state.

[0048] In contrast, previous implementations of PS-OCT required two multiplexed polarization states to illuminate the sample or a single known input polarization state combined with complex recursive reconstruction algorithms to determine sample properties for typical samples with layered architectures. The present invention overcomes this limitation and enables depth-resolved birefringence imaging with a single-input-state PS-OCT system that does not require a specific known illumination polarization state, compatible with fiber- and catheter-based imaging.

[0049] Embodiments of the present invention provide a single input state PS-OCT system that is less expensive than more complex systems that use multiple input polarization states. Birefringence imaging with PS-OCT has applications in fields such as ophthalmology, dermatology, intravascular imaging, and gastrointestinal imaging.

[0050] FIG. 1 shows a block diagram of an embodiment of a free-space optics-based single-input-state single-shot PS-OCT system 100, but as mentioned above, the same principles may be used to implement a fiber-based PS-OCT system in various embodiments. The light source element 101 shown in FIG. 1 may include one or more lasers tuned to different spectral bands. The light beam from the light source element 101 may be coupled by lens 103 into a single mode fiber 102 and passed through a polarizer 104 and a quarter wave plate 105, which may make it circularly polarized. The light beam is then split by a non-polarizing beam splitter 106 into a "reference arm" (labeled R in FIG. 1) and a "sample arm" (labeled S in FIG. 1). In the reference arm R, the light beam passes through a polarizer 107 and is relayed by lens 109 to a reflector 108, where it is reflected back along the same path. In the sample arm S, a scanning mirror 110 and an objective lens 111 define a scanning system to move the light spot generated by the light beam laterally on the sample S'. The reflected beam from the reference arm R and the backscattered beam from the sample arm S are recombined at the beam splitter 106 and detected by a polarization-sensitive detection scheme, which includes a half-wave plate 112, a section of polarization-maintaining fiber 113, a polarizing beam splitter 114, four collimation lenses 115-118 and two high-speed spectrometers 119, 120.

[0051] In another embodiment that can be implemented in conjunction with the free-space PS-OCT embodiment outlined above, Figures 2A and 2B show a data processing method for a single-input state polarization-sensitive OCT hardware configuration to extract local tissue birefringence properties, although the hardware configuration and data processing method do not necessarily have to be used in the same implementation and may be used independently of each other in various embodiments.

[0052] 2A and 2B show the geometric inference of local birefringence using the mirror state constraint, which is described in more detail below. The inference is explained in Stokes domain polarimetry, but can also be expressed in Jones formalism. The mirror state constraint is based on recent observations made by our group on the change in the light polarization state in a birefringent sample: for a sample with stacked layers of random, uniform birefringence (no attenuation), the change in the backscattered polarization state as a function of depth contains connected but discontinuous circular arcs on the Poincaré sphere. In a transposed symmetric system, all the circles on which the arcs reside contain "mirror states". passing through, which is a QU-plane reflection of the input polarization state, which corresponds to an input polarization state with reversed polarization helicity. Applying this constraint involves determining the "mirror state" to estimate the birefringence rotation circle on the Poincaré sphere between the polarization states reflected from the top and bottom surfaces of the thin sample layer of interest. Knowing the rotation circle, one can obtain the rotation axis and rotation angle that correspond to the apparent optic axis and birefringence level, respectively, for the layer of interest.

[0053] An exemplary sample is shown in FIG. 2A, which includes three layers with different optical axes. In FIG. 2B, the round trip polarization state change in the sample layer of interest is shown. In this example, we assume that the layer of interest is the third layer, which is the lowest / deepest layer of the three layers, and is shown in FIG. 2A. As shown in FIG. 2B, polarization states 201 and 203 are measured from light backscattered from the top and bottom surfaces of the layer of interest. Using mirror state 204, a rotation circle 202 is constructed, which is the birefringence rotation circle of the layer of interest. Specifically, the rotation angle is the local retardation and the rotation axis is the apparent optical axis.

[0054] Table 1 shows the individual steps of the data processing method for recovering depth-resolved birefringence information from measurements using a single input state PS-OCT system. In various embodiments, the spectral fringes recorded by the spectrometers 119 and 120 are pre-processed using algorithms typically used in conventional Fourier domain OCT, including steps such as background subtraction, k-space resampling and / or dispersion compensation. The pre-processed fringes from the spectrometers corresponding to a scan of the xy plane are denoted as H(x,y,λ), V(x,y,λ).

[0055] [Table 1]

[0056] A birefringent phantom with layers and regions of different optic axis orientation and birefringence is imaged, with birefringent regions 301 and 302 having different optic axes, as shown in Figure 3 (top). In a conventional single-input state PS-OCT implementation (bottom right), it is not possible to resolve the true local birefringence of the regions underneath the birefringent layers. However, using an embodiment of the present invention (bottom left panel), the true birefringence can be identified in the phantom.

[0057] Embodiments of the present invention can provide higher resolution images with the same contrast as conventional two-input state PS-OCT. The associated costs are significantly lower than current devices, and such an apparatus can provide resolution that is not limited by polarization modulators. For example, FIG. 4 shows the same phantom imaged by dual-input PS-OCT (left) and the present invention (right). Furthermore, embodiments of the present invention enable local birefringence contrast with single-input state single-shot polarization-sensitive OCT. Known single-input PS-OCT procedures do not reconstruct local birefringence contrast, but instead provide cumulative birefringence contrast, which cannot visualize useful local tissue structure information.

