Multi-voltage phase recovery device and method based on photorefractive effect

Abstract

A multi-voltage phase recovery device and method based on a photorefractive effect are characterized in that the photorefractive effect is utilized to replace a non-linear effect under a strong light condition, diffraction images after an optical system under different non-linear effect degrees are collected through a photoelectric detector, a reconstruction algorithm based on non-linear inversion is used, and a multi-voltage phase recovery model is obtained. Reconstructing amplitude and phase information of the sample to be measured; compared with a traditional linear multi-plane phase recovery method, the method has the advantages that the multiple diffraction images can be obtained only by changing the voltage without mechanical displacement, the axial mechanical displacement error and the transverse alignment error of the multiple images can be avoided, and therefore the precision is higher, and the convergence speed is higher.

Description

TECHNICAL FIELD

[0001] The present application relates to the phase defect detection of optical elements, in particular to a multi-voltage phase recovery device and method based on photorefractive effect. BACKGROUND

[0002] The defects contained in the elements in the high-power laser system can induce downstream light field modulation, which may lead to laser-induced damage, and the damage points can cause further deterioration of the beam quality as new defect sources, so the precise detection of the phase defects of the elements is of great significance to the improvement of the load capacity of the laser device.

[0003] At present, the detection methods of the phase defects of optical elements mainly include atomic force microscope detection method, line scanning differential imaging method, phase shift interference measurement method, and multi-plane coherent diffraction imaging. The atomic force microscope detection method is to use a needle tip sensitive to micro force to approach the sample, and after the deformation under force, the laser reflection to the displacement sensor generates current. The needle tip scans the sample surface, and the sample topography is obtained by computer feedback to detect defects. In order to improve the detection efficiency, the Lawrence Livermore National Laboratory proposed the line scanning differential imaging method to realize the rapid detection of the full field of view, and then the phase shift diffraction interference method was used to precisely characterize the defects found in the line scanning differential imaging method. The phase shift interference method is a point diffraction interference method, which uses the principle of interference to directly measure the phase. A known phase change is introduced into the optical path, and the phase of each measurement point can be calculated by the change of the interference pattern. The multi-plane coherent diffraction imaging is a method of obtaining the phase information of the sample from the scattering intensity of the light field through iteration. For the above traditional methods, the atomic force microscope detection method is not suitable for large-aperture element detection due to its high cost and low detection speed; the line scanning differential imaging method cannot detect strong-axis symmetric defects and has low detection resolution; the phase shift diffraction interference method is sensitive to vibration during the interference process and has a complex optical path; the multi-plane coherent diffraction imaging method solves the problem of vibration sensitivity in the interference method and has a simple optical path, but the mechanical displacement error in the axial direction and the horizontal alignment error of multiple images occur when different intensity images are collected, which affects the accuracy of the calculation results. For example, the multi-plane coherent diffraction imaging method proposed in the prior art (see Pedrini G, Osten W, Zhang Y. Wave-front reconstruction from a sequence of interferograms recorded at different planes[J]. Optics Letters, 2005, 30(8):833-835.) has a simple structure and is easy to implement, but the method is affected by mechanical displacement error and alignment error, and it is difficult to achieve high-precision phase recovery results. SUMMARY

[0004] The present application aims to overcome the deficiencies of the prior art, and provide a multi-voltage phase recovery device and method based on photorefractive effect. The method is characterized by using photorefractive effect to replace nonlinear effect under strong light conditions, collecting diffraction images after the optical system under different degrees of nonlinear effect by a photodetector, and using a reconstruction algorithm based on nonlinear inversion to reconstruct the amplitude and phase information of the sample to be measured. Compared with the traditional linear multi-plane phase recovery method, the method can obtain multiple diffraction images without mechanical displacement by only changing the voltage, and can avoid axial mechanical displacement error and lateral alignment error of multiple images, so the advantage of the present application is higher precision and faster convergence speed.

