Snapshot Mueller Matrix Polarization Imaging Device and Method Based on Photo-Oriented Liquid Crystal Chip

Abstract

The present invention discloses a snapshot Mueller matrix polarization imaging device and method based on an optically oriented liquid crystal chip. The device includes a light source unit, a polarization modulation unit based on an optically oriented liquid crystal chip, a sample measurement unit, a polarization demodulation unit based on an optically oriented liquid crystal chip, a light detector unit, and a computer. The present invention generates polarized light with a polarization state distributed in a specific manner in space through the light source unit and the polarization modulation unit. After the polarized light acts on the sample to generate signal light, and after the signal light passes through the polarization demodulation unit, the signal image collected by the light detector through a single exposure can be obtained, and the spatial distribution of all Mueller matrix elements of the sample to be measured can be calculated by the computer. The present invention only needs a single exposure of the light detector to complete the measurement, with a fast measurement speed; at the same time, it does not require mechanical rotation elements, electrically tunable elements, etc., and has a simple and compact structure; it has the advantages of high spatial resolution, high measurement accuracy, and low manufacturing cost.

Description

Technical Field

[0001] The present invention belongs to the field of polarization optical measurement, and particularly relates to a snapshot Mueller matrix polarization imaging device and method based on a photo-aligned liquid crystal chip. Background Art

[0002] Mueller matrix polarization imaging technology detects objects by measuring the spatial distribution of Mueller matrix elements of the target object, and can provide hidden information that cannot be obtained by traditional imaging technology. The Mueller matrix is a 4×4 matrix that can comprehensively describe the polarization changes after the interaction between light and the object, including birefringence, dichroism, depolarization and other characteristics. Therefore, Mueller matrix polarization imaging technology can obtain the most complete polarization information of the target object, which not only has important scientific significance, but also has broad application prospects in the fields of biomedicine, industrial inspection, environmental monitoring, etc.

[0003] Mueller matrix polarization imaging technology is mainly divided into two categories: time-sharing type and snapshot type. The time-sharing type technology collects multiple photos through timing control of mechanical or electrically tunable elements to reconstruct the Mueller matrix. Although the principle is simple and the spatial resolution is high, the sampling time is long, and multiple measurements affect the result accuracy, which is only suitable for the analysis of static or slowly time-varying samples. The snapshot type technology generally uses the method of polarization spatial modulation, and only needs to use a single detector for one exposure to complete sampling, and then obtains the spatial distribution of the Mueller matrix. It has the advantages of fast measurement speed and compact structure. Existing solutions mainly include snapshot Mueller matrix polarization imaging technology based on polarization gratings or birefringent crystals and snapshot Mueller matrix polarization imaging technology based on optical metasurfaces, but they generally have problems such as low spatial resolution, low measurement accuracy, high processing cost, and complex structure. Therefore, their practical applications are greatly limited. Summary of the Invention

[0004] The purpose of the present invention is to provide a snapshot Mueller matrix polarization imaging device and method based on a photo-aligned liquid crystal chip in view of the deficiencies of the prior art. Compared with other snapshot Mueller matrix polarization imaging technologies, this device has the advantages of high spatial resolution, high measurement accuracy, low cost, and simple and compact structure.

[0005] The technical solution for realizing the purpose of the present invention is as follows:

[0006] A snapshot Mueller matrix polarization imaging device based on a photo-aligned liquid crystal chip includes a light source unit, a polarization modulation unit, a sample measurement unit, a polarization demodulation unit, a light detector unit, and a computer;

[0007] The light source unit, the polarization modulation unit, the sample measurement unit, the polarization demodulation unit, and the light detector unit are sequentially arranged along the optical path direction;

[0008] The light source unit is sequentially provided with a monochromatic light source and a first lens along the optical path direction; the polarization modulation unit is sequentially provided with a first polarizer and a first liquid crystal chip along the optical path direction; the light source unit and the polarization modulation unit generate polarized light with a polarization state distributed in a specific manner in space;

[0009] The sample measurement unit is sequentially provided with a second lens, a sample stage and a third lens along the optical path direction, and the sample is placed on the sample stage;

