CN115219434A - Lens-free coaxial holographic Mueller matrix imaging system and imaging method - Google Patents
Lens-free coaxial holographic Mueller matrix imaging system and imaging method Download PDFInfo
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Abstract
The invention discloses a lens-free coaxial holographic Mueller matrix imaging system and an imaging method. According to the method, different polarization state combinations are formed by controlling the voltages of the liquid crystal phase retarders in the polarization state generator and the polarization state analyzer, so that different coaxial holographic diffraction patterns are obtained by the image sensor, and the Mueller matrix image of the sample to be measured can be obtained based on the coaxial holographic diffraction patterns. The method eliminates the vibration error caused by mechanical rotation in the traditional rotary wave plate method-based Mueller matrix microscopic imaging technology, and obviously improves the imaging speed.
Description
Technical Field
The invention relates to the field of Mueller matrix imaging, in particular to a lens-free coaxial holographic Mueller matrix imaging system and an imaging method.
Background
Mueller matrix imaging (Mueller matrix imaging) is a quantitative imaging modality that can reveal the structure and optical properties of a medium. Mueller matrix imaging, as a label-free, non-invasive mode of detection, has unique advantages in viewing biological tissues, materials, and other samples, and can provide a complete mathematical characterization of the polarization properties of the subject. As an important research direction of the mueller matrix imaging, the mueller matrix microscope has been widely studied in recent years. Mueller matrix microscopes were developed based on conventional optical microscopes by adding polarization generators and analyzers (PSG and PSA) in the light path. However, such lens-based imaging approaches suffer from some significant drawbacks, such as their limited spatial bandwidth product (difficult to have both large field of view and high resolution), bulky volume, and high cost of polarizing optics (e.g., using a stress-free objective lens). Furthermore, PSG and PSA are generally composed of a rotatable linear polarizer and a rotatable quarter-wave plate, which causes problems of mechanical vibration, slow imaging speed, and the like.
Compared with the traditional lens-based microscope, the lens-free holographic imaging has the outstanding advantages of large space bandwidth product, low cost, compact volume and the like. It has therefore been rapidly combined with other imaging techniques such as fluorescence imaging, phase contrast, optical flow, etc. Of the various previous studies on lensless holographic imaging, few have been combined with polarization imaging, and a few related efforts include quantitative measurement of birefringence parameters of polarization sensitive materials based on lensless holography, on-chip differential interference contrast microscopy using birefringent crystals and lensless digital holography, and lensless imaging of plant samples using cross-polarized illumination. However, as a special polarization imaging method capable of detecting the complete polarization characteristics of a sample, no report has been found so far to integrate the mueller matrix imaging technology into a lensless coaxial holographic imaging system to provide a high-resolution and large-field mueller matrix image. Indeed, the lensless holographic imaging technique is particularly well suited for mueller matrix imaging with its unique advantages.
Disclosure of Invention
The invention aims to provide a lens-free coaxial holographic Mueller matrix imaging system and an imaging method based on a liquid crystal variable phase retarder (LCVR), and aims to solve the problems of small space bandwidth product, high cost and large volume caused by an imaging lens of a Mueller matrix microscope in the prior art; and the problems of low imaging speed, mechanical vibration, alignment error and the like caused by adopting a mechanical rotation mode for polarization modulation.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
the utility model provides a coaxial holographic muller matrix imaging system of no lens, includes light source, image sensor, still includes polarization state generator, polarization state analyzer, light source, polarization state generator, polarization state analyzer, image sensor share the optical axis distribution in proper order, and the sample that awaits measuring is arranged in between polarization state generator, the polarization state analyzer, wherein:
the polarization state generator comprises a first linear polarizer, a first liquid crystal phase retarder and a second liquid crystal phase retarder which are sequentially distributed along a common optical axis of an optical path, an included angle between the polarization direction of the first linear polarizer and the fast axis direction of the first liquid crystal phase retarder is set to be 45 degrees, and an included angle between the polarization direction of the first linear polarizer and the fast axis direction of the second liquid crystal phase retarder is set to be 0 degree;
the polarization state analyzer is structurally symmetrical to the polarization state generator, and comprises a third liquid crystal phase retarder, a fourth liquid crystal phase retarder and a second linear polarizer which are sequentially distributed along a light path common-optical axis, so that an included angle between the polarization direction of the second linear polarizer and the fast axis direction of the third liquid crystal phase retarder is 0 degree, and an included angle between the polarization direction of the second linear polarizer and the fast axis direction of the fourth liquid crystal phase retarder is 45 degrees;
emergent light of the light source sequentially passes through a first linear polarizer, a first liquid crystal phase retarder and a second liquid crystal phase retarder in the polarization state generator and then reaches a sample to be detected, diffracted light is formed by the sample to be detected, the diffracted light sequentially passes through a third liquid crystal phase retarder, a fourth liquid crystal phase retarder and a second linear polarizer in the polarization state analyzer and then reaches an image sensor, and a coaxial holographic diffraction pattern is formed on the image sensor.
