Fast Composite Polarization System and Method with Real-Time Self-Calibration

Abstract

The present invention discloses a fast composite polarization system and method capable of real-time self-calibration, which includes a PSA composed of 2 liquid crystal variable phase retarders and 1 polarizer, a PSG composed of 1 turntable, a quarter achromatic waveplate and 1 polarizer, a collimating lens, a condenser lens, a light source and a detector. Two detection windows are provided at the sample to be measured in the system. One window is for placing the sample to be measured, and the other window is for air. The light beam emitted by the light source is collimated by the collimating lens, and the collimated light beam enters the PSG. The light beam generates various set polarized lights through the PSG. The polarized light enters the detection window. Part of the light passes through the window of the sample to be measured, carrying the polarization information of the sample to be measured, and the other part passes through the air window. Then the two light beams are analyzed by the PSA and finally detected by the detector through the condenser lens. The present invention can accelerate the polarization measurement speed and achieve automatic system calibration after each measurement, realizing fast and high-precision polarization detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical system calibration, and particularly relates to a fast composite polarization system and method capable of real-time self-calibration. Background Art

[0002] Polarized light can interact with a sample to obtain more microscopic structure information of the sample, so it is applied to many detection fields. In the past 20 years, polarization measurement technology has received extensive attention. It has become a detection technology with great research value and is widely used in fields such as medicine, astronomy, and remote sensing.

[0003] With the development of science and technology, fast and high-precision polarimeters have become the mainstream of development. The Mueller matrix polarimeter mainly consists of a polarization state generator PSG (Polarization State Generator) and a polarization state analyzer PSA (Polarization State Analyzer). The measurement of the polarization aberration of the sample is completed by modulating the PSG and PSA. Most current polarimeters are based on the measurement scheme of double rotating wave plates. The double rotating wave plate Mueller polarimeter has a simple structure and high precision, but the rotating wave plate polarimeter has a slow measurement speed. It usually takes 1 to 2 minutes to complete the measurement of the Mueller matrix of the sample, with low efficiency, unable to measure dynamic samples and obtain the Mueller matrix. The liquid crystal variable retarder LCVRs (LiquidCrystal Variable Retarders) changes the phase retardation through voltage and has a regulation speed of milliseconds. Replacing the rotating wave plate with liquid crystal can increase the measurement speed by more than an order of magnitude. However, the problems existing in the liquid crystal variable retarder itself affect the measurement accuracy of the liquid crystal polarimeter: (1) The long-term stability of the liquid crystal is not good. After a period of time, the relationship between voltage and retardation will change; (2) The temperature stability of the liquid crystal is not good. After the temperature changes, the relationship between voltage and retardation changes; (3) The liquid crystal does not have the achromatic aberration characteristic, and the retardation of light with different wavelengths under the same voltage is different. The above three aspects will all cause phase retardation errors in the liquid crystal, ultimately reducing the measurement accuracy of the liquid crystal polarimeter and limiting the application wavelength of the liquid crystal. In order to accelerate the measurement speed and improve the measurement accuracy, it is necessary to calibrate the LCVR polarimeter. The current mainstream calibration method for the Mueller matrix polarimeter is the eigenvalue calibration method. This method can calibrate the system more comprehensively, but it requires measuring standard devices, takes a long time, and it is difficult to realize the self-calibration of the LCVR polarimeter. Summary of the Invention

[0004] In view of this, the present invention provides a fast composite polarization system and method capable of real-time self-calibration, which can accelerate the polarization measurement speed and achieve automatic system correction after each measurement, solve the system accuracy problem caused by the instability of LCVR delay modulation, and achieve fast and high-precision polarization detection.

[0005] The technical solution of the present invention is implemented as follows:

[0006] A fast composite polarization system capable of real-time self-calibration includes a polarization state analyzer (PSA) composed of 2 liquid crystal variable phase retarders and 1 polarizer, a polarization state generator (PSG) composed of 1 turntable, a quarter-wave achromatic waveplate and 1 polarizer, a collimating lens, a condenser lens, a light source and a detector. In order to achieve real-time self-calibration, two detection windows are provided at the position of the sample to be measured in the system. One window is for placing the sample to be measured, and the other window is for air. The light source emits a light beam, which is collimated by the collimating lens. The collimated light beam enters the polarization state generator, that is, the PSG. The light beam becomes linearly polarized light through the PSG polarizer, and then the fast axis azimuth angle of the achromatic waveplate is changed by the turntable and is further modulated to generate a variety of set polarized lights. This polarized light enters the detection window. Part of the light passes through the window of the sample to be measured and carries the polarization information of the sample to be measured. The other part passes through the air window. Then the two light beams pass through the polarization light analyzer, that is, the PSA. The light beam is analyzed after passing through the liquid crystal variable phase retarder with different delay amounts generated by voltage control and the polarizer in the PSA, and finally is detected by the detector through the condenser lens.

[0007] Furthermore, the system is controlled by a self-calibration automatic control program and has a visualization interface for measurement results.