[0010] The polarization demodulation unit is sequentially provided with a second liquid crystal chip and a second polarizer along the optical path direction; the light detector unit is sequentially provided with a fourth lens and a light detector along the optical path direction; the polarization demodulation unit and the light detector unit perform full Stokes polarization imaging on the signal light emitted by the sample measurement unit;

[0011] In the direction along the optical path, the photosensitive surfaces of the first liquid crystal chip, the sample, the second liquid crystal chip and the light detector are in an optically conjugate position; in the direction perpendicular to the optical path, each sub-pixel of the first liquid crystal chip and each super-pixel of the second liquid crystal chip are in an optically conjugate position;

[0012] The computer is connected to the light detector and acquires the signal image detected by the light detector; based on the image acquired by the light detector during one-shot photographing exposure, the computer calculates the spatial distribution of all Mueller matrix elements of the sample, realizing snapshot Mueller matrix polarization imaging.

[0013] The optical path of the described device is defined in a right-handed rectangular coordinate system XYZ, where the main optical axis is parallel to the Z axis, the positive direction of the Z axis is the light propagation direction, and the transmission axis of the first polarizer is parallel to the X axis.

[0014] Both the first liquid crystal chip and the second liquid crystal chip are liquid crystal cells with a micro-structure fabricated by using liquid crystal optical orientation technology and filled with nematic liquid crystal. The micro-structures of the first liquid crystal chip and the second liquid crystal chip are both composed of super-pixels that are periodically repeated in space. Each super-pixel is internally divided into at least four sub-pixels with equal areas. In each sub-pixel, the nematic liquid crystal is uniformly oriented in a certain specific direction, and in different sub-pixels, the orientation directions of the liquid crystal are different.

[0015] For the described device, the micro-structures of the first liquid crystal chip and the second liquid crystal chip are both composed of super-pixels that are periodically repeated in space. Each super-pixel is internally divided into four sub-pixels with equal areas; in each sub-pixel, the liquid crystal is uniformly oriented, and the director n is parallel to the surface. For the first liquid crystal chip and the second liquid crystal chip, the angle between the director n of the liquid crystal in sub-pixel i and the X axis is θ 1 = 51.69°, and the angle between the director n of the liquid crystal in sub-pixel ii and the X axis is θ2 = 164.88°, and the angles between the liquid crystal director n and the X-axis in sub-pixel iii are all θ 3 = 128.31°, and the angles between the liquid crystal director n and the X-axis in sub-pixel iv are all θ 4 = 15.12°; the phase delays introduced by the first liquid crystal chip 202 and the second liquid crystal chip 401 are both 228.19°.

[0016] The optical detector is a planar array image sensor that can capture a complete two-dimensional image of the signal light within one exposure period.

[0017] A measurement method using the snapshot Mueller matrix polarization imaging device based on the photo-aligned liquid crystal chip described above, including the steps of calculating the spatial distribution of all Mueller matrix elements of the sample from the image collected by one-time photographing and exposure by the optical detector:

[0018] In the XYZ coordinate system, let the Mueller matrix of the first polarizer be M P1 , and the Mueller matrix of the second polarizer be M P2 , the Mueller matrix of each sub-pixel of the first liquid crystal chip be , the Mueller matrix of each sub-pixel of the second liquid crystal chip be , j = 1, 2, 3, …, n, the Mueller matrix of the sample be M sample , the Stokes parameters of the light emitted by the monochromatic light source be S 0 , the Stokes parameters of the light conjugate to the i th sub-pixel of the first liquid crystal chip emerging from the sample be , i = 1, 2, 3, …, m, the Stokes parameters of the light conjugate to the j th sub-pixel of the second liquid crystal chip incident on the photosensitive surface of the optical detector be , 𝑗 = 1, 2, 3, …, n; denote The first element of be I (j) , which can be measured by the optical detector; denote ; let The first row of be , denote ; then the Mueller matrix of the sample is M sample , the light intensity I measured by the optical detector (j) , j = 1, 2, 3, …, n, and is calculated by the following formula

[0019] ;

[0020] Among them, is M PSG the Moore-Penrose pseudoinverse of

[0021] ;

[0022] is M PSA the Moore-Penrose pseudoinverse of

[0023] Advantages of the present invention:

[0024] (1) The snapshot Mueller matrix polarization imaging device based on the photo-aligned liquid crystal chip provided by the present invention only needs to use a single photo-detector for one exposure to complete sampling, and then calculates the spatial distribution of all Mueller matrices of the sample. It has a fast measurement speed and is suitable for observing dynamic samples.

