An apparatus and method for measuring stress distribution in KDP crystal structures.

Abstract

This invention proposes an apparatus and method for measuring the stress distribution of a KDP crystal structure. The apparatus includes: a laser, a convex lens, a first linear polarizer, a crystal sample to be tested, a vertical lifting stage, a horizontal displacement stage, a second linear polarizer, a CCD image sensor, a screen, and a computer. The laser, convex lens, first linear polarizer, crystal sample to be tested, second linear polarizer, and screen are arranged sequentially according to the optical path. The crystal sample to be tested is placed on the vertical lifting stage, which is placed on the horizontal displacement stage. The crystal sample to be tested is a Z-cut, double-sided polished KDP crystal. The CCD image sensor is connected to the computer. This apparatus and method are used to measure the stress distribution of a Z-cut KDP crystal structure. It is simple, low-cost, has no requirements on sample quality, size, or shape, and has wide adaptability. It can quickly, effectively, accurately, and quantitatively measure the structural stress and stress distribution within the crystal.

Description

Technical Field

[0001] This invention relates to the field of crystal optical testing, and more specifically to an apparatus and method for measuring the stress distribution of a KDP crystal structure. Background Technology

[0002] KDP(KH2PO4) and its isomer DKDP(KD) x H 2-x KDP (Polyoxaloacetic acid) crystals, due to their unique properties, are the only nonlinear crystals suitable for use as electro-optic switches and frequency converters in inertial confinement fusion (ICF) systems. Since traditional growth methods require 1-2 years to grow KDP-type crystals to the required size for ICF systems, rapid growth methods have been developed. However, during the rapid growth of KDP crystals, structural stresses are inevitably generated, which do not disappear after crystal growth ends. These stresses restrict the application of KDP crystals in practical applications, such as reducing the extinction ratio of electro-optic switches and the frequency doubling efficiency when used as frequency doubling devices. Therefore, accurately measuring the structural stress and its distribution in KDP crystals is of great significance for guiding actual KDP growth, evaluating their optical quality, and selecting high-quality KDP crystal regions suitable for the needs of ICF systems.

[0003] In the study of crystal structure stress, X-ray diffraction and neutron diffraction are generally used to determine the magnitude of crystal structure stress by observing the shift of diffraction peaks. However, X-ray diffraction has a limited penetration depth into crystals, and the surface flatness of the bulk crystal under test has a significant impact on the test results. Furthermore, the shift of X-ray and neutron diffraction peaks provides lattice information at the crystal test point, making it difficult to obtain quantitative information on the internal structural stress and its distribution within the crystal.

[0004] Chinese patent document CN111474192A discloses a neutron diffraction measurement method and system for tracking the second-order stress distribution of a specific orientation, comprising: Step 1: determining the orientation of the grain to be tracked and its {hkl} crystal plane group to be measured based on the crystal structure characteristics of the sample; Step 2: determining the spatial diffraction geometric position of each {hkl} crystal plane to be measured through texture calibration or theoretical calculation; Step 3: measuring the {hkl} crystal plane to be measured using neutron diffraction; Step 4: calculating the three-dimensional stress tensor of the tracked grain orientation and outputting the result; Step 5: if an in-situ experiment is performed, after changing the in-situ environment, returning to Step 3 and continuing to obtain the in-situ three-dimensional stress tensor of the tracked grain orientation and outputting the result. This invention solves the problem of measuring the second-order stress tensor of different grain orientations within polycrystalline materials by using neutron diffraction technology to track specific grain orientations. However, neutron diffraction requires the sample to have sufficient single-crystal quality, necessitating additional sample preparation steps. Sample size and shape are also limited. Furthermore, neutron flux is typically low, and neutron diffraction experiments require a relatively long time to obtain sufficient data. This results in a long experimental cycle, limiting the availability and efficiency of the experiments. In addition, neutron diffraction experiments require specialized equipment, such as neutron sources and neutron scattering instruments, which are usually expensive and difficult to obtain.

[0005] Therefore, there is an urgent need for a simple, low-cost, fast, efficient method that is not limited by sample quality, size, or shape, has wide applicability, and can accurately measure the internal stress distribution of KDP crystal structures. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes an apparatus and method for measuring the stress distribution in KDP crystal structures. The apparatus and method of this invention are used to measure the stress distribution in Z-cut KDP crystal structures. They are simple, low-cost, have no requirements regarding sample quality, size, or shape, and are widely adaptable. They can quickly, effectively, accurately, and quantitatively measure the structural stress and stress distribution within the crystal.