Furthermore, the device also comprises a collimation beam expander, wherein the common optical axis of the collimation beam expander is arranged between the light source and the polarization state generator, and emergent light of the light source reaches the polarization state generator after passing through the collimation beam expander.
Further, in the polarization state generator, the first linear polarizer, the first liquid crystal phase retarder, and the second liquid crystal phase retarder are stacked to form an integral structure.
Further, in the polarization analyzer, a third liquid crystal phase retarder, a fourth liquid crystal phase retarder and a second linear polarizer are laminated to form an integral structure.
Furthermore, the image sensor is attached to the whole structure of the polarization state analyzer.
The image sensor further comprises a thermoelectric cooler, and the thermoelectric cooler is attached to the non-light-receiving surface of the image sensor.
A Mueller matrix imaging method of a lensless coaxial holographic Mueller matrix imaging system comprises the following steps:
setting voltages applied to a first liquid crystal phase retarder and a second liquid crystal phase retarder in a polarization state generator to be V1 and V2 respectively, and setting generated phase delays to be delta 1 and delta 2 respectively; voltages applied to the third and fourth liquid crystal phase retarders in the polarization state analyzer are respectively V3 and V4, and the resulting phase retardations are respectively δ 3 and δ 4. Then:
when delta 1= pi +2N pi, (N epsilon N), the polarization state generator generates horizontal linear polarization light, which is marked as PSG (H);
when delta 1=2n pi, (N epsilon is N), the polarization state generator generates vertical linearly polarized light, and is marked as PSG (V);
when δ 1= δ 2= π/2+ N π, (N ∈ N), the polarization state generator generates +45 ° linearly polarized light, denoted as PSG (+ 45 °);
when delta 1= pi/2 + N pi, delta 2= -pi/2 + N pi (N belongs to N), the polarization state generator generates linearly polarized light with a degree of-45 degrees, and the linearly polarized light is marked as PSG (-45 degrees);
when δ 1= π/2+ N π, δ 2= π + N π (N ∈ N), the polarization generator generates left-handed circularly polarized light, denoted as PSG (L);
when δ 1= π/2+ N π, δ 2= N π (N ∈ N), the polarization state generator produces right-handed circularly polarized light, denoted PSG (R);
the polarization analyzer detects horizontally linearly polarized light when δ 4= π +2N π, (N ∈ N), noted PSA (H);
when delta 4=2n pi, (N ∈ N), the polarization analyzer detects the vertically linearly polarized light, which is recorded as PSA (V);
the polarization analyzer detects +45 ° linearly polarized light when δ 3= δ 4= π/2+ N π, (N ∈ N), noted as PSA (+ 45 °);
when delta 3= pi/2 + N pi, delta 4= -pi/2 + N pi (N epsilon N), the polarization analyzer detects linearly polarized light at-45 degrees, which is recorded as PSA (-45 degrees);
the polarization analyzer detects the left-handed circularly polarized light when δ 4= π/2+ N π, δ 3= π + N π (N ∈ N), and records as PSA (L);
when delta 4= pi/2 + N pi, delta 3= N pi (N ∈ N), the polarization analyzer detects the right-handed circularly polarized light, denoted as PSA (R);
if the polarization state generator and the polarization state analyzer are paired in pairs, 36 pairs of different polarization states can be generated, which are: PSG (H) -PSA (H), PSG (H) -PSA (V), PSG (H) -PSA (+ 45 °), PSG (H) -PSA (-45 °), PSG (H) -PSA (L), PSG (H) -PSA (R), PSG (V) -PSA (H), PSG (V) -PSA (V), PSG (V) -PSA (+ 45 °), PSG (V) -PSA (-45 °), PSG (V) -PSA (L), PSG (V) -PSA (R), PSG (+ 45 °) -PSA (H), PSG (+ 45 °), PSA (V) PSG (+ 45 degree) -PSA (+ 45 degree), PSG (+ 45 degree) -PSA (-45 degree), PSG (+ 45 degree) -PSA (L), PSG (+ 45 degree) -PSA (R), PSG (-45 degree) -PSA (H), PSG (-45 degree) -PSA (V), PSG (-45 degree) -PSA (+ 45 degree, PSG (-45 degree) -PSA (L), PSG (-45 degree) -PSA (R), PSG (L) -PSA (H), PSG (L) -PSA (V)), PSG (L) -PSA (+ 45 degree), PSG (L) -PSA (-45 degree), PSG (L) -PSA (L), PSG (L) -PSA (R), PSG (R) -PSA (H), PSG (R) -PSA (V), PSG (R) -PSA (+ 45 degree), PSG (R) -PSA (-45 degree), PSG (R) -PSA (L), PSG (R) -PSA (R).
