阅读说明：本技术 一种空间调制偏振检测系统及设计方法 (Spatial modulation polarization detection system and design method ) 是由 李艳秋 宁天磊 周国栋 于 2021-07-13 设计创作，主要内容包括：本发明提供一种空间调制偏振检测系统及设计方法,用于对待测样品的偏振特性进行检测,属于光学设计技术领域,该系统包括光源、偏振态产生器、空间光滤波器、准直透镜、相位延迟器、偏振态分析器、聚焦透镜、探测器及数据处理模块；该系统还包括四分之一波片,所述四分之一波片设置于相位延迟器与准直透镜之间；所述四分之一波片的快轴方位角为所述由所述偏振态系数对比度最大来确定。本申请选取四分之一波片的最优快轴方位角使强度分布模型中的模型参数有最大的对比度,以使得拟合解算偏振态过程中有全局最优解,来提高偏振态的解算精度。(The invention provides a spatial modulation polarization detection system and a design method, which are used for detecting the polarization characteristic of a sample to be detected, and belong to the technical field of optical design, the system comprises a light source, a polarization state generator, a spatial light filter, a collimating lens, a phase retarder, a polarization state analyzer, a focusing lens, a detector and a data processing module; the system also comprises a quarter-wave plate, wherein the quarter-wave plate is arranged between the phase retarder and the collimating lens; the fast axis azimuth angle of the quarter-wave plate is The above-mentioned Determined by the maximum contrast of the polarization state coefficient. The method selects the optimal fast axis azimuth angle of the quarter-wave plate to ensure the strengthThe model parameters in the distribution model have the maximum contrast, so that a global optimal solution exists in the process of fitting and solving the polarization state, and the solving precision of the polarization state is improved.)

1. A space modulation polarization detection system is used for detecting the polarization characteristics of a sample to be detected and comprises a light source, a polarization state generator, a space optical filter, a collimating lens, a phase retarder, a polarization state analyzer, a focusing lens, a detector and a data processing module; the system is characterized by further comprising a quarter-wave plate, wherein the quarter-wave plate is arranged between the phase retarder and the collimating lens; the fast axis azimuth angle of the quarter-wave plate isThe above-mentionedDetermined by the maximum contrast of the polarization state coefficient.

2. The spatially modulated polarization detection system of claim 1, wherein the polarization state analyzer has a polarizer pass axis and a fast axis azimuth angle of the polarization plate in a horizontal direction.

3. A design method of a space modulation polarization detection system is used for detecting the polarization characteristic of a sample to be detected, and is characterized by comprising the following specific processes:

a quarter-wave plate is arranged between a phase retarder and a collimating lens in the spatial modulation polarization detection system;

fixing the azimuth angles of the transmission axis of the polarization plate and the fast axis of the polarization wave plate in the polarization state analyzer in the horizontal direction;

adjusting the fast axis azimuth angle theta of the quarter-wave plate for multiple times, and calculating the polarization coefficient v of the polarization state corresponding to each adjustment₁、ν₂、ν₃；

Obtaining the fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches the maximum

Adjusting the fast axis azimuth angle of the quarter-wave plate to

4. The design method of the spatial modulation polarization detection system of claim 3, wherein the phase retarder uses a vortex half-wave plate.

5. The method of claim 4 wherein the polarization state coefficient v is a function of the spatial modulation polarization detection system₁、ν₂、ν₃Is expressed as follows:

wherein theta represents the fast axis azimuth of the quarter-wave plate,indicating the fast axis azimuth of the vortex plate.