[0008] A calibration measurement method for a fast composite polarization system capable of real-time self-calibration is to place reference samples: air, a quarter-wave plate and a polarizer between the PSG and the PSA for measurement respectively, and use the eigenvalue method to calibrate the systems on both sides of the reference sample to obtain the instrument matrix G of the PSG and the instrument matrix A of the PSA; since the PSG is composed of a turntable and a quarter-wave achromatic waveplate, its instrument matrix is relatively stable and does not change with temperature, time and wavelength. Therefore, the polarized light generated by the calibrated PSG is used as the reference light; then, when measuring the polarization characteristics of the sample to be measured each time, the light intensity matrix I is obtained by using the air window and combined with the Stokes four-point calibration method. According to the following formula, the instrument matrix A of the PSA composed of LCVR is obtained:

[0009] A = I·G -1 (1)

[0010] The calibration of the LCVR system is completed.

[0011] Furthermore, the data of the window of the sample to be measured is calculated by using the instrument matrix A of the resolved PSA and the instrument matrix G of the PSG, realizing high-precision measurement of the polarization characteristics of the sample to be measured.

[0012] Beneficial effects:

[0013] 1. The fast composite polarization system with real-time self-calibration disclosed by the present invention adopts a composite polarization system combining a rotating quarter-wave plate and a liquid crystal phase retarder. A liquid crystal phase retarder with a response time of milliseconds is used to replace the traditional turntable to construct the PSA. It only takes a few seconds to measure the Mueller matrix of a sample once, greatly improving the measurement speed. Moreover, a rotating quarter-wave plate with a stable polarization state is used to form the PSG, and combined with simultaneous measurement of double windows, real-time self-calibration of the system is realized.

[0014] 2. By using the real-time self-calibration technology of the present invention, not only can the problem of liquid crystal modulation stability be solved, but also the existing optical path does not need to be changed, no other errors will be caused, and each device in the whole system is calibrated, improving the measurement accuracy.

[0015] 3. The real-time self-calibration method of the present invention is simple to operate. It combines the automatic calibration of the PSA composed of liquid crystals, the calculation of the polarization characteristics of the sample, and the display of the Mueller results of the sample. Moreover, the system can quickly measure the sample by using this program, featuring fast speed, high precision, and easy operation. Description of the drawings

[0016] Figure 1 It is a schematic structural diagram of the fast composite polarization system with real-time self-calibration of the present invention.

[0017] Among them, 101 - light source, 102 - collimating lens, 103 - polarizer, 104 - quarter-wave achromatic plate and turntable, 105 - sample stage, 106 - liquid crystal variable phase retarder 2, 107 - liquid crystal variable phase retarder 1, 108 - polarizer, 109 - condenser lens, 110 - CCD detector.

[0018] Among them, 103 and 104 constitute the polarization state generator PSG, and 106, 107 and 108 constitute the polarization state analyzer PSA.

[0019] Figure 2 It is a flow chart of the real-time self-calibration control program of the system of the present invention. Specific embodiments

[0020] The following are detailed descriptions of the present invention by way of examples in conjunction with the drawings.

[0021] Figure 1Schematic diagram of a fast composite polarization system device capable of real-time self-calibration according to the present invention, including a light source, a polarization state generator PSG, a condenser lens L1, a collimator lens L2, a polarization state analyzer PSA, and a detector CCD. The polarization state generator PSG consists of a polarizer P1 and a quarter-wave plate R1, where the polarizer P1 is in the front and the quarter-wave plate R1 is in the back; the polarization state analyzer PSA consists of two liquid crystal variable phase retarders LCVR1 and LCVR2 and a polarizer P2. The liquid crystal variable phase retarders are in the front and the polarizer P2 is in the back. The measurement of the Mueller matrix of the sample is completed by adjusting the PSG and PSA. Among them, the instrument matrices of the PSG and PSA are G and A respectively. The light beam emitted by the light source is collimated by the collimating lens. The collimated light beam enters the polarization state generator, that is, the PSG, and is modulated to generate a specific polarized light. The light beam becomes a linearly polarized light through the polarizer of the PSG, and then the fast axis azimuth angle of the achromatic wave plate is changed by the turntable, and is further modulated to generate a variety of specific polarized lights. This polarized light enters the detection window. Part of the light passes through the window of the sample to be measured, carrying the polarization information of the sample to be measured, and the other part passes through the air window. Then the two light beams pass through the polarization light analyzer, that is, the PSA. The light beam is analyzed after passing through the liquid crystal variable phase retarder with different delay amounts generated by voltage control and the polarizer in the PSA, and finally is detected by the detector through the condenser lens.