and 4, based on the 36-frame intensity image of the sample to be tested obtained in the step 3, calculating by using a calculation method provided by Dee.g. Qinghua university Du's et al in Journal of Innovation in Optical Health Science 2014, volume 7, entitled "Characteristic features of Mueller matrix patterns for polarization modeling of biological tissues", to obtain a Mueller matrix image of the sample to be tested.
Further, in step 1, the polarization state combination includes a horizontal linear polarization state, a vertical linear polarization state, a +45 ° linear polarization state, a-45 ° linear polarization state, a left-handed circular polarization state, and a right-handed circular polarization state.
Specifically, according to the theory proposed by the Peinado a et al in the article entitled "Optimization and performance criterion of a Stokes polarization based on a two-way variable counters" in the 10 th article of the Optics Express 2010 volume 18, the multivariate equations corresponding to different polarization state combination methods have different condition numbers, and when the number of polarization states used by the polarization state generator and the polarization state analyzer is greater than or equal to 4, if the polyhedron surrounded by the points corresponding to the polarization states on the poincare is a regular polyhedron, the condition number of the equation set obtained by measuring the polarization states of the group can reach the minimum value
And the variance of the measured values becomes smaller and smaller as the number of polarization states increases. To minimize the condition number of the system of linear equations, we should choose points that can be connected as a regular polyhedron on the Poincare sphere to measure. Meanwhile, although a smaller variance can be obtained by increasing the number of measurements, doing so affects the overall measurement speed. In order to take account of condition number and measurement speed, the inventionThe invention selects and uses 36 times of imaging measurement methods, namely six polarization states, namely a horizontal linear polarization state, a vertical linear polarization state, a + 45-degree linear polarization state, a-45-degree linear polarization state, a left-hand circular polarization state and a right-hand circular polarization state of the polarization state generator and the polarization state analyzer are matched with each other. The corresponding points of the six polarization states on the Poincare sphere form a regular octahedron, the minimum condition number rule is met, and the variance is reduced while the measurement speed of the Mueller matrix is guaranteed.
Further, in step 3, an intensity image of a sample to be measured of the sample to be measured is calculated from the coaxial holographic diffraction Pattern according to a back propagation angular spectrum reconstruction method based on an angular spectrum theory, which is proposed in the Introduction to Fourier Optics fourth edition, published by j.w. Goodman et al in 2017, in combination with an auto-focusing algorithm, which is proposed by j.l. Pech-Pacheco et al in the 2010 Proceedings 15th International Conference on Pattern Recognition, entitled "diamond auto-focusing in scattering field micro-Optics: a synthetic study".
Further, in step 2, temperature control is provided for the image sensor by a thermoelectric cooler.