[0007] The technical solution of the present invention is as follows:

[0008] An apparatus for measuring the stress distribution of a KDP crystal structure includes: a laser, a convex lens, a first linear polarizer, a crystal sample to be tested, a vertical lifting stage, a horizontal displacement stage, a second linear polarizer, a CCD image sensor, a screen, and a computer; the laser, convex lens, first linear polarizer, crystal sample to be tested, second linear polarizer, and screen are arranged sequentially according to the optical path; the crystal sample to be tested is placed on the vertical lifting stage, and the vertical lifting stage is placed on the horizontal displacement stage; the crystal sample to be tested is a Z-cut, double-sided polished KDP crystal; the CCD image sensor and the computer are connected.

[0009] According to a preferred embodiment of the present invention, a 532nm laser with stable power is selected to ensure that the CCD image sensor can collect a stable interference pattern. Since the refractive index of KDP crystal is different for different wavelengths of light, and the photoelastic coefficient at 532nm is known, a 532nm laser is used.

[0010] According to a preferred embodiment of the present invention, the focal length of the convex lens is 2-3 cm, and its function is to convert the parallel light beam into a conical light beam. The focal point of the convex lens is adjusted to be inside the crystal sample to be tested, so as to ensure that the optical axis tilt angle caused by structural stress can be calculated subsequently using the distance between the crystal sample and the screen.

[0011] According to a preferred embodiment of the present invention, the polarization direction of the first linear polarizer is horizontal, and the polarization direction of the second linear polarizer is vertical. The polarization directions of the first and second linear polarizers are orthogonal to achieve orthogonal polarization interference. The second linear polarizer is close to the crystal sample to be tested to ensure that a complete wide-angle conical interference pattern can be collected on the screen, thereby measuring the fringe spacing of the pattern to reflect stress information.

[0012] According to the present invention, the horizontal displacement stage is driven by a stepper motor to realize the X-axis (lateral scanning) of the crystal sample to be tested, and the vertical lifting stage is driven by a stepper motor to realize the Y-axis (longitudinal scanning) of the crystal sample to be tested.

[0013] According to the present invention, the crystal sample to be tested must be a Z-cut, double-sided polished KDP crystal. Since the conical interference fringes of a Z-axis KDP crystal are black crosses, when the fringe distortion becomes a hyperbola, the spacing between the vertices of the hyperbola can be used to measure the structural stress. However, the conical interference fringes in other directions are more complex, and the change in fringe spacing is difficult to directly use for calculating stress information. Furthermore, the higher the polishing precision of the crystal sample to be tested, the clearer the interference fringes and the more accurate the measurement results; preferably, the polishing precision is 0-0.5λ, where λ is the wavelength of the incident wave when detecting the polishing precision.

[0014] According to a preferred embodiment of the present invention, a sufficient distance L must be separated between the screen and the focal point of the convex lens, and the size of L must be much larger than the focal length of the convex lens, so as to magnify the fringe spacing in the interference pattern and improve the measurement accuracy. Preferably, the screen is provided with a grid scale, and the side length of the smallest square is 1 mm.

[0015] According to a preferred embodiment of the present invention, a CCD image sensor is used to acquire interference patterns, is placed directly opposite the screen, and is focused on the screen to ensure that the smallest unit of the grid on the screen can be clearly distinguished when the computer collects images.

[0016] The method for measuring the stress distribution of a KDP crystal structure using the above-mentioned apparatus includes the following steps:

[0017] (1) Place the crystal sample to be tested on the vertical lifting platform, turn on the recording mode of the laser and CCD image sensor, start the horizontal displacement stage, and collect the interference pattern video of one row of the crystal sample to be tested.

[0018] (2) Return the horizontal displacement stage to its original position, start the vertical lifting stage to raise the crystal sample to be tested by a certain distance δy, turn on the recording mode of the laser and CCD image sensor, start the horizontal displacement stage, and collect the interference pattern video of one row of the crystal sample to be tested.

[0019] (3) Repeat step (2) until all points of the crystal sample to be tested have been collected as interference pattern videos;

[0020] (4) Use video processing software to divide the interference pattern video acquired in step (3) into m*n interference pattern images, and ensure that the test points corresponding to each image are evenly distributed along the row and column directions on the actual crystal sample to be tested.