[0005] The technical solution of the present application is as follows: A multi-voltage phase recovery device based on photorefractive effect is provided, which comprises a laser; a polarization beam splitter, a sample to be measured, a nonlinear medium, an imaging lens group and a photodetector arranged in sequence along the output light path direction of the laser; further comprising a voltage output device electrically connected with the nonlinear medium, for applying multiple different voltages to the nonlinear medium to change the photorefractive effect intensity thereof; and a computer in communication connection with the photodetector, for receiving multiple light intensity image data collected by the photodetector under different voltages and executing a phase recovery algorithm; wherein the polarization direction of the polarization light emitted by the polarization beam splitter is consistent with the optical axis direction of the nonlinear medium.

[0006] Further, the photodetector is arranged on an electrically controlled displacement table, the electrically controlled displacement table is connected with the computer and controlled to move by the computer, and the electrically controlled displacement table is used to control the movement of the photodetector to collect the intensity distribution at a fixed position away from the rear surface of the nonlinear medium.

[0007] Further, the nonlinear medium is a photorefractive crystal, and electrodes are adhered to both sides of the nonlinear medium and connected with the voltage output device. By adhering electrodes to the nonlinear medium, the voltage applied to the nonlinear medium is controlled by the voltage output device. The photorefractive effect occurs in the nonlinear medium due to the voltage applied thereto and the influence of the incident light field. The change of the refractive index in the medium can be approximately expressed as: wherein, represents the linear refractive index of the crystal, which is 2.2817 and does not change with voltage, represents the polarization electro-optic coefficient, is the electric field applied to the crystal axis, is the normalized light field intensity, The fitting coefficient was experimentally calibrated to 0.6. The applied electric field was adjusted by changing the voltage magnitude. The change causes the change in the refractive index of the medium. Changes occur, and the normalized light field intensity changes. It will also cause uneven changes in the refractive index of the medium. The altered photorefractive effect can replace the nonlinear effect under strong light.

[0008] Furthermore, one of the core functions of the imaging lens group is to provide a defined and precisely repeatable initial optical reference plane for the entire measurement system, namely the conjugate image plane of the back surface of the nonlinear medium 4. The position of the photodetector's detection surface is adjusted so that it is precisely located at this initial image plane; by moving the photodetector along the optical path, light intensity distributions equivalent to a series of different propagation distances from the back surface of the nonlinear medium can be collected. All subsequent equivalent propagation distances are calculated starting from this reference plane, ensuring the consistency and accuracy of the measurement coordinate system.

[0009] Furthermore, the photodetector is mounted on an electrically controlled displacement stage, which is connected to the computer. The electrically controlled displacement stage is used to drive the photodetector to move a set distance along the optical path and fix it after the detection surface of the photodetector is aligned with the conjugate image plane of the back surface of the nonlinear medium. This allows the photodetector to be located at a fixed, non-zero equivalent propagation distance during data acquisition, so as to collect light intensity distribution with a specific diffraction effect.

[0010] This invention also proposes a method for phase recovery using the aforementioned multi-voltage phase recovery device based on photorefractive effect, comprising the following steps: Data acquisition steps: By adjusting the output voltage of the voltage output device, the photoelectric detector... Multiple diffraction intensity images corresponding to different nonlinear effect intensities were obtained at the instrument. Phase retrieval step: In the computer, based on the multiple diffraction intensity images, using a method containing... An iterative phase recovery algorithm based on a nonlinear propagation model reconstructs the phase information of the sample under test. The applied voltage affects the nonlinear propagation model by changing the refractive index distribution of the nonlinear medium.

[0011] Furthermore, a calibration step is included before the data acquisition step: Adjust the position of the detection surface of the photodetector so that it coincides with the image plane formed by the imaging lens group on the rear surface of the nonlinear medium.

[0012] Furthermore, the data acquisition steps specifically include: After completing the calibration steps, the photodetector is moved a set distance from the image plane position along the optical path transmission direction and then fixed by computer-controlled electronic displacement stage. Keeping the photodetector in this position, perform the operation of applying different voltages and acquiring corresponding diffraction intensity images.