Compared with the prior art, the invention has the advantages that:
the invention adopts a liquid crystal phase retarder without mechanical motion to form a polarization state generator and a polarization state analyzer, utilizes voltage control to produce a group of polarization state combinations, and combines a lens-free imaging technology to realize the acquisition of digital coaxial holograms under different polarization states, thereby calculating and generating a Mueller matrix image.
The invention eliminates the vibration error caused by mechanical rotation of the traditional Mueller matrix microscopic imaging technology based on the wave plate rotation method, and meanwhile, the response time of the liquid crystal phase delayer under the voltage control is far shorter than the time required by the mechanical rotation in the wave plate rotation method, thereby obviously improving the imaging speed. Because the lens-free imaging technology is combined, the whole system has the advantages of large space bandwidth product, compact structure and low cost, and is very suitable for constructing a lens-free on-chip polarization imaging system.
Drawings
Fig. 1 is a schematic structural view of a lensless coaxial holographic mueller matrix imaging system of the present invention.
FIG. 2 is a schematic diagram of a polarization state generator according to the present invention.
Fig. 3 is an exploded view of a portion a of fig. 1.
Detailed Description
The invention is further illustrated by the following examples in conjunction with the drawings.
As shown in fig. 1, fig. 2, and fig. 3, the lensless coaxial holographic mueller matrix imaging system of this embodiment includes an optical fiber coupled semiconductor laser 1 as a light source, a collimating beam expander 2, a polarization state generator 3, a polarization state analyzer 5, an image sensor 6, and a thermoelectric cooler 7. The optical fiber coupling semiconductor laser 1, the collimation beam expander 2, the polarization state generator 3, the polarization state analyzer 5 and the image sensor 6 are distributed on a common optical axis in sequence, and the sample 4 to be detected is arranged between the polarization state generator 3 and the polarization state analyzer 5.
Specifically, the polarization state generator 3 and the polarization state analyzer 5 are identical in structure and symmetrical in structure. The polarization state generator 3 comprises a first linear polarizer 31, a first liquid crystal phase retarder 32 and a second liquid crystal phase retarder 33 which are sequentially distributed along an optical path coaxial axis, and the first linear polarizer 31, the first liquid crystal phase retarder 32 and the second liquid crystal phase retarder 33 are laminated to form an integral layer structure. The polarization state analyzer 5 comprises a third liquid crystal phase retarder 51, a fourth liquid crystal phase retarder 52 and a second linear polarizer 53 which are distributed along the optical path in a coaxial manner, and the same third liquid crystal phase retarder 51, fourth liquid crystal phase retarder 52 and second linear polarizer 53 are laminated to form an integral layer structure.
In this embodiment, the angle between the polarization direction of the first linear polarizer 31 in the polarization state generator 3 and the fast axis direction of the first liquid crystal retarder 32 is 45 °, the angle between the polarization direction of the first linear polarizer 31 and the fast axis direction of the second liquid crystal retarder 33 is 0 °, the angle between the polarization direction of the second linear polarizer 53 in the polarization state analyzer 5 and the fast axis direction of the third liquid crystal retarder 51 is 0 °, and the angle between the polarization direction of the second linear polarizer 53 and the fast axis direction of the fourth liquid crystal retarder 52 is 45 °.
The sample 4 to be measured is attached to the third liquid crystal phase retarder 51 in the polarization state analyzer 5, and the image sensor 6 is attached to the second linear polarizer 53 in the polarization state analyzer 5. The thermoelectric cooler 7 is in close contact with the non-light-receiving surface of the image sensor 6, and the temperature of the image sensor 6 can be controlled by the arrangement circuit of the thermoelectric cooler 7.
In this embodiment, the optical fiber coupled semiconductor laser 1 is used as an illumination device, a light beam emitted from the optical fiber coupled semiconductor laser 1 is collimated and expanded by the collimating and expanding device 2, and then is polarized by the polarization state generator 3 to form polarized light, the polarized light interacts with the sample 4 to be measured when reaching the sample to be measured to form diffracted light, and the diffracted light forms a coaxial holographic diffraction pattern on the image sensor 6 after passing through the polarization state generator 5. The introduction of the collimating beam expander 2 is to ensure perpendicularity when incident on the polarization state generator 3 and the polarization state analyzer 5, to eliminate the angle dependence of the phase retardation of the liquid crystal phase retarder, and to ensure the unit imaging magnification.