[0021] (5) Using image processing software, measure the distance d between the vertices of the hyperbola in the interference pattern obtained in step (4); then use the following formula to convert the distance d between the vertices of the hyperbola in each interference pattern into the absolute value of shear strain S. 21 (Since the physical quantity strain is dimensionless, this invention uses strain to represent stress, and stress and strain are essentially equivalent.)

[0022]

[0023] Where β is the dielectric isolation ratio, p is the strain-optical coefficient, and for a KDP crystal with an incident wavelength of 532 nm, β2 = 0.4389, β3 = 0.4639, and p 66 = -0.064; L is the distance between the screen and the focal point of the convex lens; d is the distance between the vertices of the hyperbola in the interference pattern image;

[0024] This formula is derived from the formula for the optic axis tilt angle in the wide-angle conic interferometry pattern by combining the perturbation of stress on the refractive index ellipsoid of a single optical axis crystal. In this invention, only the absolute value of the strain is used for the shear strain value, and the direction of the strain is not discussed.

[0025] (6) The absolute values ​​of shear strain S corresponding to all interference pattern images 21 The shear stress distribution of the crystal sample under test is plotted using a plotting program, which is an m*n two-dimensional matrix.

[0026] According to a preferred embodiment of the present invention, in step (1), after the crystal sample to be tested is placed on the vertical lifting platform, it must be left for 5 minutes before the laser is turned on to start the measurement, so as to eliminate the influence of the stress generated by the hand or tool clamping when placing the sample on the structural stress.

[0027] According to the present invention, in step (1), the moving speed of the horizontal displacement stage should be as slow as possible, generally below 1 mm / s.

[0028] According to the present invention, in step (2), δy is the step size of the longitudinal scan, which is related to the longitudinal dimension of the crystal sample to be tested, and is taken as δy = y / n, where y is the longitudinal width of the crystal sample to be tested, and n is the number of longitudinal data points of the stress distribution map. When the resolution requirement of the stress distribution map is high, δy can be reduced to obtain a clearer stress distribution image. Preferably, δy is less than or equal to 2 mm, and more preferably less than or equal to 1 mm.

[0029] According to the present invention, in step (4), the video processing software can be Adobe Premiere Pro (Windows / Mac), iMovie (Mac), Kdenlive (Linux), etc.

[0030] According to the present invention, in step (4), the ratio of m to n should be the same as the length-to-width ratio of the actual crystal, so as to ensure that the stress distribution diagram obtained from step (6) can intuitively show the stress distribution of the actual crystal.

[0031] According to the present invention, in step (5), the image processing software can be Adobe Photoshop (Windows / Mac), GIMP (Windows / Mac / Linux), PhotoScape X (Windows / Mac), etc.

[0032] According to a preferred embodiment of the present invention, in step (5), the method for measuring the hyperbola vertex spacing d in the interference pattern image is as follows: the number of pixels spanned by the hyperbola vertex spacing in the interference pattern image and the number of pixels spanned by the smallest unit square on the screen are measured, and the hyperbola vertex spacing d is obtained by comparison.

[0033] According to the present invention, in step (6), the plotting program can be based on existing technology; a two-dimensional array can be created using the numpy module of the Python language in the integrated development environment Spyder, and the absolute values ​​of shear strain at each point of the crystal can be filled into the two-dimensional array, and finally the matplotlib module can be used for plotting.

[0034] The technical features and beneficial effects of this invention are as follows:

[0035] 1. The measuring device and method of this invention are designed for crystals that have undergone X-ray orientation. Utilizing the interference pattern distortion caused by strain during the growth of a uniaxial crystal in a conical interferogram, the relationship between the fringe spacing and the structural stress of the crystal is derived from the theory of the photoelastic effect. This leads to a method for detecting the structural stress and its distribution in KDP crystals. This invention achieves quantitative detection of the structural stress distribution in KDP crystals, overcoming the current difficulties in quantitative stress distribution testing of uniaxial KDP crystals. Based on the photoelastic effect, this invention has broad application prospects and can provide testing for the structural stress and its distribution in Z-cut KDP crystals of different sizes, shapes, and masses, demonstrating wide adaptability. The method of this invention does not require expensive equipment, is low-cost, and simple; it can quickly, effectively, accurately, and quantitatively measure the internal structural stress and its distribution in crystals.

[0036] 2. The photoelastic effect can be used to obtain information about the structural stress and stress distribution within a crystal. For optically isotropic materials, such as transparent amorphous materials and cubic crystals, the phase retardation phenomenon caused by stress birefringence can be utilized in experiments, and a combination of polarizers can be used to directly observe the magnitude and distribution of structural stress. However, for the KDP crystal under test in this invention, the birefringence caused by the crystal's inherent optical anisotropy prevents the polarizer from directly observing the crystal stress information.