[0013] Furthermore, the iterative phase retrieval algorithm performs the following iterative process: a. Initialize an initial estimate representing the complex amplitude distribution of the sample under test; b. Enter a global iterative loop. Each iteration includes sequentially processing all diffraction intensity images acquired under different voltages. The processing procedure is as follows: b1 Forward propagation of the light field: The currently estimated complex amplitude distribution of the sample is first propagated linearly in the forward direction to the front surface of the nonlinear medium, then propagated nonlinearly in the forward direction through the nonlinear medium under the nonlinear effect corresponding to the current voltage, and finally propagated linearly in the forward direction to the detection surface of the photodetector to obtain a calculated complex amplitude distribution of the detection surface; b2 Amplitude Constraint: On the detection surface, the amplitude of the calculated complex amplitude distribution is replaced with the square root of the current corresponding acquired light intensity image, while retaining its phase, thereby updating the complex amplitude distribution of the detection surface; b3 Back-propagation of the light field: The updated complex amplitude distribution of the detector surface is first propagated back linearly to the back surface of the nonlinear medium, then propagated back nonlinearly through the nonlinear medium, and finally propagated back linearly to the plane where the sample is located to update the complex amplitude distribution estimate of the sample. Repeat steps b1 to b3 for the next diffraction intensity image under different voltages until all images have been processed once, completing one global iteration. c. After completing one global iteration, determine whether the estimated complex amplitude distribution of the sample satisfies the preset convergence condition; If satisfied, the final sample phase information is output. If the conditions are not met, the updated estimate is used to restart the next global iteration loop.

[0014] Furthermore, in steps b1 and b3, the nonlinear propagation process in the nonlinear medium is achieved by solving the following nonlinear Schrödinger equation using the step-by-step Fourier method.

[0015] The calculation process of the step-by-step Fourier method is as follows: The length of the nonlinear medium Divided into Segment, for each segment of distance The calculations include: First step, before calculation linear diffraction of 1 / 2; Second step, calculate the whole section nonlinear effect; Third step, calculate the linear diffraction of 1 / 2 after .

[0016] Further, the judgment basis of the convergence condition in step c is that after one global iteration, the error function value between the calculated reconstructed light field and the collected light intensity data is calculated, and when the error function value is less than a preset threshold or iteration no longer makes it significantly decrease, it is determined that the convergence condition is met.

[0017] Compared with the prior art, the beneficial technical effects of the present application are as follows: In the traditional multi-plane phase defect detection method, due to the mechanical displacement error and alignment error between the collection planes, the final phase recovery result accuracy is not high, and the convergence speed is slow in the iteration process, usually hundreds or even thousands of times are needed to achieve the convergence effect. Compared with the traditional multi-plane phase defect detection method, the image collection method of the present application is to collect multiple diffraction intensity images under different voltage conditions at the same position surface, which can eliminate the influence of mechanical displacement error and alignment error on the result accuracy, and the phase reconstruction method based on nonlinear inversion is used, the phase matching condition in nonlinear propagation plays the role of linear filter, which can effectively reduce the influence of noise on phase recovery, so the convergence speed is faster. The fundamental reason is that the multiple mechanical displacements which are easy to introduce errors in the traditional method are changed into a preset fixed parameter, and the voltage change through accurate electric control is used as the core variable to obtain multi-dimensional information, so the displacement and alignment error is avoided at the data collection source. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a structural schematic diagram of a multi-voltage phase recovery device based on photo-induced refractive effect; Figure 2 is a flowchart of a phase recovery algorithm (iteration process) in a multi-voltage phase recovery method based on photo-induced refractive effect; Figure 3 is a phase recovery result graph (part of the area) of the sample to be measured by the present application; wherein, Figure 3 (a) is a three-dimensional phase distribution graph of the phase recovery result of the sample to be measured, Figure 3 (b) is a one-dimensional phase distribution graph of the phase recovery result of the sample to be measured; Figure 4 is a comparison graph of the iteration convergence curve of the present application and the multi-plane phase recovery method. DETAILED DESCRIPTION

[0019] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings: The present specific embodiment will elaborate the multi-voltage phase retrieval device and method based on photorefractive effect. The core of the technical solution is to provide a device that enables the acquisition of multiple diffraction light intensity images by changing the voltage applied on the nonlinear medium at a single fixed position of the photodetector; and based on this, a method for recovering the phase of the sample by using an iterative algorithm containing nonlinear effects is provided. The solution aims to overcome the mechanical displacement error and image alignment error introduced by the need to move the detector to multiple different positions in the traditional multi-plane phase retrieval technology.