[0058] Polarization offers an attractive contrast mechanism for a variety of applications, from remote sensing to biomedical optics. Traditionally, multiple input polarization states have been required to fully characterize the polarization properties of a sample. It has been observed that the polarization state of backscattered or reflected light, when measured with identical illumination and detection paths, often coincides with the used input polarization state, "mirrored" by the horizontal plane of the Poincaré sphere. Here, we investigate the predisposition of this mirror state and show how it constrains the depth-dependent changes in polarization state measured with polarization-sensitive optical coherence tomography (PS-OCT), thereby enabling the analysis of depth-resolved tissue birefringence even when using a single input polarization state.

[0059] Early investigations into the polarization properties of single-mode fibers have reported aspects of the polarization mirror state, but its manifestation has not been elucidated. To investigate the polarization mirror state, we measured round-trip signals through a 1.5 m long single-mode optical fiber, as shown in Figure 5. We measured the time-varying polarization state resulting from using a polarization controller to change the fiber birefringence by randomly moving the controller paddle position. The polarization state, visualized as a Stokes vector normalized in the Q, U, and V coordinates of the Poincaré sphere, clearly changes multiple times through a particular state u' (Figure 5(b)). By iterating with different emission polarization states u measured by reflecting the light onto a detector before entering the fiber, we identify the state u' = D u (where D = diag(1,1,-1)) as the input state mirrored by the horizontal QU plane, which we designate as the polarization mirror state (Figure 5(c) and (d)). Of note, all measurements are made in fixed coordinates of the receiver and are independent of the orientation of the illumination path coordinates.

[0060] To understand the mirror state phenomenon, consider a general retarder M(x) whose retardation varies as a function of x (e.g., the paddle position of a polarization controller). This may be preceded by a fixed element A. The coupled system shown in Figure 6(a) transforms an input polarization state u into an output state v, which can be expressed as

[0061]

number

[0062] Here, P=D A T M T D M A, and all vectors and matrices are in the rotation group SO(3). We have chosen to follow the convention of preserving the orientation of the spatial xy coordinates, regardless of the direction of light propagation. In a reciprocal medium, the backpropagation through an element M is D M T ·D (see below). The round trip transmission P is D transposed symmetric P=D·P T It is important to note that ·D, which makes P a linear retarder. The round trip effectively cancels the optical rotation or circular retardation. The effect of P on the input state can be described by a rotation vector ω(P) in the QU plane of the Poincaré sphere, whose direction indicates the axis of rotation and whose length defines the amount of rotation. Taking into account these ambiguities about 2π, the rotation vectors of all possible linear retarders are restricted to a circle of radius π in the QU plane (Figure 6b). When moving the paddles of the polarization controller, ω(P) traces a complex path in the QU plane, which is shown by the green lines in Figure 6b for a simulation of the synchronous motion of the three paddles.

[0063] There is only one rotation vector in the QU-plane that rotates a given input state u to any output state v. This rotation vector is defined by the intersection of the QU-plane with the plane that bisects u and v. For u to pass through v, ω(P) must change through this specific point in the π circle of the QU-plane. The only exception is that there exists a continuum of rotation vectors that map u to the mirror state u' = D u, because in this case the QU-plane coincides with the bisection plane. The rotation vector lies on a curve r in the QU-plane (the red curve in Figure 6(b); see below for details). Every intersection of ω(P) with r corresponds to v changing through the mirror state u', explaining the frequent occurrence of changes.

[0064] We then used PS-OCT to measure the polarization state of the backscattered light in a scattering sample as a function of its round-trip depth. On the scale of the spatial resolution of OCT, tissue can be modeled as a sequence of uniform linearly birefringent layers with different optic axis orientations. M(x) represents a linear retarder with retardance that increases linearly with depth x, and D M T The parameter A is the combination of the system components and the preceding organizational layers. The resulting rotation vector ω(P) represents a regular curve across the circle π (blue curve in Figure 6(b)). Every possible trace crosses the curve r exactly once, and is guaranteed to cross u' periodically. To study the evolution of v in more detail, we take the derivative with respect to x, u=P T Replace (x) v.

[0065]

number

[0066] P.P. T = I, so (δP / δx) P T =-P·(δP T / δx) is the skew ratio is symmetric and can be expressed as the cross product operator τ×, which is constant for retardance that grows linearly with x (see below for details). Thus, v varies with constant velocity on the circle that passes through u′ on the Poincaré sphere.

[0067] For experimental validation, we prepared a scattering phantom consisting of three linearly birefringent layers with different optic axis orientations (Figure 7(a)). Referring to Figure 7, (b)-(d) show the depth-dependent polarization states measured using PS-OCT for three different input polarization states. As expected, fitting circles to the change in polarization state within each layer shows close agreement, with all circles varying through the polarization mirror state u'.

[0068] A persistent challenge in single-input state PS-OCT is to measure the local retardation of the sample, i.e., the derivative of the retardance P(x) with depth, which is given by the norm of |τ| and is proportional to the birefringence of the sample. According to equation (2), we obtain the following: In the following equation, we used |v|=1.