[0022] The calibration of the polarization system includes the calibration of the PSG and PSA, and then the automatic calibration of the liquid crystal composition PSA alone. The specific steps are as follows:

[0023] Step 1: Use a polarimeter to measure air, a quarter-wave plate with a fast axis direction of 30°, a polarizer with a transmission axis in the horizontal direction, and a polarizer with a transmission axis in the vertical direction respectively, and obtain the corresponding light intensity matrices I0, I1, I2, and I3 in sequence. The Mueller matrix of air is the unit matrix, denoted as matrix E, so I0:

[0024] I0 = A·E·G = A·G(2)

[0025] where the subscript i = 1, 2, 3 represents the number of the reference sample, M i is the Mueller matrix of the reference sample, and the light intensity I i of other samples:

[0026] I i = A·M i ·G(3)

[0027] Left-multiply the inverse matrix of I0 by I i , and the corresponding matrix C i can be obtained:

[0028]

[0029] It can be seen that Ci Similar to M i which has the same eigenvalues, the Mueller matrix M of the reference sample is calculated by solving the eigenvalues of C i ; i ;

[0030] The instrument matrix G of the PSG can be obtained by matrix operation; the PSA instrument matrix A can be solved using Equation (2):

[0031] A = I0·G -1 (5)

[0032] Since the PSG is composed of achromatic quarter-wave plates and its polarization characteristics are constant and do not change with temperature, time, or wavelength, the polarized light generated by it can be used as a reference light with a known polarization state. Before each measurement of the sample to be measured, using the Stokes four-point calibration method, that is, controlling the PSG to rotate 4 times at set angles to generate 4 linearly independent known polarization states, and the Stokes parameters form a matrix S. The light intensity matrix I is measured using the PSA composed of liquid crystals psa , then the instrument matrix A of the PSA is:[[]]

[0033] A = I psa ·S -1 (6)

[0034] Since the rotation angle setting of the PSG is the same as the angle during ECM calibration, the instrument matrix G of the PSG calibrated in ECM is the same as the Stokes parameter matrix, that is: A = I psa ·G -1 (6)

[0035] The calibration of the PSA composed of liquid crystals is completed.

[0036] Program this formula method using a programming language, such as the G language of Labview, and write it into the self-calibration control program. Only the first time use the ECM calibration method to solve the instrument matrix G of the PSG, and then after each measurement, the self-calibration control program can complete the calibration of the PSA according to the measured air window light intensity data, realizing the real-time self-calibration of the liquid crystal variable phase retarder polarimeter.

[0037] The present invention realizes the real-time self-calibration of the composite polarization system through the combination of eigenvalue calibration, Stokes four-point calibration method, and self-calibration control program, which is of great significance for improving the polarization measurement accuracy and compensating for the defects of the liquid crystal phase retarder.

[0038] While accelerating the polarization measurement speed, the present invention solves the problem that the modulation of the retardation amount of the liquid crystal variable phase retarder in the liquid crystal variable phase retarder polarimeter is unstable and vulnerable to the environment. The calibration of the entire polarization system is completed by using eigenvalue calibration, and the real-time self-calibration of the liquid crystal polarimeter is completed by combining polarization theory calculation and a self-calibration control program, improving the stability and measurement accuracy of the polarization system.

[0039] In summary, the above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A composite polarization system capable of real-time self-calibration, characterized in that, It includes a polarization state analyzer PSA composed of two liquid crystal variable phase retarders and one polarizer, a polarization state generator PSG composed of one turntable, a quarter achromatic waveplate and one polarizer, a collimating lens, a condenser lens, a light source and a detector. There are two detection windows set at the sample to be measured in the system. One window is for placing the sample to be measured, and the other window is for air. The light beam emitted by the light source is collimated by the collimating lens. The collimated light beam enters the PSG. The light beam becomes linearly polarized light through the polarizer of the PSG. Then, the azimuth angle of the fast axis of the achromatic waveplate is changed by the turntable and is further modulated to generate various set polarized lights. This polarized light enters the detection window. Part of the light passes through the window of the sample to be measured and carries the polarization information of the sample to be measured. The other part passes through the air window. Then, the two light beams pass through the PSA. After the light beam passes through the liquid crystal variable phase retarder with different retardation amounts generated by voltage control and the polarizer in the PSA, it is analyzed. Finally, it is detected by the detector through the condenser lens.

2. The real-time self-calibrating composite polarization system according to claim 1, characterized in that, The system is controlled by a self-calibrating automatic control program and has a visualization interface for measurement results.

3. A calibration measurement method applied to the system described in claim 1, characterized in that, The reference samples: air, quarter-wave plate and polarizer are respectively placed between the PSG and the PSA for measurement. The eigenvalue method is used to calibrate the systems on both sides of the reference sample to obtain the instrument matrix G of the PSG and the instrument matrix A of the PSA. The polarized light generated by the calibrated PSG is used as the reference light. Then, when measuring the polarization characteristics of the sample to be measured each time, the light intensity matrix I is obtained by using the air window in combination with the Stokes four-point calibration method. According to the following formula, the instrument matrix A of the PSA composed of LCVR is obtained: A = I·G -1 (1) The calibration of the LCVR system is completed.

4. The calibration measurement method according to claim 3, characterized in that, The data of the sample window to be measured is calculated by using the solved instrument matrix A of the PSA and the instrument matrix G of the PSG, and the high-precision measurement of the polarization characteristics of the sample to be measured is realized.

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