[0020] First, refer to Figure 1 , Figure 1 The structure of the multi-voltage phase retrieval device based on photorefractive effect is shown in the figure. As can be seen from the figure, the multi-voltage phase retrieval device based on photorefractive effect includes a laser 1, a polarization beam splitter 2, a sample to be measured 3, a nonlinear medium 4, a voltage output device 5, an imaging lens group 6, a photodetector 7, and a computer 8. The electrically controlled displacement stage of the photodetector 7 is connected to the computer 8. By adhering electrodes to the nonlinear medium 4, the voltage applied to the nonlinear medium 4 is controlled by the voltage output device 5, and the direction of the polarized light emitted by the polarization beam splitter 2 is consistent with the c-axis direction of the nonlinear medium 4.

[0021] The electrically controlled displacement stage is controlled by the computer 8, and the photodetector 7 moves a distance from the conjugate surface position with the back surface of the nonlinear medium 4 to the direction of light transmission. The light intensity distribution collected by the photodetector 7 is equivalent to the diffraction light intensity distribution after propagating a corresponding distance in free space from the back surface of the nonlinear medium 4.

[0022] In this embodiment: the laser is a laser with a wavelength of 532 nm, the nonlinear medium 4 has a size of 5 mm x 5 mm x 10 mm, the photodetector 7 is a CCD with a resolution of 1400 pixel x 1600 pixel, and each pixel has a side length of 5.5 µm. The sample to be measured 3 is a phase-type resolution plate, which carries the structural information of the sample after being irradiated by the illuminating light, and the phase transmittance function that needs to be reconstructed.

[0023] The method for phase retrieval using the above imaging device includes the following steps: 1) Data recording: a. Device setup and calibration: set up the phase retrieval device according to the requirements described above. Start the laser 1 and adjust its output power to be stable. Then, perform the calibration step: finely adjust the position of the photodetector 7 on the electrically controlled displacement stage so that the detection surface of the photodetector 7 and the image surface formed by the imaging lens group 6 through the back surface of the nonlinear medium 4 are accurately overlapped.

[0024] It should be noted that the advantage of "avoiding mechanical displacement errors" described in this invention refers to the repeated and multiple mechanical displacements performed in traditional multi-plane phase retrieval methods to obtain multiple diffraction images. In this invention, the electrically controlled displacement stage is used only once before data acquisition. Its purpose is to position the photodetector 7 from a known reference position (i.e., the image plane conjugate with the back surface of the nonlinear medium 4, where the equivalent propagation distance is zero) to a preset, fixed non-zero equivalent propagation distance.

[0025] The setting of this fixed non-zero equivalent propagation distance is of significant technical importance: at the image plane where the equivalent propagation distance is zero, the diffraction effect of the light field is weakest, and the phase information contained in the acquired light intensity image may be insufficient to support high-precision inversion. By introducing a known, non-zero fixed equivalent propagation distance, it is equivalent to superimposing a definite linear diffraction effect into the system. Subsequently, while keeping the detector position absolutely unchanged (i.e., the equivalent propagation distance is fixed), the refractive index of the nonlinear medium 4 is adjusted by changing the voltage (i.e., changing the nonlinear effect). In this way, the differences between multiple diffraction images are entirely caused by controllable voltage changes (changes in the nonlinear effect), rather than unreliable mechanical displacements. The algorithm utilizes both the known fixed linear diffraction (determined by the equivalent propagation distance) and the variable nonlinear effect (determined by the voltage) as dual constraints, thereby achieving phase recovery with higher accuracy and faster convergence than traditional multi-plane methods without the need for multiple mechanical displacements.

[0026] In short, traditional methods acquire information by changing multiple equivalent propagation distances (displacements), while this invention obtains a better combination of information by fixing one equivalent propagation distance and changing multiple voltages. This fundamentally avoids the errors caused by multiple displacements.