The Mueller matrix imaging method based on the lensless coaxial holographic Mueller matrix imaging system comprises the following steps:
then, a set of appropriate driving voltages is applied to the polarization state generator 3 and the polarization state analyzer 5 in sequence to obtain a set of different polarization state combinations including a horizontal linear polarization state, a vertical linear polarization state, a +45 ° linear polarization state, a-45 ° linear polarization state, a left-handed circular polarization state, and a right-handed circular polarization state, each of the polarization state combinations of the polarization state generator 3 and the polarization state analyzer 5 being paired two by two.
In particular, the sets of different polarization states are based on the theory set forth by Peinado A et al in the Optics Express 2010, vol.18, no. 10, entitled "Optimization and Performance criteria of a Stokes polarimeter based on two variable routersThe multiple equation sets corresponding to the combination method have different condition numbers, and when the number of the polarization states used by the polarization state generator and the polarization state analyzer is more than or equal to 4, if the polyhedron surrounded by the points corresponding to the polarization states on the Poincare sphere is a regular polyhedron, the condition numbers of the equation sets obtained by measuring the polarization states of the polarization states can reach the minimum value
And the variance of the measured values becomes smaller and smaller as the number of polarization states increases. To minimize the condition number of the system of linear equations, we should choose points that can be connected as a regular polyhedron on the Poincare sphere to measure. Meanwhile, although a smaller variance can be obtained by increasing the number of measurements, doing so affects the overall measurement speed. In order to take account of condition number and measurement speed, 36 times of imaging measurement methods are selected and used in the invention, namely six polarization states of a horizontal line polarization state, a vertical line polarization state, a + 45-degree linear polarization state, a-45-degree linear polarization state, a left-hand circular polarization state and a right-hand circular polarization state of the polarization state generator and the polarization state analyzer are matched with each other. The corresponding points of the six polarization states on the Poincare sphere form a regular octahedron, the minimum condition number rule is met, and the variance is reduced while the measurement speed of the Mueller matrix is guaranteed.
Setting voltages applied to a first liquid crystal phase retarder and a second liquid crystal phase retarder in a polarization state generator to be V1 and V2 respectively, and setting generated phase delays to be delta 1 and delta 2 respectively; applying voltages V3 and V4 to the third liquid crystal retarder and the fourth liquid crystal retarder in the polarization state analyzer respectively, and generating phase retardations δ 3 and δ 4 respectively, then:
when delta 1= pi +2N pi, (N epsilon N), the polarization state generator generates horizontal linear polarization light, which is marked as PSG (H);
when delta 1=2n pi, (N ∈ N), the polarization state generator generates vertical linearly polarized light, which is marked as PSG (V);
when δ 1= δ 2= π/2+ N π, (N ∈ N), the polarization state generator generates +45 ° linearly polarized light, denoted as PSG (+ 45 °);
when delta 1= pi/2 + N pi, delta 2= -pi/2 + N pi (N belongs to N), the polarization state generator generates linearly polarized light with a degree of-45 degrees, and the linearly polarized light is marked as PSG (-45 degrees);
when δ 1= π/2+ N π, δ 2= π + N π (N ∈ N), the polarization state generator produces left-handed circularly polarized light, denoted PSG (L);
when δ 1= π/2+ N π, δ 2= N π (N ∈ N), the polarization state generator produces right-handed circularly polarized light, denoted PSG (R);