6. The design method of the spatially modulated polarization detection system of claim 3 or 5, wherein the fast axis azimuth angle corresponding to the quarter-wave plate is obtained when the contrast ratio of the polarization state coefficient reaches a maximumThe specific process comprises the following steps:

constructing an optimal polarization state analysis model, and calculating a fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches the maximum according to the model

The optimal polarization state analysis model is as follows:

wherein，ν_imaxRepresenting model parameters v_iThe larger value of (i ═ 1,2,3), mean (ν)_imax) Representation v_imaxAverage value of v_iminRepresenting model parameters v_iMedium to small value, mean (v)_imin) Representation v_imaxMean value, the difference between the two mean values representing the model parameter v_iThe contrast of (2).

7. The method of claim 6 wherein the model parameter v is a design parameter of the spatially modulated polarization detection system_iThe larger value in (b) means that the model parameter v is_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) Per 2 elements, model parameters v_iThe smaller value in (b) means the model parameter v_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) And 2 elements.

8. The method of claim 6 or 7, wherein the model calculates fast axis azimuth angles corresponding to the quarter-wave plates when the contrast ratio of polarization state coefficients reaches a maximumThe quick searching method comprises the following steps:

setting an initial value of theta;

setting the amount of change of a loss function

Increasing said θ when the calculated loss function is greater than zero, and conversely decreasing said θ;

increasing or decreasing the θ to:where α represents the learning rate.

Technical Field

The invention belongs to the technical field of polarization measurement, and particularly relates to a spatial modulation polarization detection system and a design method.

Background

Polarization is an important property of light, and polarized light is very sensitive to microstructure characteristics in samples such as optical elements, materials, biological tissues and the like. The interaction of light with the sample can cause refraction, reflection, scattering, etc. to change the polarization state of the incident light. The change capability of the sample on the polarization state of light is represented by a Mueller matrix, the Mueller matrix contains all polarization information of the sample, can be further decomposed into polarization parameters which are closely related to the microstructure of the sample, have practical physical significance, can be quantized, such as depolarization, phase delay, dichroism, fast axis direction angle, optical rotation and the like, and can be used for obtaining the polarization characteristics and the structural parameters of the sample. Polarization measurement is an important tool for analyzing polarization characteristics of light and samples, and has been widely applied to the fields of biomedicine, quantum communication, laser radar and the like.

As shown in fig. 1, the currently existing spatial modulation polarization detection system includes a light source 101, a polarization state generator 102, a spatial light filter 103, a collimating lens 104, a phase retarder 105, a polarization state analyzer 106, a focusing lens 107, a detector 108, and a data processing module 109; a light beam to be measured sequentially passes through a polarization state generator 102, a spatial light filter 103, a collimating lens 104, a spatially varying phase retarder 105 and a polarization state analyzer 106, and then the light beam is imaged to a detector 108 by a focusing lens 107; the detector 108 receives the intensity distribution of a light to be measured, and sends the obtained intensity map to the data processing module 109 for data processing, so as to further provide the polarization information of the light to be measured. In the system, when the fast axis azimuth angle of the polarization state analyzer is set at different positions, the detected results are different, so that the orientation of the fast axis azimuth angle of the polarization state analyzer determines the accuracy of system measurement.

Patent document CN110806266A discloses a selection method of a polarization analyzer in a polarization detection system, which determines a suitable polarization analyzer by calculating a partial derivative of polarization probability density and determining whether the partial derivative has a unique zero point, but the method has the following problems:

firstly, when the wave plates of the polarization state analyzer are rotated for multiple times to obtain the fast axis azimuth angles of the wave plates, and the partial derivatives of the obtained polarization probability density functions have unique zero points, the method cannot judge which unique zero point corresponds to the optimal fast axis azimuth angle of the wave plates, so that when a plurality of polarization state analyzers are given, only a proper polarization state analyzer can be selected, and the optimal polarization state analyzer cannot be found by traversing all polarization state analysis modes.