[0027] b. Setting the detection position: After calibration, the computer 8 controls the electrically controlled displacement stage to drive the photodetector 7 to move from the image plane position along the optical path transmission direction. The set distance is fixed.

[0028] c. Multi-voltage data acquisition: Keeping the photodetector 7 in a fixed position, voltage output devices 5 sequentially apply voltages to the nonlinear medium 4. The values ​​of the different voltages are as follows: After each voltage stabilizes, the photodetector 7 acquires the corresponding diffraction intensity image, obtaining a total of [number missing] images. Amplitude diffraction intensity image And stored in computer 8, in this example .

[0029] 2) Data processing: in the computer 8, based on the collected multiple diffraction light intensity images, a phase recovery algorithm is executed to reconstruct the phase information of the sample 3 to be measured. The iterative execution process of the algorithm is as shown in Figure 2 , and the specific steps are as follows: wherein the different voltage values applied by the voltage output device 5 determine the change of the refractive index of the nonlinear medium through the formula , which determines the change of the refractive index of the nonlinear medium 4, which will be directly reflected in the subsequent iterative algorithm of the nonlinear forward and inverse propagation process.

[0030] The computer 8 first sets the initial value of the complex amplitude of the sample 3 to be measured , wherein represents the object plane, i.e., the sample plane, is the number of iterations, is the number of times the initial light field goes through forward and backward propagation and returns to the initial plane in one iteration, and the initial value of the object plane complex amplitude distribution is set to , the light field amplitude is set to 1, the light field phase is set to 0, and the initial voltage parameter in the nonlinear transmission process is , the distance between the plane where the sample 3 to be measured is located and the front surface of the nonlinear medium 4 is set to , and the distance between the detection surface of the photodetector 7 and the image plane (i.e., the conjugate image plane of the rear surface of the nonlinear medium 4) is set to .

[0031] ① Perform forward propagation of the light field. The length of the nonlinear medium 4 is set to , the linear propagation operator is set to , and the nonlinear propagation operator is set to , After linear spatial propagation , the front surface of the nonlinear medium 4 is reached, and then nonlinear spatial propagation is performed for a distance of , followed by linear transmission for a distance of , i.e. , and the complex amplitude distribution of the photodetector 7 detection surface is obtained: wherein represents the image plane, i.e., the photodetector plane.

[0032] The linear propagation operator is calculated by the formula wherein: the object plane coordinate is , the object plane light field distribution is , and after spatial propagation for a distance of , the coordinate of the image plane is reached The light field distribution on the image plane is . , denote Fourier transform and inverse Fourier transform, respectively; , is the frequency domain coordinate; is the spatial frequency transfer function. The expression of is: , , wherein, is the imaginary unit, the linear spatial wave number , the light wavelength ; the calculation width of the image plane light field ; , are the sampling point numbers, the total sampling point number .

[0033] The specific calculation of the nonlinear propagation operator is realized by solving the Schrödinger equation by the distributed Fourier method, and the Schrödinger equation is as follows: wherein, is the light field distribution, ; the wave number in the nonlinear medium 4 ; , is the intensity amplitude of the light field. Based on the equation, a split-step Fourier method is used for solving, in which the transmission of the nonlinear medium is subdivided into a plurality of transmission processes, in each of which the linear transmission and the part changed by the nonlinear refractive index are considered separately, and in the case that the step distance is small enough, the calculation result can be considered as the approximate result of the nonlinear transmission. The specific calculation process is that the nonlinear medium 4 with a length of L is divided into N segments, wherein N is an integer greater than or equal to 1, and each segment has a length of L / N. The length of each segment of the medium is L / N, and in the example, L / N = 1 mm. The three steps for calculating the diffraction and nonlinear effects of each segment are as follows: The three steps for calculating the diffraction and nonlinear effects of each segment are as follows: The first step is to calculate the linear spatial diffraction of the light field before the distance L / N, wherein is the light field on the incident plane of the distance L / N; The second step is to calculate the nonlinear effect process of the light beam transmission of the distance L / N, wherein is the light field on the incident plane of the distance L / N.​​​​ The third step is to calculate the remaining... The diffraction process, Repeat the previous step to calculate. Nonlinear transmission in nonlinear medium 4 can be completed in one step.