the polarization analyzer detects horizontally linearly polarized light when δ 4= π +2N π, (N ∈ N), noted PSA (H);
when delta 4 is not greater than 2n pi, (N epsilon is N), the polarization analyzer detects vertical linear polarization and records as PSA (V);
the polarization analyzer detects +45 ° linearly polarized light, denoted as PSA (+ 45 °), when δ 3= δ 4= π/2+ N π, (N ∈ N);
the polarization analyzer detects-45 DEG linearly polarized light, marked as PSA (-45 DEG), when delta 3= pi/2 + N pi, delta 4= -pi/2 + N pi (N epsilon N);
the polarization analyzer detects the left-handed circularly polarized light when δ 4= π/2+ N π, δ 3= π + N π (N ∈ N), noted PSA (L);
when delta 4= pi/2 + N pi, delta 3= N pi (N ∈ N), the polarization analyzer detects the right-handed circularly polarized light, denoted as PSA (R);
if the polarization state generator and the polarization state analyzer are paired in pairs, 36 pairs of different polarization states can be generated, which are: PSG (H) -PSA (H), PSG (H) -PSA (V), PSG (H) -PSA (+ 45 °), PSG (H) -PSA (-45 °), PSG (H) -PSA (L), PSG (H) -PSA (R), PSG (V) -PSA (H), PSG (V) -PSA (V), PSG (V) -PSA (+ 45 °), PSG (V) -PSA (-45 °), PSG (V) -PSA (L), PSG (V) -PSA (R), PSG (+ 45 °) -PSA (H), PSG (+ 45 °) PSA (V) & lt & gt) PSG (+ 45 degree) -PSA (+ 45 degree), PSG (+ 45 degree) -PSA (-45 degree), PSG (+ 45 degree) -PSA (L), PSG (+ 45 degree) -PSA (R), PSG (-45 degree) -PSA (H), PSG (-45 degree) -PSA (V), PSG (-45 degree) -PSA (+ 45 degree, PSG (-45 degree) -PSA (L), PSG (-45 degree) -PSA (R), PSG (L) -PSA (H), PSG (L) -PSA (V)), PSG (L) -PSA (+ 45 degree), PSG (L) -PSA (-45 degree), PSG (L) -PSA (L), PSG (L) -PSA (R), PSG (R) -PSA (H), PSG (R) -PSA (V), PSG (R) -PSA (+ 45 degree), PSG (R) -PSA (-45 degree), PSG (R) -PSA (L), PSG (R) -PSA (R).
And 2, during each polarization state pairing period, enabling the image sensor 6 to collect 36 frames of coaxial holographic diffraction patterns corresponding to the sample 4 to be detected.
And 3, adopting a back propagation method based on an angular spectrum method, and simultaneously using a proper automatic focusing algorithm to judge the distance from the sample 4 to be detected to the image sensor 6 in the coaxial holographic diffraction pattern, thereby reconstructing an intensity image of the object in the coaxial holographic diffraction pattern of the sample 4 to be detected.
Specifically, according to a back propagation angular spectrum reconstruction method based on an angular spectrum theory, which is proposed in the book from Introduction to Fourier Optics fourth edition, published in 2017 by j.w. Goodman et al, in combination with an auto-focusing algorithm, which is proposed in the book entitled "atomic automatic in bright field micro: a comprehensive study" in Proceedings 15th International Conference on Pattern Recognition, 2010 by j.l. Pech-Pacheco et al, an intensity image of a sample to be measured of the sample to be measured is calculated from a coaxial holographic diffraction Pattern.
And 4, based on the 36 frames of intensity images of the sample to be detected obtained in the step 3, calculating by adopting a calculation method provided by Du's moth of Qinghua university in the article of Journal of Innovation in Optical Health Science 2014, no. 7, no. 1, entitled "Characteristic defects of muller matrix patterns for polarization analysis model of biological tissues", so as to obtain a Mueller matrix image of the sample to be detected.
In addition, a temperature control circuit based on a thermoelectric cooler 7 is added to the bottom of the image sensor 6 and set to room temperature to prevent the image sensor 6 from heating up and interfering with the polarization state analyzer 5 placed in close proximity thereto.
The embodiments of the present invention are described only for the preferred embodiments of the present invention, and not for the limitation of the concept and scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the design concept of the present invention shall fall into the protection scope of the present invention, and the technical content of the present invention which is claimed is fully set forth in the claims.