Secondly, setting the constraint conditions as follows: selecting a variable fast axis azimuth angle of a wave plate of the polarization state analyzer, wherein the transmission axis direction of a polaroid is fixed in the horizontal direction; therefore, in the technology, only the influence of different fast axis azimuth angles of the wave plate of the polarization state analyzer on polarization detection is considered, and the influence of different rotation angles of the polarizer is not considered, and the polarization state analysis mode obtained by the method only obtains the proper fast axis azimuth angle of the quarter-wave plate in the polarization state analyzer, so that the proper polarizer angle cannot be obtained.

A thesis document 'spatial modulated polar based on a vortex retro and Fourier analysis' discloses a spatial modulation polarimeter using a vortex half-wave plate as a spatial variation modulator. However, this method cannot achieve spatial modulation of a circular polarization component using only a vortex half-wave plate, and thus cannot directly detect s of stokes₃Parametric, polarization state detection of only fully polarized light is enabled, and polarization state detection of partially polarized light is not enabled. Therefore, design optimization of the spatial modulator vortex half-wave plate is required to realize accurate measurement of all normalized stokes parameters.

Disclosure of Invention

In view of this, the present invention provides a spatial modulation polarization detection system and a design method thereof, which can be used to accurately measure the polarization characteristics of a sample to be measured.

The technical scheme for realizing the invention is as follows:

a space modulation polarization detection system is used for detecting the polarization characteristics of a sample to be detected and comprises a light source, a polarization state generator, a space optical filter, a collimating lens, a phase retarder, a polarization state analyzer, a focusing lens, a detector and a data processing module; theThe system further comprises a quarter-wave plate, wherein the quarter-wave plate is arranged between the phase retarder and the collimating lens; the fast axis azimuth angle of the quarter-wave plate isThe above-mentionedDetermined by the maximum contrast of the polarization state coefficient.

Furthermore, the azimuth angles of the transmission axis of the polarization plate and the fast axis of the polarization wave plate in the polarization state analyzer are horizontal directions.

A design method of a spatial modulation polarization detection system is used for detecting the polarization characteristic of a sample to be detected, and comprises the following specific processes:

a quarter-wave plate is arranged between a phase retarder and a collimating lens in the spatial modulation polarization detection system;

adjusting the fast axis azimuth angle theta of the quarter-wave plate for multiple times, and calculating the polarization coefficient v of the polarization state corresponding to each adjustment₁、ν₂、ν₃；

Obtaining the fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches the maximum

Adjusting the fast axis azimuth angle of the quarter-wave plate to

Further, the phase retarder of the invention adopts a vortex half-wave plate.

Further, the invention fixes the azimuth angles of the transmission axis of the polarization plate and the fast axis of the polarization wave plate in the polarization state analyzer to the horizontal direction.

Furthermore, the polarization state coefficient v of the invention₁、ν₂、ν₃Is expressed as follows:

wherein theta represents the fast axis azimuth of the quarter-wave plate,indicating the fast axis azimuth of the vortex plate.

Furthermore, when the contrast ratio of the obtained polarization state coefficient reaches the maximum, the fast axis azimuth angle corresponding to the quarter-wave plateThe specific process comprises the following steps:

constructing an optimal polarization state analysis model, and calculating a fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches the maximum according to the model

The optimal polarization state analysis model is as follows:

wherein, v_imaxRepresenting model parameters v_iThe larger value of (i ═ 1,2,3), mean (ν)_imax) Representation v_imaxAverage value of v_iminRepresenting model parameters v_iMedium to small value, mean (v)_imin) Representation v_imaxMean value, difference table of two mean valuesModel parameter v_iThe contrast of (2).

Furthermore, the model parameter v of the invention_iThe larger value in (b) means that the model parameter v is_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) Per 2 elements, model parameters v_iThe smaller value in (b) means the model parameter v_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) And 2 elements.