[0069]

number

[0070] α is the angle between the rotation vector τ and the polarization state v, which is needed to estimate the local retardation. Due to the change from v to u', both δv / δx and (v-u') are in the same plane, perpendicular to τ. Therefore, the direction is given by the cross product τ0=(δv / δx)×(u'- v), and sinα = |τ0 × v| / |τ0|, and after some algebraic manipulations, the following equation can be calculated:

[0071]

number

[0072] To verify the ability of the polarized mirror state to reconstruct local retardation, we imaged a tissue-like phantom consisting of a long birefringent band followed by four parallel elements with different birefringence levels and different optic axis orientations than the long band (Figure 8(a) and (d)). As shown in Figure 8, (b) and (e) represent accumulated retardation images from either side of the sample, corresponding to the retardation of P(x) reconstructed with a conventional algorithm for single-input state PS-OCT. As shown in Figure 8, (c) and (f) show images of local retardation reconstructed using the polarized mirror state. While the accumulated retardation is difficult to interpret, the local retardation reveals individual sample segments with their different levels of birefringence and was recovered regardless of the sample orientation.

[0073] Figures 8(g)-(j) show local retardation imaging of an excised porcine retina. The intensity image visualizes the layered structure of the retina (Figure 8(g)). Local retardation reveals birefringence of the retinal nerve fiber layer (RNFL), clearly distinguishing it from the less birefringent subsequent layers (Figure 8(h)). RNFL birefringence has been investigated in relation to its potential to diagnose early degeneration of the RNFL associated with glaucoma.

[0074] Previous strategies for reconstructing local birefringence from single-input-state PS-OCT rely on the inherent symmetry of the imaging system. Importantly, the change in v through the mirror state persists even in imaging systems with additional fixed single-path system components B and C in only the illumination or detection paths, as is often the case in fiber-based systems. These change the observed polarization state to v(x) = C P(x) B u. While P(x) is essentially D-transposition symmetric and therefore a linear retarder, the presence of B and C modifies the overall transmission to become an overall retarder. However, B u only defines a modified input state u' = B u, which, when transmitted through P(x), changes iteratively through mirror state D u'. The unitary matrix C simply rotates this change in polarization state, mapping the apparent mirror state to the observed mirror state C D B u. In the presence of fixed, possibly separate, system components, the “input” polarization state observed at the receiver in the absence of sample transmission is C·B·u. Mirror The state phenomenon is that the change in polarization state involving propagation through a sample P(x) leads to the mirror state C D B u = C D C T (C B u), where C D C T defines a new symmetry plane, which is translated from the QU-plane defined by D under the similarity transformation by C. The reconstruction of the local retardation only requires the mirror states, not the input states or the symmetry plane.

[0075] The remaining challenge appears whenever v aligns with u', inhibiting the reconstruction of the local retardation and resulting in an artificially high local retardation (white arrow in Figure 8c). With circularly polarized input light, a half-wave retardation is required to achieve this alignment, which is not typical for many biological samples. However, some tissues feature significant birefringence, and control of the input state is not always possible. The resulting artifacts can be avoided by introducing a moderate amount of polarization mode dispersion (PMD) into the system and using spectral binning in the reconstruction. As shown in Figure 9, τ is reconstructed for each individual spectral bin along with a confidence map representing the alignment of v with u'. Because PMD distributes the input polarization state across the spectral bins, it is highly unlikely that v and u' will be aligned simultaneously in all bins. After aligning the rotation vectors across the spectral bins to compensate for PMD, the rotation vectors are weighted with a confidence metric and vectorially averaged (see below for details). The averaged norm of τ finally provides the artifact-free local sample retardation, as shown for a tissue-like phantom in FIG. 9(i).

[0076] Figure 9 shows a demonstration of the application of mirror state constraints in a fiber-based system and the removal of alignment-induced errors by using the wavelength dependence of the polarization state with spectral binning. (a) shows a simplified schematic diagram of the sample arm of a fiber-based interferometric imaging system. L1-L2 are lenses, PC is a polarization controller, M is a scanning mirror, and SMF is a single-mode fiber. (b) shows the principle of spectral binning, in which the entire spectrum is divided into nine bins by multiplying nine Hanning windows. (c) shows the illumination state of polarization (SOP) for each spectral bin. The SOP variation is introduced by the PMD induced by the polarization controller. (d)-(i) show cross-sectional images of a two-layer phantom reconstructed with different spectral bins. (d) shows the local retardation resolved using the 1st spectral bin. (e) shows the local retardation resolved using the 9th spectral bin. (f) shows the reliability metric map for the 1st spectral bin. (g) shows the confidence metric map for the 9th spectral bin. (h) shows the cross-sectional birefringence image reconstructed using the entire spectrum. (i) shows the cross-sectional birefringence image reconstructed with the spectral binning method. The scale bars shown in (i) for (d)-(i) are 100 μm vertically and 400 μm horizontally.

[0077] Thus, presented herein are devices, methods and systems for obtaining polarization information from a sample based on the unique properties of mirror polarization states when measuring backscattered light along identical illumination and detection paths without polarization dependent losses. In PS-OCT, the mirror states suppress depth-dependent polarization state changes and enable local retardation imaging that was previously unavailable in PS-OCT without significantly more complex measurements using multiple input states.