Claims (10)
1. The utility model provides a coaxial holographic Mueller matrix imaging system of no lens, includes light source, image sensor, its characterized in that still includes polarization state generator, polarization state analyzer, light source, polarization state generator, polarization state analyzer, image sensor share the optical axis in proper order and distribute, and the sample that awaits measuring is arranged in between polarization state generator, the polarization state analyzer, wherein:
the polarization state generator comprises a first linear polaroid, a first liquid crystal phase retarder and a second liquid crystal phase retarder which are sequentially distributed along a light path common-axis, wherein an included angle between the polarization direction of the first linear polaroid and the fast axis direction of the first liquid crystal phase retarder is set to be phi 45 degrees, and an included angle between the polarization direction of the first linear polaroid and the fast axis direction of the second liquid crystal phase retarder is set to be phi 0 degrees;
the polarization state analyzer is structurally symmetrical to the polarization state generator, and comprises a third liquid crystal phase retarder, a fourth liquid crystal phase retarder and a second linear polarizer which are sequentially distributed along the optical path common-axis, so that an included angle between the polarization direction of the second linear polarizer and the fast axis direction of the third liquid crystal phase retarder is phi 0 degrees, and an included angle between the polarization direction of the second linear polarizer and the fast axis direction of the fourth liquid crystal phase retarder is phi 45 degrees;
emergent light of the light source sequentially passes through a first linear polaroid, a first liquid crystal phase retarder and a second liquid crystal phase retarder in a polarization state generator and then reaches a sample to be detected, diffracted light is formed by the sample to be detected, and the diffracted light sequentially passes through a third liquid crystal phase retarder, a fourth liquid crystal phase retarder and a second linear polaroid in a polarization state analyzer and then reaches an image sensor, so that a coaxial holographic diffraction pattern is formed on the image sensor.
2. The lens-free coaxial holographic Mueller matrix imaging system of claim 1, further comprising a collimating beam expander, wherein a common optical axis of the collimating beam expander is disposed between the light source and the polarization state generator, and emergent light of the light source reaches the polarization state generator after passing through the collimating beam expander.
3. The lensless in-line holographic mueller matrix imaging system of claim 1, wherein the polarization state generator comprises a first linear polarizer, a first liquid crystal phase retarder, and a second liquid crystal phase retarder stacked together as a unitary structure.
4. The lensless coaxial holographic mueller matrix imaging system of claim 1, wherein the polarization analyzer comprises a third liquid crystal phase retarder, a fourth liquid crystal phase retarder, and a second linear polarizer laminated together as a unitary structure.
5. The lensless coaxial holographic mueller matrix imaging system of claim 4, wherein the image sensor is affixed to an integral structure of the polarization analyzer.
6. The lensless coaxial holographic mueller matrix imaging system of claim 1, further comprising a thermoelectric cooler proximate to a non-illuminated surface of the image sensor.
7. A Mueller matrix imaging method based on the lensless coaxial holographic Mueller matrix imaging system of any one of claims 1-6, comprising the steps of:
step 1, respectively applying different driving voltages to liquid crystal phase retarders in a polarization state analyzer and a polarization state generator to obtain different polarization state combinations of the polarization state generator and the polarization state analyzer, wherein each polarization state in the polarization state combinations of the polarization state generator and the polarization state analyzer is paired pairwise;
step 2, when the image sensors are matched in each polarization state, acquiring a coaxial holographic diffraction pattern corresponding to a sample to be detected;
step 3, respectively calculating the intensity images of the samples to be detected according to the plurality of coaxial holographic diffraction patterns obtained in the step 2;
and 4, calculating the Mueller matrix image of the sample to be detected by using the intensity images of the samples to be detected obtained in the step 3.
8. The mueller matrix imaging method of claim 7, wherein in step 1, the combination of polarization states comprises a horizontal linear polarization state, a vertical linear polarization state, a +45 ° linear polarization state, a-45 ° linear polarization state, a left-handed circular polarization state, and a right-handed circular polarization state.
9. The mueller matrix imaging method according to claim 7, wherein in step 3, an intensity image of the sample to be measured is calculated from the coaxial holographic diffraction pattern by using a back propagation method based on an angular spectrum method in combination with an auto-focusing algorithm.
10. The mueller matrix imaging method of claim 7, wherein in step 2, the image sensor is provided with temperature control by a thermoelectric cooler.
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