Further, the model calculates the fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches the maximumThe quick searching method comprises the following steps:

setting an initial value of theta;

setting the amount of change of a loss function

Increasing said θ when the calculated loss function is greater than zero, and conversely decreasing said θ;

increasing or decreasing the θ to:where α represents the learning rate.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a schematic diagram of a conventional spatial modulation polarization detection system;

FIG. 2 is a schematic diagram of a spatially modulated polarization detection system designed in accordance with the present invention;

201-a light source, 202-a polarization state generator, 203-a spatial light filter, 204-a collimating lens, 205-a quarter wave plate, 206-a vortex half wave plate, 207-a polarization state analyzer, 208-a focusing lens, 209-a detector and 210-a data processing module.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

It should be noted that, in the case of no conflict, the features in the following embodiments and examples may be combined with each other; moreover, all other embodiments that can be derived by one of ordinary skill in the art from the embodiments disclosed herein without making any creative effort fall within the scope of the present disclosure.

It is noted that various aspects of the embodiments are described below within the scope of the appended claims. It should be apparent that the aspects described herein may be embodied in a wide variety of forms and that any specific structure and/or function described herein is merely illustrative. Based on the disclosure, one skilled in the art should appreciate that one aspect described herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented and/or a method practiced using any number of the aspects set forth herein. Additionally, such an apparatus may be implemented and/or such a method may be practiced using other structure and/or functionality in addition to one or more of the aspects set forth herein.

The design idea of the invention is as follows: when the light to be detected penetrates through the system, the intensity distribution difference on the detection surface is larger, the contrast is larger, the convergence of the polarization state is better when the polarization state is solved by using a fitting algorithm, and the solved polarization state is more accurate. The detector receives the intensity distribution of the light to be measured, sends the obtained intensity map to the data processing module for data processing, and further calculates the polarization information of the light to be measured. The intensity distribution of the detection surface is related to the intensity distribution of the Moeller matrix of the quarter-wave plate, the vortex half-wave plate and the polarization state analyzer and the polarization state of incident light, the vortex half-wave plate and the polarization state analyzer are relatively fixed, the azimuth angle of the fast axis of the quarter-wave plate is adjusted for multiple times, and when the contrast reaches the maximum value under a certain azimuth angle, the higher the accuracy of the polarization state obtained under the azimuth angle is.

Based on the design concept, the spatial modulation polarization detection system provided by the embodiment of the application is used for detecting the polarization characteristic of a sample to be detected, and comprises a light source, a polarization state generator, a spatial light filter, a collimating lens, a phase retarder, a polarization state analyzer, a focusing lens, a detector and a data processing module; the system also comprises a quarter-wave plate, wherein the quarter-wave plate is arranged between the phase retarder and the collimating lens; the fast axis azimuth angle of the quarter-wave plate isThe above-mentionedDetermined by the maximum contrast of the polarization state coefficient.

In an embodiment of the present invention, the azimuth angles of the transmission axis of the polarization plate and the fast axis of the polarization wave plate in the polarization analyzer are horizontal directions.

Based on the design concept, the design method of the spatial modulation polarization detection system in the embodiment of the application is used for detecting the polarization characteristic of a sample to be detected, and the specific process is as follows:

as shown in fig. 2, a quarter-wave plate is arranged between the phase retarder and the collimating lens in the spatial modulation polarization detection system;

fixing the azimuth angles of the transmission axis of the polarization plate and the fast axis of the polarization wave plate in the polarization state analyzer in the horizontal direction;

adjusting the fast axis azimuth angle theta of the quarter-wave plate for multiple times, and calculating the polarization coefficient v of the polarization state corresponding to each adjustment₁、ν₂、ν₃；

ObtainingWhen the polarization state coefficient contrast reaches the maximum, the fast axis azimuth angle corresponding to the quarter-wave plate