[0078] Experimental setup A single-input state polarization-sensitive spectral domain optical coherence tomography (PS-SD-OCT) system was used for the specific embodiments disclosed herein. Unpolarized light from a supercontinuum source was linearly polarized and, unless otherwise noted, achromatic. The light was adjusted to circular polarization with a quarter-wave plate and split into the reference and sample arms by a free-space beam splitter. A linear polarizer oriented at 45° in the reference arm defined the reference polarization state independent of the source polarization. In each of the interferometer arms, the light was directed to the sample and reference mirrors, respectively, in free space using a flip mirror, or in another embodiment was first coupled through 1.5 m of single mode fiber. For polarization management, a polarization controller with three paddles was applied to each fiber. For sample imaging, the light was focused with a 30 mm focal length lens to a FWHM spot diameter of approximately 8 μm and scanned in both lateral directions with a galvanometer mirror. At the exit port of the beam splitter, a half-wave plate precisely aligned the polarization axes of the sample and reference light with the slow and fast axes of the polarization-maintaining (PM) fiber. The polarization state of the PM fiber was then split using a polarizing beam splitter towards two identical custom-built spectrometers, which recorded the interference patterns of the horizontally (h) and vertically (v) polarized components, respectively. The detected source bandwidth was approximately 160 nm, centered at 840 nm, and an axial resolution of approximately 2.5 μm was measured in air. The recorded spectra were preprocessed by background subtraction, alignment between the two spectrometers, interpolation to a linear wavenumber k, and compensation for dispersion imbalance between the sample and reference arms. Details of the optical system are described below.

[0079] Data Processing In the first demonstration of the polarizing mirror state, light was passed through a fiber and reflected off a mirror in the sample arm. The paddle of the polarization controller in the sample arm was dynamically moved while recording the interference signal. The preprocessed interference pattern f h,v Analytic signal of (k) was constructed by setting the negative delays to zero. f' h,v (k)=FT -1 {FT {f h,v (k)}·H(z)}, where H(z) is the Heaviside step function, F T is the Fourier transform, z is the Fourier transform variable, and corresponds to the difference in optical path length. Next, the Stokes parameter [IQUV] T =[|f' h | 2 +|f' v | 2 |f' h | 2 -|f' h | 2 2Re{f' h f' v *}-2lm{f' h f' v *}] T Calculate ( * indicates the complex conjugate). A Gaussian kernel of 1 / 5 of the entire recorded spectral width is used to measure the center wavenumber k c Strikes around The normalized three-component Stokes vector s = [QUV] T / ( Q 2 +U 2 +V 2 ) 1 / 2 was calculated. The measurements are comparable to those obtained when using a conventional polarimeter in combination with a monochromatic laser source. The input polarization state was determined without any fiber segments in the two arms.

[0080] To change the polarization state in the three-layer phantom, the preprocessed spectral fringe data was Fourier transformed to obtain the tomographic image t h,v (z)=FT{f h,v(k)} was obtained, input into the Stokes parameters as above, spatially filtered with a 2D Gaussian kernel of width 20 μm in the axial direction and 80 μm in the lateral direction, and normalized to obtain the 3-component Stokes vector s(z). To process the retinal data, filtering was also performed in the second lateral direction using the same kernel.

[0081] For the reconstruction of the local retardation, we implemented equation (4) by adopting the pre-calibrated polarization mirror state u' and approximating v = (s[n+l] + s[n]) / 2 and δv / δx = (s[n+l] - s[n]) / Δz, where n is the pixel index along the depth z = nΔz and Δz is the axial sampling distance. To avoid high-frequency noise caused by taking the difference between adjacent points, the reconstructed rotation vector τ(z) was averaged axially over a rectangular Gaussian window of length 20 pixels, and then its norm was calculated as the local retardation image, scaled to degrees of retardation per depth (° / μm). For comparison, the cumulative retardation was calculated as the sum of s(z) and s(z) at each depth. surf ), z surf is the axial position of the sample surface within each depth profile.

[0082] To suppress artifacts when the mirror state coincides with the apparent optical axis, we multiply the preprocessed spectrum by a Hanning window han(k,m) of width Δk / N centered at mΔk / (2N) in the available k support (Δk,m∈[l,2N-1],N=5), resulting in nine spectral bins from which we can calculate the binned Stokes vector s(z,m). We also take the degree of polarization DOP=<(Q 2 +U 2 +V 2 ) 1 / 2 / I〉, where 〈〉 denotes the average over the spectral bins, and Q, U, V, and I are the spatially filtered Stokes components before normalization, which later serve as a metric to identify meaningful polarization signals. After performing the same local retardation processing as above for each bin, we obtained the rotation vector τ(z,m). These all represent the same sample birefringence, but their relative orientations may be offset due to system PMD introduced by the polarization controller paddle of the sample arm fiber. The rotation R(m) required to align each bin vector with the central bin N in the least-squares sense is given by:

[0083]

number

[0084] In the above equation, w(x,z)=1-v T ·u' is the projection distance from the mirror state R(m) is a weight that represents the reliability of a given Stokes vector. We assume that R(m) is constant throughout the B-scan and sum it over all points with sufficient DOP>0.8 and signal strength SNR>5dB.

number

number

[0085] Birefringence phantom Bands were cut from a scattering film of acrylonitrile butadiene styrene (ABS) and stretched after heating above the glass transition temperature. A three-layer phantom was assembled from three birefringent bands oriented at approximately 30° and 60° to the first layer. The second sample consisted of one long band on top of four short segments with different birefringence. The angle between the stretch directions of the bands in the first and second layers was approximately 45°. To provide a non-birefringent scattering matrix, the bands were embedded in ultrasound gel mixed with polystyrene beads (Polysciences Inc., 300 nm diameter).