[0025] (2) The use of the photo-aligned liquid crystal chip to achieve structured polarization illumination and full Stokes polarization imaging not only has a flexible spatial modulation method, can greatly reduce the condition number of the measurement matrix by optimizing the liquid crystal orientation pattern, significantly improve the measurement accuracy of the system, but also greatly reduces the processing difficulty, reduces the processing cost, and improves the processing efficiency compared with the optical metasurface solution.

[0026] (3) The optical path structure design that uses a liquid crystal chip conjugated with the sample to be measured, while keeping the system structure simple and compact, does not involve diffraction and spatial frequency spectrum analysis, has a high light energy utilization rate, a high utilization rate of the camera frame, and the spatial resolution of imaging can reach the theoretical maximum value of the snapshot type solution.

[0027] Of course, not all of the beneficial effects described above can be achieved by any technical solution of the present invention. Brief Description of the Drawings

[0028] Figure 1 is a schematic structural diagram of an embodiment of the device of the present invention.

[0029] Figure 2 is a schematic structural diagram of the liquid crystal chip in an embodiment of the device of the present invention.

[0030] In the figure, light source unit 1, polarization modulation unit 2, sample measurement unit 3, polarization demodulation unit 4, photo-detector unit 5, computer 6, white light source 101, color filter 102, first lens 103, first polarizer 201, first liquid crystal core 202, 301, sample stage 302, third lens 303, second liquid crystal chip 401, second polarizer 402, fourth lens 501, photo-detector 502. Detailed Embodiments

[0031] Referring to the accompanying drawings, the present invention will be further described in detail below in conjunction with specific embodiments.

[0032] In an exemplary embodiment, as Figure 1 shown, a snapshot Mueller matrix polarization imaging device based on a photo-aligned liquid crystal chip is provided, which includes a light source unit 1, a polarization modulation unit 2, a sample measurement unit 3, a polarization demodulation unit 4, a light detector unit 5, and a computer 6 that are sequentially connected along the optical path.

[0033] In the light source unit 1, a monochromatic light source composed of a white light source 101 and a color filter 102, and a first lens 103 are sequentially provided along the optical path direction. In the polarization modulation unit 2, a first polarizer 201 and a first liquid crystal chip 202 are sequentially provided along the optical path direction. The light source unit 1 and the polarization modulation unit 2 generate polarized light with a polarization state distributed in a specific manner in space. In the sample measurement unit 3, a second lens 301, a sample stage 302, and a third lens 303 are sequentially provided along the optical path direction, and the sample is placed on the sample stage 302. In the polarization demodulation unit 4, a second liquid crystal chip 401 and a second polarizer 402 are sequentially provided along the optical path direction. In the light detector unit 5, a fourth lens 501 and a light detector 502 are sequentially provided along the optical path direction. The polarization demodulation unit 4 and the light detector unit 5 perform full Stokes polarization imaging on the signal light emitted by the sample measurement unit 3. The computer 6 is connected to the light detector 502 to collect the signal image detected by the light detector 502.

[0034] The optical path of the device is defined in a right-handed rectangular coordinate system XYZ, where the main optical axis is parallel to the Z axis (the positive direction of the Z axis is the light propagation direction), the transmission axis of the first polarizer 201 is parallel to the X axis, and the angle between the transmission axis of the second polarizer 402 and the X axis is 0°.