Adjusting the fast axis azimuth angle of the quarter-wave plate to

In the embodiment, the quarter-wave plate is arranged in the system, different model parameters can be generated due to the quarter-wave plates with different fast axis azimuth angles, the detection result of the polarization state to be detected is directly influenced by the spatial variation characteristic of the model parameters, if the model parameters have no spatial variation characteristic or have small spatial variation characteristic (small contrast), the intensity distribution difference corresponding to different polarization states is not obvious, in the process of resolving the polarization state to be detected by using a fitting algorithm, the resolution of the polarization state is not converged, and therefore the resolution error of the polarization state is large. In the embodiment of the application, the optimal fast axis azimuth angle of the quarter-wave plate is selected to enable the model parameters (namely polarization state coefficients) in the intensity distribution model to have the maximum contrast, so that a global optimal solution exists in the process of fitting and solving the polarization state, and the solving precision of the polarization state is improved. The system designed by the design method of the embodiment of the application can accurately measure the polarization state of the polarized light.

In one embodiment of the present application, the phase retarder employs a vortex half-wave plate.

In this embodiment, the quarter-wave plate is combined with the vortex half-wave plate, and the optimal fast axis azimuth angle of the quarter-wave plate is selected by using an optimization algorithm for maximizing the contrast of model parameters. The invention solves the problem that the original space modulation polarimeter can not obtain the space modulation circular polarization component, realizes the space modulation of the full polarization component, and can directly solve the normalized Stokes parameter. For the detection of the polarization state of the light to be detected, the method does not need to rotate a complex optical device, and can realize high-precision real-time dynamic measurement.

In one embodiment of the present application, in the present system, the polarizationCoefficient of state v₁、ν₂、ν₃Is expressed as follows:

wherein theta represents the fast axis azimuth of the quarter-wave plate,representing the fast axis azimuth of the vortex wave plate; since the fast axis of the vortex wave plate is varied, the fast axis azimuth angleIs a matrix, and can be obtained by actual measurement.

In this embodiment, the polarization state coefficient v₁、ν₂、ν₃The derivation process of the expression is:

acquiring a Mueller matrix value M of a spatial variation vortex half-wave plate;

the Mueller matrix value of the spatial variation vortex half-wave plate in the step is measured through experiments, and the polarization state of the light to be measured entering the spatial variation vortex half-wave plate is assumed to beT represents matrix transposition, and the polarization state of the light beam emitted from the space-varying vortex half-wave plate is

The transmission axis direction of a polaroid and the fast axis azimuth angle of a wave plate of the polarization analyzer are fixed in the horizontal direction, and the Mueller matrix of the polaroid and the wave plate can be represented as follows:

setting the fast axis azimuth angle of the vortex wave plate asThe Mueller matrix of the vortex half-wave plate is denoted M₂：

Step two, establishing a detection surface intensity distribution model, multiplying the Mueller matrix and a Stokes vector of the light to be detected by utilizing a Mueller-Stokes theoretical relationship, obtaining the polarization state of emergent light of a polarization state analyzer, and taking out a first Stokes component of the polarization state of the emergent light to express an intensity value I of the detection surface as follows:

where the intensity distribution referred to is the relative intensity, a grey value representation of the image is acquired using the detector. Considering device processing and alignment errors and system weak variation, in order to make up for system errors, the Mueller matrix value of the spatial variation vortex half-wave plate can be measured through experiments, and the measured experimental value is used for substituting a theoretical value and being brought into the intensity distribution model, so that device processing and alignment errors are calibrated.

The formula in step two can be expressed as follows:

wherein, v₁,ν₂,ν₃Can be expressed as:

model parameter v₁、ν₂、ν₃Is the state of polarization(s)₁,s₂,s₃) The physical quantity related to the mueller matrix of the selected spatial variation modulator and the polarization analysis mode of the polarization analyzer can map the polarization state of the incident light to an intensity distribution through the above formula, and the intensity distributions corresponding to different polarization states are accompanied by different spatial variation characteristics. After the intensity map of the detection surface is obtained, the polarization state of incident light can be solved through a fitting algorithm, and the solving precision of the polarization state depends on the model parameter v of a theoretical model₁、ν₂、ν₃Whether or not it has strong spatial variation properties to achieve a full polarization component(s)₁,s₂,s₃) Spatial modulation of (2). V is₁、ν₂、ν₃It is strong spatial variation that indicates a large contrast in the model parameters.