[0086] Retinal Imaging Pig eyes were collected from a butcher and imaging was performed within 2 hours after collection of the eyeball. The eye was then dissected using a microscope, the lens removed, and the vitreous chamber emptied and injected with a sufficient amount of saline to prevent specular reflections while scanning the retina. The scanning area was centered on the optic nerve head.

[0087] Mueller matrix for reverse transmission through a reciprocal medium In this section, we derive the deterministic (i.e. non-depolarizing) Mueller matrix for the inverse transmission through a reciprocal element from the corresponding Jones representation. If A and B denote the front and rear interfaces of this element, then the Jones matrix J AB describes the forward propagation through this element (illustrated in Figure 10). If we want to maintain the orientation of the spatial xy coordinates regardless of the propagation direction, the inverse propagation is BA =J T AB is given by the transpose of

[0088] A matrix J in Jones form can be converted to the corresponding Mueller matrix M using

[0089]

number

[0090] The transpose of a Jones matrix corresponds to the transpose of the corresponding Mueller matrix, and the last elements excluding the diagonal elements are The signs of the rows and columns of are reversed. This represents the inverse transmission through the reciprocal elements. For a pure retardation matrix R in SO(3) form, this is R BA =D·R T AB D becomes D=d iag(1,1,-1).

[0091] Determining the rotation vector The effect of the linear retarder is the rotation vector ω=φ p=φ [cosα,sinα,0] T where p has unit length, α is the azimuth angle with respect to the Q axis, and φ is the rotation angle. The corresponding SO(3) rotation matrix is:

[0092]

number

[0093] Here, the input state u=[u1,u2,u3] T In some state v=[v1,v2,v3] T We are looking for a linear retarder P(φ,α) that maps to the rotation axis given by the intersection of the QU plane with the plane that bisects u and v.

[0094]

number

[0095] In general, u and v clearly define a single rotation axis. Only in the case of mirror polarization states do both the numerator and denominator vanish, but α is undefined and can take any value. To find the amount of rotation around this axis needed to map u to v, project both vectors onto a plane orthogonal to the axis of rotation and evaluate their relative angle.

[0096]

number

[0097] where p=[cosα,sinα,0] T and p T ·u=p T ·v. QU Hei A defined in-plane rotation vector ω maps u to a generic v. If and only if the mirror state v = u' exists for any azimuthal rotation in the QU-plane, which when wrapped to φ∈(-π,π) traces a continuous curve in the QU-plane (the red curve in Figure 11(b)).

[0098] The shape and orientation of this trace is determined by u. If we assume that the azimuth and elevation angles of u are η and ε, respectively, where η∈[0,2π],ε∈[-π / 2,π / 2], then the maximum rotation angle φ=π occurs when the rotation vector is aligned with the projection of u onto the QU plane, which is τ in Figure 11(a). An The minimum rotation angle is given by the rotation axis τ Bn is τ An In this case, the orbit coincides with the longitude of the sphere, and ω B =2|ε| (ε is the elevation angle of u) As visualized in FIG. 11(b), the long axis of the trace is aligned with the azimuth angle of the input state u, and its curvature is determined by the elevation angle of u.

[0099] Change in polarization state via mirror state According to equation (2), the change in polarization state v is determined by τ×. Substituting P into the expression in equation (1), we obtain

[0100]

number

[0101] q× is not only skew-symmetric, but also D-transpose-symmetric, restricting q to the QU plane. However, D AT M T A similarity transformation of q× by D will in general rotate the apparent optical axis q out of the QU plane. Reintegrating ∂v / ∂x in equation (2) for a given x gives a first-order approximation to the change in v.

[0102]

number

[0103] where v=D A T M T ·D·M·A·u and the properties of the matrix exponential function exp(A·B· A -1 )=A exp(B) A -1 where Δx determines the amount of retardance of the linear retarder exp(Δx·q×). As explained above, it is always possible to find a suitable retardance for a given orientation of q in the QU plane to map M·A·u to its mirror state D·M·A·u, such that v=D·u. Thus, a first-order approximation of the change in the polarization state at a given depth lies on a circle that varies (periodically) through the mirror states. Since q× is constant in a uniformly birefringent sample layer, the first-order change does not depend on depth in this layer, and the polarization state closely follows the approximation circle. Remarkably, the apparent rotation axis τ is the same as that of D·A T M T ·D is related to the true sample optical axis q× via a similarity transformation.