[0035] Both the first liquid crystal chip 202 and the second liquid crystal chip 401 are liquid crystal cells with microstructures fabricated using liquid crystal optical control orientation technology and filled with E7 nematic liquid crystal, and the liquid crystal thickness is equal everywhere inside the liquid crystal cell. The microstructures of the first liquid crystal chip 202 and the second liquid crystal chip 401 are both composed of superpixels that are periodically repeated in space, and each superpixel is divided into four sub-pixels with equal areas, as Figure 2 shown. In each sub-pixel, the liquid crystal is uniformly oriented, and the director n is parallel to the surface. For the first liquid crystal chip 202 and the second liquid crystal chip 401, the angle between the liquid crystal director n and the X axis in sub-pixel i is θ 1 = 51.69° (positive in the counterclockwise direction), the angle between the liquid crystal director n and the X axis in sub-pixel ii is θ 2 = 164.88°, and the angle between the liquid crystal director n and the X axis in sub-pixel iii is θ3 = 128.31°, and the angle between the liquid crystal director n in sub-pixel iv and the X-axis is θ 4 = 15.12°. The phase retardation introduced by the first liquid crystal chip 202 and the second liquid crystal chip 401 is both 228.19°.

[0036] Along the optical axis direction, the photosensitive surfaces of the first liquid crystal chip 202, the sample, the second liquid crystal chip 401, and the optical detector 502 are in optically conjugate positions; perpendicular to the optical axis direction, each sub-pixel of the first liquid crystal chip 202 and each super-pixel of the second liquid crystal chip 401 are in optically conjugate positions.

[0037] The optical detector 502 can use a black and white camera. The spatial distribution of all Mueller matrix elements of the sample can be calculated by the following method from the image collected by one exposure of the optical detector 502:

[0038] In the XYZ coordinate system, let the Mueller matrix of the first polarizer 201 be M P1 , and the Mueller matrix of the second polarizer 402 be M P2 , the Mueller matrix of each sub-pixel of the first liquid crystal chip 202 be , the Mueller matrix of each sub-pixel of the second liquid crystal chip 401 be , ( j = 1, 2, 3, …, n), the Mueller matrix of the sample be M sample , the Stokes parameters of the light emitted by the monochromatic light source be S 0 , the Stokes parameters of the light conjugate to the i th sub-pixel of the first liquid crystal chip 202 emerging from the sample be (i = 1, 2, 3, …, n), the Stokes parameters of the light incident on the photosensitive surface of the optical detector 502 conjugate to the j th sub-pixel of the second liquid crystal chip 401 be (𝑗 = 1, 2, 3, …, n); denote 's first element as I (j) , which can be measured by the optical detector 502; denote ; let 's first row be , denote ; then the Mueller matrix of the sample is M sample , and the light intensity I (j) ( j = 1, 2, 3, …, n) that can be measured by the optical detector 502 is calculated by the following formula

[0039] ;

[0040] wherein, is M PSG the Moore-Penrose pseudoinverse of,

[0041] ;

[0042] is M PSA the Moore-Penrose pseudoinverse of.

[0043] The described device only needs to use a single photodetector for one exposure to complete sampling, and then calculates the spatial distribution of all Mueller matrices of the sample, with a fast measurement speed. The spatial resolution of the imaging of the described measuring device is excellent, reaching the theoretical upper limit of the snapshot Mueller matrix polarization imaging technology (reducing the spatial resolution of the 4-fold linear dimension and increasing the measurement speed by 16 times). At the same time, M PSG and M PSA both have a condition number of 1.73, so the described measuring device has excellent measurement accuracy.

[0044] The embodiments in the above description can be further combined or replaced, and the embodiments are only descriptions of the preferred embodiments of the present invention, not limitations on the concept and scope of the present invention. Without departing from the design idea of the present invention, various changes and improvements made by those of ordinary skill in the art to the technical solutions of the present invention all belong to the protection scope of the present invention. The protection scope of the present invention is given by the appended claims and any equivalents thereof.