[0037] 3. In this invention, for the Z-cut KDP crystal to be tested, due to the presence of structural stress, the interference pattern of the uniaxial crystal in its wide-angle conical interference pattern is distorted into a hyperbola (as shown in the attached figure). Figure 1 The vertices of the hyperbola actually represent the outcrops of the crystal's optical axis. Therefore, the distance d between the vertices of the hyperbola can be measured (as shown in the attached figure). Figure 1 Divide by the distance L between the converging point of the cone beam and the screen (as shown in the attached diagram). Figure 1 The tilt angle V of the biaxial crystal optical axis is obtained. This optical axis tilt angle is determined by the crystal refractive index ellipsoid, which is determined by the structural stress in the crystal through the elasto-optic effect (while keeping other physical fields constant during the measurement process). By combining the above relationships, the direct relationship between the fringe spacing and crystal stress in the wide-angle conic light interference can be obtained, which is the formula used in step (5).

[0038] 4. In this invention, the stress test for crystal samples is a non-destructive test. In addition, this device and method can also provide structural stress and distribution tests for other Z-cut uniaxial crystals with similar elastic-optical coefficients to KDP. Attached Figure Description

[0039] Figure 1This is a schematic diagram of the optical path between the crystal sample to be tested and the screen in this invention; wherein, the z-axis is the direction of the ideal KDP crystal optical axis, L is the distance between the screen and the focal point of the convex lens, and d is the distance between the vertices of the hyperbola in the interference pattern image.

[0040] Figure 2 This is a schematic diagram of the device for measuring the stress distribution of a Z-cut KDP crystal structure according to the present invention; wherein, 1 is a laser, 2 is a convex lens, 3 is a first linear polarizer, 4 is the crystal sample to be tested, 5 is a vertical lifting stage, 6 is a horizontal displacement stage, 7 is a second linear polarizer, 8 is a CCD image sensor, and 9 is a screen. Wherein, L and... Figure 1 The L in the text is consistent.

[0041] Figure 3 This is a stress distribution diagram of a Z-cut KDP crystal structure measured in Example 2.

[0042] Figure 4 This is a stress distribution diagram of a Z-cut KDP crystal structure measured in Example 3. Detailed Implementation

[0043] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but is not limited thereto.

[0044] Furthermore, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods; and unless otherwise specified, the equipment and materials described are all commercially available.

[0045] Example 1

[0046] An apparatus for measuring the stress distribution in a KDP crystal structure, such as... Figure 2 As shown, the system includes: a laser (1), a convex lens (2), a first linear polarizer (3), a crystal sample to be tested (4), a vertical lifting stage (5), a horizontal displacement stage (6), a second linear polarizer (7), a CCD image sensor (8), a screen (9), and a computer; the laser (1), convex lens (2), first linear polarizer (3), crystal sample to be tested (4), second linear polarizer (7), and screen (9) are arranged sequentially according to the optical path; the crystal sample to be tested (4) is placed on the vertical lifting stage (5), and the vertical lifting stage (5) is placed on the horizontal displacement stage (6); the crystal sample to be tested (4) is a Z-cut double-sided polished KDP crystal; the CCD image sensor (8) is connected to the computer.

[0047] The laser (1) is a 532nm laser with stable power to ensure that the CCD image sensor can collect a stable interference pattern. Since the KDP crystal has different refractive indices for different wavelengths of light, and the photoelastic coefficient at 532nm is known, a 532nm laser is used.

[0048] The focal length of the convex lens (2) is 2.5 cm, and its function is to convert the parallel beam into a conical beam. The focal point of the convex lens (2) is adjusted to be inside the crystal sample (4) to ensure that the optical axis tilt angle caused by structural stress can be calculated using the distance between the crystal sample and the screen.