[0104] 12 shows a setup that may be used in various embodiments disclosed herein, where SCL is a supercontinuum laser, L1-L15 are lenses, P1-P2 are polarizers, Q1 is a quarter wave plate, H1 is a half wave plate, BS is a beam splitter, PBS is a polarizing beam splitter, SP is a spectrometer, M1-M5 are mirrors, FM1-FM2 are flip mirrors, PC1-PC2 are polarization controllers, SMF is single mode fiber, PMF is polarization maintaining fiber, and DC is dispersion compensation. SCL: Supercontinuum laser is DK-3460, manufactured by NKT Photonics Inc.; P1, P2: Polarizer is LPNIRE100-B, manufactured by Thorlabs Inc.; Q1: 1 / 4 wave plate is AQWP10M-980, manufactured by Thorlabs Inc.; H1: 1 / 2 wave plate is AHWP10M-980, manufactured by Thorlabs Inc.; BS: Non-polarizing beam splitter (50 / 50) is BS014, manufactured by Thorlabs Inc.; PBS: Polarizing beam splitter (50 / 50) is PBS202, manufactured by Thorlabs Inc.; DC: Dispersion compensation is BK7 glass plate. SP: Spectrometer, each custom spectrometer consists of a transmission grating (Wasatch TG: 1200 lines / mm), a camera lens (L9: Nikon AF 85mm f / l.8D) and a line scan CCD (E2V AVIIVA EV71YEM4CL2010-BA9). SMF: Single mode fiber is 780HP, Thorlabs Inc. PMF: Polarization maintaining fiber is P3-780PM-FC-2, Thorlabs Inc. PC: Polarization controller is a 3-paddle polarization controller with 1, 2 and 1 fiber loops on the 3 paddles respectively.

[0105] 13, an example system 1300 for determining the retardance of a layer of a sample is illustrated in accordance with some embodiments of the disclosed subject matter. As shown in FIG. 13, a computing device 1310 can receive polarization data from a detector 1302, such as a detector associated with a PS-OCT system. In some embodiments, the computing device 1310 can execute at least a portion of a retardance determination system 1304 to determine the retardance of a layer of a sample based on the polarization data received from the detector 1302. Additionally or alternatively, in some embodiments, the computing device 1310 can communicate information regarding the polarization data received from the detector 1302 to a server 1320 via a communication network 1306, and can execute at least a portion of the retardance determination system 1304 to determine the retardance of a layer of a sample based on the polarization data received from the detector 1302. In some such embodiments, the server 1320 can return information indicative of the output of the retardance determination system 1304 to the computing device 1310 (and / or any other suitable computing device). This information may be transmitted and / or presented to a user (eg, an operator, a clinician, etc.) and / or stored (eg, as part of a medical record associated with the subject).

[0106] In some embodiments, the computing device 1310 and / or the server 1320 may be any suitable computing device or combination of such devices, such as a desktop computer, a laptop computer, a smartphone, a tablet computer, a wearable computer, a server computer, a virtual machine running on a physical computing device, etc.

[0107] In some embodiments, the detector 1302 may be any suitable detector for use in performing polarization-sensitive optical coherence tomography (PS-OCT). In some embodiments, the interference detector 1302 may be located locally to the computing device 1310. For example, the detector 1302 may be incorporated into the computing device 1310 (e.g., the computing device 1310 may be configured as part of a device for detecting light as part of a PS-OCT system). As another example, the detector 1302 may be connected to the computing device 1310 by a cable, a direct wireless link, or the like. Additionally or alternatively, in some embodiments, the detector 1302 may be located locally to the computing device 1310 and / or remotely from the computing device 1310 and may communicate polarization information to the computing device 1310 (and / or the server 1320) via a communications network (e.g., the communications network 1306).

[0108] In some embodiments, the communication network 1306 may be any suitable communication network or any combination of suitable communication networks. For example, the communication network 1306 may include a Wi-Fi network (which may include one or more wireless routers, one or more switches, etc.), a peer-to-peer network (e.g., a Bluetooth® network), a cellular network (e.g., a 3G network, a 4G network, a 5G network, etc., conforming to any suitable standard, such as CDMA, GSM, LTE, LTE Advanced, WiMAX®, etc.), a wired network, etc. In some embodiments, the communication network 1306 may be a local area network, a wide area network, a public network (e.g., the Internet), a private or semi-private network (e.g., a corporate or university intranet), any other suitable type of network, or any suitable combination of any other suitable type of networks. The communication links illustrated in FIG. 13 may each be any suitable communication link or combination of such communication links, such as a wired link, an optical fiber link, a Wi-Fi® link, a Bluetooth® link, a cellular link, etc.

[0109] FIG. 14 illustrates example hardware 1400 that may be used to implement a computing device 1310 and a server 1320 in accordance with some embodiments of the disclosed subject matter. As illustrated in FIG. 14, in some embodiments, the computing device 1310 may include a processor 1402, a display 1404, one or more inputs 1406, one or more communication systems 1408, and / or a memory 1410. In some embodiments, the processor 1402 may be any suitable hardware processor, such as a central processing unit, a graphics processing unit, or a combination of such processors. In some embodiments, the display 1404 may include any suitable display device, such as a computer monitor, a touch screen, a television, or the like. In some embodiments, the input 1406 may include any suitable input device and / or sensor that may be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, or the like.

[0110] In some embodiments, communications system 1408 may include any suitable hardware, firmware, and / or software for communicating information over communications network 1306 and / or any other suitable communications network. For example, communications system 1408 may include one or more transceivers, one or more communications chips and / or chipsets, etc. In a more specific example, communications system 1408 may include a Wi-Fi -The device may include hardware, firmware, and / or software that can be used to establish a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, and the like.