[0034] ② Replacement For the first The square root of the light intensity collected is the amplitude. The updated amplitude image plane, i.e., the light field distribution of the seven surfaces of the photodetector, is obtained as follows: ③ Perform backpropagation of the light field. Set the linear backpropagation operator as... The nonlinear inverse propagation operator is The light field obtained by replacing the amplitude with forward propagation Reverse linear transmission After reaching the surface of the nonlinear medium 4, it then undergoes nonlinear transmission over a distance. Then, the transmission distance is reversed. After that, ,get The linear backpropagation operator calculation method modifies the distance by replacing it with its inverse, while the nonlinear backpropagation operator calculation method modifies the calculation... The method is as follows: First step calculation Diffraction in linear space: in yes Light field at the incident surface at a distance.

[0035] The second step is to calculate beam propagation. The nonlinear effect process; The third step is to calculate the remaining... The diffraction process.

[0036] Repeat the calculation process from step one to step three above. In one pass, the entire length of the nonlinear medium 4 can be completed. Nonlinear transmission.

[0037] ④ Determine at this time Is it equal to If they are not equal, then Increment by 1, changing the voltage parameters of the nonlinear transmission process. , repeat steps ①, ②, ③ to continue to judge and value relative size; if equal, then the diffraction plane light intensity value obtained at this time and the actual first amplitude diffraction light intensity value relative error size , the formula is as follows: Wherein is the actual image collected in position pixel value, is the image after iteration in position pixel value, the relative error is calculated by the actual first image and the first image after iteration, is the total number of pixels of the image 1024x1024. If is not less than the set value , then add 1, 1, repeat steps ①, ②, ③ to continue to judge and value relative size; if less than the set value , the target object plane light field Wherein is the phase distribution of the object plane, i.e. the sample 3 to be measured.

[0038] Figure 3 Part of the phase recovery results of the present example are shown, and the phase sample is a phase resolution plate. First, the photodetector 7 collects the diffraction light intensity map under different voltage application conditions, and the amplitude of the collection plane is constrained by the obtained diffraction light intensity map. Through linear and nonlinear propagation between the sample plane and the collection plane, the phase three-dimensional distribution of the sample plane is obtained Figure 3 (a) and one-dimensional distribution Figure 3 (b). Figure 4 The iteration error convergence curves of the multi-plane method and the present method for phase recovery are shown. The present method converges after about 50 iterations, and the multi-plane method needs to be iterated nearly 300 times to converge. It can be seen that the convergence speed of the present method for phase recovery is faster than that of the traditional multi-plane phase recovery method.

Claims

1. A multi-voltage phase recovery device based on photorefractive effect, characterized in that, include: Laser (1); A polarization beam splitter (2), a sample to be tested (3), a nonlinear medium (4), an imaging lens group (6), and a photodetector (7) are arranged sequentially along the output optical path of the laser (1). A voltage output device (5) electrically connected to the nonlinear medium (4) is used to apply multiple different voltages to the nonlinear medium (4) to change its photorefractive effect intensity; And a computer (8) connected in communication with the photodetector (7) to receive multiple light intensity image data collected by the photodetector (7) under different voltages and to execute a phase recovery algorithm; The polarized light emitted from the polarization beam splitter (2) is oriented in a direction parallel to the optical axis of the nonlinear medium (4). The direction remains consistent.

2. The multi-voltage phase recovery device based on photorefractive effect according to claim 1, characterized in that, The photodetector (7) is mounted on an electrically controlled displacement stage, which is connected to the computer (8) and its movement is controlled by the computer (8).

3. The multi-voltage phase recovery device based on photorefractive effect according to claim 1, characterized in that, The nonlinear medium (4) is a photorefractive crystal with electrodes attached to both sides and connected to the voltage output device (5).

4. The apparatus according to claim 2, characterized in that, The electrically controlled displacement stage is used to: after the detection surface of the photodetector (7) is aligned with the conjugate image plane of the rear surface of the nonlinear medium (4), drive the photodetector (7) to move a set distance along the optical path and fix it, so that the photodetector (7) is located at a fixed, non-zero equivalent propagation distance during data acquisition.