In an embodiment of the present application, when the contrast of the polarization coefficient reaches the maximum, the fast axis azimuth angle corresponding to the quarter-wave plate is obtainedThe specific process comprises the following steps:

constructing an optimal polarization state analysis model, and calculating a quarter-wave plate when the polarization state coefficient contrast reaches the maximum according to the modelCorresponding fast axis azimuth

The optimal polarization state analysis model is as follows:

wherein, v_imaxRepresenting model parameters v_iThe larger value of (i ═ 1,2,3), mean (ν)_imax) Representation v_imaxAverage value of v_iminRepresenting model parameters v_iMedium to small value, mean (v)_imin) Representation v_imaxMean value, the difference between the two mean values representing the model parameter v_iThe contrast of (2). Argmax represents the maximum function that takes the maximum value of contrast as the fast axis azimuth θ of the quarter-wave plate changes. The optimal theta can be selected by the above formulaAnd representing, namely selecting an optimal polarization state analysis mode.

In this embodiment, the polarization coefficient contrast is determined as: the average value of the larger value in the polarization coefficient matrix is reduced to the average value of the smaller value, the difference between the two values corresponds to the contrast of the model parameters, and as the polarization coefficients have 3, namely v₁、ν₂、ν₃Therefore, the difference is calculated for each polarization coefficient, the larger the sum of the three polarization coefficient differences is, the larger the contrast ratio is, the better the contrast ratio is, the contrast ratio calculated by the method can well reflect the light intensity difference of the detection surface, the larger the contrast ratio is, the larger the difference is, the better the convergence of the polarization state is, and the more accurate the detection result is.

In another embodiment of the present application, the model parameter v_iThe larger value in (b) means that the model parameter v is_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) Per 2 elements, model parameters v_iThe smaller value in (b) means the model parameter v_iN is contained₁×n₂Element, element value is larger (n)₁×n₂) And 2 elements.

For example, the determination method of the larger and smaller gray values of the upper model parameter is as follows: model parameter v_iThe method is essentially an intensity map (assuming that the pixels of the intensity map are 40 × 40), each pixel has a corresponding gray value, the gray value with the larger model parameter is 800 larger pixel values among 1600 pixel points, the gray value with the smaller model parameter is 800 smaller pixel values among 1600 pixel points, and the division of the gray values of the larger and smaller pixels can be realized by a sorting algorithm in MATLAB.

In yet another embodiment of the present application, the model calculates a fast axis azimuth angle corresponding to the quarter-wave plate when the polarization state coefficient contrast reaches a maximumThe quick searching method comprises the following steps:

setting an initial value of theta;

setting the amount of change of a loss function

Increasing said θ when the calculated loss function is greater than zero, and conversely decreasing said θ;

increasing or decreasing the θ to:where α represents the learning rate.

In the specific implementation: fast search for optimal using gradient descent methodThe gradient descent method can quickly find the corresponding model parameter with the maximum contrast along the gradient ascending direction of the contrast of the model parameterFirstly, an initial value is assigned to theta, and the invention providesThe fixed initial value is theta is 0; further, the amount of change in the loss function is set,the result obtained by the above formula is larger than zero, the model parameter contrast and theta are in a direct proportion relation, and theta should be increased in the iteration process to maximize the model parameter contrast; conversely, θ should be decreased; and further updating theta of iteration each time, finding the maximum value of the model parameter, wherein the updating iteration is shown as the following formula:

where theta is_nDenotes θ taken in the nth iteration, and α denotes the learning rate, i.e., the step size of the optimization. Theta of the next iteration can be obtained through the formula, and the optimal theta can be found through multi-step iteration

In summary, the above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.