[0111] In some embodiments, memory 1410 may include any suitable storage device or may include devices that may be used to store instructions, values, etc. that processor 1402 may use to present content using display 1404, for example, to communicate with server 1320, such as via communication system(s) 1408. Memory 1410 may include any suitable volatile memory, non-volatile memory, storage, or any suitable combination thereof. For example, memory 1410 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid state drives, one or more optical drives, etc. In some embodiments, memory 1410 may be encoded with a computer program for controlling the operation of computing device 1310. In such embodiments, processor 1402 may execute at least a portion of the computer program to present content (e.g., images, user interfaces, graphics, tables, etc.), receive content from server 1320, transmit information to server 1320, etc.

[0112] In some embodiments, the server 1320 may include a processor 1412, a display 1414, one or more inputs 1416, one or more communication systems 1418, and / or memory 1420. In some embodiments, the processor 1412 may be any suitable hardware processor, such as a central processing unit, a graphics processing unit, or a combination of these processors. In some embodiments, the display 1414 may include any suitable display device, such as a computer monitor, a touch screen, a television, or the like. In some embodiments, the input 1416 may include any suitable input device and / or sensor that may be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, or the like.

[0113] In some embodiments, communications system 1418 may include any suitable hardware, firmware, and / or software for communicating information over communications network 1306 and / or any other suitable communications network. For example, communications system 1418 may include one or more transceivers, one or more communications chips and / or chipsets, etc. In more specific examples, communications system 1418 may include hardware, firmware, and / or software that can be used to establish a Wi-Fi® connection, a Bluetooth® connection, a cellular connection, an Ethernet connection, etc.

[0114] In some embodiments, memory 1420 may include any suitable storage device or may include a device that may be used to store instructions, values, etc. that processor 1412 may use to present content using display 1414, for example, to communicate with one or more computing devices 1310, etc. Memory 1420 may include any suitable volatile memory, non-volatile memory, storage, or any suitable combination thereof. For example, memory 1420 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid state drives, one or more optical drives, etc. In some embodiments, memory 1420 may be encoded with a server program for controlling the operation of server 1320. In such embodiments, the processor 1412 may execute at least a portion of a server program to send information and / or content (e.g., tissue identification and / or classification results, user interfaces, etc.) to one or more computing devices 1310, receive information and / or content from one or more computing devices 1310, receive instructions from one or more devices (e.g., a personal computer, a laptop computer, a tablet computer, a smartphone, etc.), etc.

[0115] In some embodiments, any suitable computer-readable medium may be used to store instructions for performing the functions and / or processes described herein. For example, in some embodiments, computer-readable media may be transitory or non-transient. For example, non-transient computer-readable media may include media such as magnetic media (hard disks, floppy disks, etc.), optical media (compact disks, digital video disks, Blu-ray® disks, etc.), semiconductor media (RAM, flash memory, electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), any suitable media that does not lack similarity of non-transient or permanent in transmission, and / or any suitable tangible media. As another example, transitory computer-readable media may include networks, wires, conductors, optical fibers, circuits, or any suitable media that lack similarity of transitory and permanent in transmission, and / or any suitable intangible media.

[0116] It should be noted that the term mechanism, as used herein, can encompass hardware, software, firmware, or any suitable combination thereof.

[0117] FIG. 15 illustrates an example process 1500 for determining the retardance of a layer of a sample according to some embodiments of the disclosed subject matter. As shown in FIG. 15, at 1502, the process 1500 can send a first portion of polarized light to a sample arm of an optical system and a second portion of polarized light to a reference arm of the optical system. At 1504, the process 1500 can combine the first return light returned from the sample arm and the second return light returned from the reference arm. At 1506, the process 1500 can detect the combined light along a first polarization state and a second polarization state to generate polarization data, where the second polarization state may be different from the first polarization state. At 1508, the process 1500 can determine the polarization states of light returning from the top and bottom surfaces of the layer of the sample based on the detection of the combined light. Finally, at 1510, the process 1500 can determine the retardance of the layer of the sample based on the determined polarization states.

[0118] It should be understood that the above steps of the process of Figure 15 may be performed or executed in any order or sequence, including but not limited to the order and sequence shown and described in the figures. Also, some of the above steps of the process of Figure 15 may be performed or executed substantially simultaneously or in parallel, as desired, to reduce latency and processing times.

[0119] Those skilled in the art will recognize that many other changes and modifications to the above specific embodiments of the invention are possible within the scope of the invention, and therefore, all statements herein should be interpreted as illustrative and not restrictive.

Claims

1. 1. A method for determining the retardance of a layer of a sample, comprising: directing a first portion of polarized light into a sample arm of an optical system and a second portion of said polarized light into a reference arm of said optical system; a step of combining a first return light returned from the sample arm and a second return light returned from the reference arm; detecting, with a detector, the combined light along a first polarization state and a second polarization state different from the first polarization state to generate polarization data; determining, with a processor coupled to the detector, a polarization state of light returning from the top and bottom surfaces of the sample layer based on detection of the combined light; and determining, with the processor, a retardance of the layer of the sample based on the determined polarization state.