5. A multi-voltage phase recovery method based on photorefractive effect, employing the apparatus as described in any one of claims 1-4, characterized in that, The method includes the following steps: Data acquisition steps: By adjusting the output voltage of the voltage output device (5), the photoelectric detector... Multiple diffraction intensity images corresponding to different nonlinear effect intensities were obtained at the device (7); Phase recovery step: In the computer (8), based on the multiple diffraction intensity images, using a method containing... The iterative phase recovery algorithm of the nonlinear propagation model reconstructs the phase information of the sample to be tested (3); The applied voltage affects the nonlinear propagation model by changing the refractive index distribution of the nonlinear medium (4).

6. The multi-voltage phase recovery method based on photorefractive effect according to claim 5, characterized in that, A calibration step is included before the data acquisition step: Adjust the position of the detection surface of the photodetector (7) so that it coincides with the image plane formed by the rear surface of the nonlinear medium (4) through the imaging lens group (6).

7. The method according to claim 6, characterized in that, The data acquisition steps are as follows: After completing the calibration steps, the computer (8) controls the electronically controlled displacement stage to move the photodetector (7) a set distance from the image plane position along the optical path transmission direction. And fix it; Keeping the photodetector (7) in this position, perform the operation of applying different voltages and acquiring corresponding diffraction intensity images.

8. The multi-voltage phase recovery method based on photorefractive effect according to claim 5, characterized in that, The iterative phase recovery algorithm performs the following iterative process: a. Initialize an initial estimate representing the complex amplitude distribution of the sample under test; b. Enter a global iterative loop. Each iteration includes sequentially processing all diffraction intensity images acquired under different voltages. The processing procedure is as follows: b1 Forward propagation of the light field: The currently estimated complex amplitude distribution of the sample is first propagated linearly in the forward direction to the front surface of the nonlinear medium, then propagated nonlinearly in the forward direction through the nonlinear medium under the nonlinear effect corresponding to the current voltage, and finally propagated linearly in the forward direction to the detection surface of the photodetector to obtain a calculated complex amplitude distribution of the detection surface; b2 Amplitude Constraint: On the detection surface, the amplitude of the calculated complex amplitude distribution is replaced with the square root of the current corresponding acquired light intensity image, while retaining its phase, thereby updating the complex amplitude distribution of the detection surface; b3 Back-propagation of the light field: The updated complex amplitude distribution of the detector surface is first propagated back linearly to the back surface of the nonlinear medium, then propagated back nonlinearly through the nonlinear medium, and finally propagated back linearly to the plane where the sample is located to update the complex amplitude distribution estimate of the sample. Repeat steps b1 to b3 for the next diffraction intensity image under different voltages until all images have been processed once, completing one global iteration. c. After completing one global iteration, determine whether the estimated complex amplitude distribution of the sample satisfies the preset convergence condition; If satisfied, the final sample phase information is output. If the conditions are not met, the updated estimate is used to restart the next global iteration loop.

9. A multi-voltage phase recovery method based on photorefractive effect according to claim 8, characterized in that, In steps b1 and b3, the nonlinear propagation process in the nonlinear medium (4) is achieved by solving the following nonlinear Schrödinger equation using the step-by-step Fourier method.

10. A multi-voltage phase recovery method based on photorefractive effect according to claim 9, characterized in that, The calculation process of the step-by-step Fourier method is as follows: The length of the nonlinear medium (4) Divided into Segment, for each segment of distance The calculations include: First step, before calculation Linear diffraction of 2 / 2; The second step is to calculate the entire segment. Nonlinear effects; The third step is to calculate... Linear diffraction of 2 / 2.

11. The multi-voltage phase recovery method based on photorefractive effect according to claim 8, characterized in that... The characteristic is that the convergence condition in step c is determined by: after one global iteration, calculating the error function value between the reconstructed light field and the acquired light intensity data; when the error function value is less than a preset threshold or the iteration no longer causes it to decrease significantly, it is determined that the convergence condition is met.

Images