Claims

1. A snapshot Mueller matrix polarization imaging device based on a light-oriented liquid crystal chip, characterized in that: It includes a light source unit, a polarization modulation unit, a sample measurement unit, a polarization demodulation unit, a light detector unit and a computer; The light source unit, polarization modulation unit, sample measurement unit, polarization demodulation unit and light detector unit are arranged in sequence along the light path direction; The light source unit is provided with a monochromatic light source and a first lens in sequence along the light path; the polarization modulation unit is provided with a first polarizer and a first liquid crystal chip in sequence along the light path; the light source unit and the polarization modulation unit generate polarized light whose polarization state is distributed in a specific manner in space; The sample measurement unit is provided with a second lens, a sample stage and a third lens in sequence along the optical path direction, and the sample is placed on the sample stage; The polarization demodulation unit is provided with a second liquid crystal chip and a second polarizer in sequence along the optical path; the light detector unit is provided with a fourth lens and a light detector in sequence along the optical path; the polarization demodulation unit and the light detector unit perform full Stokes polarization imaging on the signal light emitted by the sample measurement unit; In the direction of the optical path, the photosensitive surfaces of the first liquid crystal chip, the sample, the second liquid crystal chip and the photodetector are in optical imaging conjugate positions; In a direction perpendicular to the optical path, each sub-pixel of the first liquid crystal chip and each super-pixel of the second liquid crystal chip are in an optical imaging conjugate position; The computer is connected to the photodetector to collect the signal image detected by the photodetector; based on the image collected by the photodetector in one exposure, the computer calculates the spatial distribution of all Mueller matrix elements of the sample to achieve snapshot Mueller matrix polarization imaging.

2. The device according to claim 1, characterized in that The optical path of the device is defined in a right-hand rectangular coordinate system XYZ, wherein the principal optical axis is parallel to the Z axis, the positive direction of the Z axis is the light propagation direction, and the light transmission axis of the first polarizer is parallel to the X axis.

3. The device according to claim 1 or 2, characterized in that: The first liquid crystal chip and the second liquid crystal chip are both liquid crystal boxes with microstructures that are infused with nematic liquid crystals and are made using liquid crystal light-controlled orientation technology. The microstructures of the first liquid crystal chip and the second liquid crystal chip are both composed of superpixels that are periodically repeated in space, and each superpixel is divided into at least four sub-pixels of equal area. In each sub-pixel, the nematic liquid crystal is uniformly oriented along a specific direction, and the orientation directions of the liquid crystals are different in different sub-pixels.

4. The device according to claim 3, characterized in that The microstructures of the first liquid crystal chip and the second liquid crystal chip are composed of superpixels that are periodically repeated in space. Each superpixel is divided into four subpixels of equal area. In each subpixel, the liquid crystal is uniformly oriented, and the director n is parallel to the surface. For the first liquid crystal chip and the second liquid crystal chip, the angle between the liquid crystal director n in subpixel i and the X-axis is θ 1=51.69°, the angle between the liquid crystal director n and the X axis in sub-pixel ii is θ 2=164.88°, the angle between the liquid crystal director n and the X axis in sub-pixel iii is θ 3 = 128.31°, the angle between the liquid crystal director n and the X axis in sub-pixel iv is θ 4=15.12°; the phase delay introduced by the first liquid crystal chip (202) and the second liquid crystal chip (401) is both 228.19°.

5. The device according to claim 1, characterized in that The light detector is an area array image sensor capable of capturing a complete two-dimensional image of signal light within one exposure cycle.

6. A measurement method using the snapshot Mueller matrix polarization imaging device based on the light-oriented liquid crystal chip according to claim 2, characterized in that: The method comprises the steps of calculating the spatial distribution of all Mueller matrix elements of the sample using an image collected by the light detector through a single exposure: In the XYZ coordinate system, the Mueller matrix of the first polarizer is assumed to be M P1 , the Mueller matrix of the second polarizer is M P2 , the Mueller matrix of each sub-pixel of the first liquid crystal chip is , the Mueller matrix of each sub-pixel of the second liquid crystal chip is , j =1,2,3,…,n, the Mueller matrix of the sample is M sample , the Stokes parameter of the light emitted by the monochromatic light source is S 0 , the light emitted from the sample is conjugated to the first liquid crystal chip i The Stokes parameter of the sub-pixel light is , i =1,2,3,…,m, incident on the photosensitive surface of the light detector conjugate to the second liquid crystal chip j The Stokes parameter of the light of a sub-pixel is , 𝑗=1,2,3,…,n; The first element is I (j) , which can be measured by a light detector; ;set up The first behavior ,remember ; Then the Mueller matrix of the sample is M sample , the light intensity I measured by the light detector (j) , j =1,2,3,…,n, calculated by the following formula ； in, yes M PSG The Moore-Penrose pseudoinverse, ； yes M PSA The Moore-Penrose pseudoinverse.

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