2. The method of claim 1 , further comprising determining a mirror state associated with the polarization data.

3. The method of claim 2 , wherein the mirror states comprise points on a Poincaré sphere.

4. The method of claim 2 or 3, wherein the mirror state comprises an input polarization state with reversed helicity.

5. The method of any one of claims 1 to 4, wherein the layer of the sample comprises a sub-surface layer of the sample.

6. the optical system comprises an optical coherence tomography system; 6. The method of claim 1, wherein detecting the combined light along a first polarization state and a second polarization state further comprises detecting the combined light along a first polarization state and a second polarization state using the optical coherence tomography system.

7. determining the polarization state of light returning from the top and bottom surfaces of the layer of the sample; determining a rotation angle and a rotation axis of a circle of rotation associated with the polarization states from the top and bottom surfaces of the layer of the sample; The method of any one of claims 1 to 6, further comprising determining a retardance level and an apparent optical axis based on the determination of the rotation angle and the rotation axis, respectively.

8. The sample has a plurality of layers; determining a retardance of each of the layers of the sample based on the determined polarization state; The method of any one of claims 1 to 7, further comprising generating a reconstruction of the sample based on the retardance of each of the layers of the sample.

9. 9. The method of claim 1, wherein determining the retardance of the layer of the sample further comprises determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts.

10. Determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts is based on spectral binning and includes 10. The method of claim 9, further comprising determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts.

11. The method of any one of claims 1 to 10, wherein the polarized light comprises circular polarized light.

12. the detector comprises a first detector and a second detector; detecting the combined light along a first polarization state and a second polarization state to generate polarization data further comprises sending the combined light to a polarizing beam splitter; 12. The method of claim 1, wherein the polarizing beam splitter is configured to transmit light having the first polarization state to the first detector and transmit light having the second polarization state to the second detector.

13. The method according to any one of claims 1 to 12, wherein the sample is at least one of an ophthalmological sample, a dermatological sample, an intravascular sample or a gastrointestinal sample.

14. The method of any one of claims 1 to 13, wherein the optical system comprises a polarization-sensitive optical coherence tomography (PS-OCT) system.

15. 1. An apparatus for determining the retardance of a layer of a sample, comprising: an interferometer optical system including a sample arm and a reference arm; a light source coupled to the optical system; A detector; a processor coupled to the detector; the light source is configured to send a first portion of polarized light to the sample arm and a second portion of the polarized light to the reference arm; the optical system is configured to combine a first return light from the sample arm and a second return light from the reference arm; the detector is configured to detect the combined light along a first polarization state and a second polarization state different from the first polarization state to generate polarization data; The processor, determining a polarization state of light returning from the top and bottom surfaces of the layer of the sample based on detection of the combined light by the detector; an apparatus configured to determine a retardance of the layer of the sample based on the determined polarization state.

16. The apparatus of claim 15 , wherein the processor is further configured to determine a mirror state associated with the polarization data.

17. The apparatus of claim 16 , wherein the mirror states comprise points on a Poincaré sphere.

18. 18. Apparatus according to claim 16 or 17, wherein the mirror state comprises an input polarization state with reversed helicity.

19. An apparatus according to any one of claims 15 to 18, wherein the layer of the sample comprises a sub-surface layer of the sample.

20. the optical system comprises an optical coherence tomography system; The processor, upon detecting the combined light along a first polarization state and a second polarization state, uses the optical coherence tomography system to detect the combined light along the first polarization state and the second polarization state.

20. The apparatus of claim 15, further configured to detect along a first polarization state and a second polarization state.

21. The processor, in determining the polarization state of light returning from the top and bottom surfaces of the layer of the sample, comprises: determining a rotation angle and a rotation axis of a circle of rotation associated with the polarization states from the top and bottom surfaces of the layer of the sample; The apparatus of any one of claims 15 to 20, further configured to determine a retardance level and an apparent optical axis based on determining the rotation angle and the rotation axis, respectively.

22. The sample has a plurality of layers; The processor is further configured, in determining the retardance of the layers of the sample based on the determined polarization state, to determine a retardance of each of the layers of the sample based on the determined polarization state; 22. The apparatus of claim 15, wherein the processor is further configured to generate a reconstruction of the sample based on the retardance of each of the layers of the sample.

23. 23. The apparatus of claim 15, wherein the processor is further configured to determine the retardance of the layer of the sample using a wavelength dependence of the polarization state to reduce artifacts when determining the retardance of the layer of the sample.

24. 24. The apparatus of claim 23, further configured, when determining the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts, to determine the retardance of the layer of the sample using the wavelength dependence of the polarization state to reduce artifacts based on spectral binning.

25. 25. An apparatus according to any one of claims 15 to 24, wherein the polarised light comprises circular polarised light.

26. the detector comprises a first detector and a second detector; the apparatus is configured to transmit the combined light to a polarizing beam splitter; 26. The apparatus of claim 15, wherein the polarizing beam splitter is configured to transmit light having the first polarization state to the first detector and to transmit light having the second polarization state to the second detector.

27. 27. The method according to claim 15, wherein the sample is at least one of an ophthalmological sample, a dermatological sample, an intravascular sample, or a gastrointestinal sample. The apparatus described in

28. The apparatus of any one of claims 15 to 27, wherein the optical system comprises a polarization-sensitive optical coherence tomography (PS-OCT) system.

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