[0049] The polarization direction of the first linear polarizer (3) is horizontal, and the polarization direction of the second linear polarizer (7) is vertical. The polarization directions of the first linear polarizer (3) and the second linear polarizer (7) are orthogonal to achieve orthogonal polarization interference. The second linear polarizer (7) is close to the crystal sample (4) to ensure that the screen (9) can collect a complete wide-angle conical interference pattern, and then measure the fringe spacing of the pattern to reflect stress information. The horizontal displacement stage (6) is driven by a stepper motor to achieve X-axis scanning of the crystal sample (4), and the vertical lifting stage (5) is driven by a stepper motor to achieve Y-axis scanning of the crystal sample (4). The screen (9) and the focal point of the convex lens (2) need to be separated by a sufficient distance L (1m). The size of L should be much larger than the focal length of the convex lens (2) to amplify the fringe spacing in the interference pattern and improve the measurement accuracy. A piece of graph paper is placed on the screen (9), and the size of each smallest unit square in the graph paper is 1mm. The CCD image sensor (8) is used to acquire interference patterns. It is placed directly opposite the screen (9) and focused on the screen (9) to ensure that the smallest unit of the grid on the screen (9) can be clearly distinguished when the computer collects images.

[0050] Example 2

[0051] The method for measuring the stress distribution of a KDP crystal structure using the apparatus in Example 1 includes the following steps:

[0052] (1) The KDP crystal (polished to a precision of 0.2λ, where λ is the wavelength of the incident wave when the polishing precision is detected, which is 532nm) of the Z-cut double-sided polished crystal sample (4) to be tested is placed on the vertical lifting platform (5). After it is placed for 5 minutes, the laser (1) is turned on to start the measurement, so as to eliminate the influence of the stress generated by the hand or tool clamping on the structural stress when the sample is placed. Turn on the recording mode of the laser (1) and the CCD image sensor (8), start the horizontal displacement stage (6) (moving speed of 1mm per second), and collect the interference pattern video of one row of the crystal sample (4) to be tested;

[0053] (2) Return the horizontal displacement stage (6) to its original position, start the vertical lifting stage (5) to raise the crystal sample (4) to a certain distance δy (1mm), turn on the recording mode of the laser (1) and CCD image sensor (8), start the horizontal displacement stage (6), and collect the interference pattern video of one row of the crystal sample (4).

[0054] (3) Repeat step (2) until all points of the crystal sample (4) to be tested have been collected as interference pattern videos;

[0055] (4) Use the video processing software Adobe Premiere to divide the interference pattern video acquired in step (3) into m*n interference pattern images, and ensure that the test points corresponding to each image are evenly distributed along the row and column directions on the actual crystal sample (4) to be tested; the ratio of m to n should be the same as the length and width ratio of the actual crystal, so that the stress distribution map obtained from step (6) can intuitively show the stress distribution of the actual crystal.

[0056] (5) Using the image processing software Adobe Photoshop, measure the hyperbola vertex spacing d in the interference pattern image obtained in step (4); the measurement method is as follows: measure the number of pixels spanned by the hyperbola vertex spacing in the interference pattern image and the number of pixels spanned by the smallest unit square on the screen (9), and obtain the hyperbola vertex spacing d by comparison. Then, use the following formula to convert the hyperbola vertex spacing d in each interference pattern image into the absolute value of shear strain S. 21 ;

[0057]

[0058] Where β is the dielectric isolation ratio, p is the strain-optical coefficient, and for a KDP crystal with an incident wavelength of 532 nm, β2 = 0.4389, β3 = 0.4639, and p 66 =-0.064; L is the distance between the focal points of the screen (9) and the convex lens (2); d is the distance between the vertices of the hyperbola in the interference pattern image;

[0059] (6) The absolute values ​​of shear strain S corresponding to all interference pattern images 21 The sample is arranged into an m*n two-dimensional matrix. In the integrated development environment Spyder, the numpy module of the Python language is used to create a two-dimensional array, and the absolute values ​​of shear strain at each point of the crystal are filled into the two-dimensional array. Finally, the matplotlib module is used to plot the shear stress distribution of the crystal sample (4) to be tested.

[0060] The shear stress distribution diagram is attached. Figure 3 As shown, this figure is a distribution diagram of stress in a KDP crystal structure with an actual diameter of 4.4cm*4.5cm (since the physical quantity strain is dimensionless, this invention uses strain to represent stress, and stress and strain are essentially equivalent). The horizontal and vertical axes in the figure correspond to the actual size of the crystal, and the color depth on the distribution diagram reflects the magnitude of the stress value.

[0061] Example 3

[0062] The method for measuring the stress distribution of the KDP crystal structure using the apparatus in Example 1 is as described in Example 2, except that δy in step (2) is increased to 2 mm. Other steps and conditions are the same as in Example 2.

[0063] Stress distribution diagram is attached. Figure 4 For and attached Figure 3 When testing the same crystal sample, increasing δy in step (2) results in a sparser number of measurement points, leading to a smaller amount of actual two-dimensional matrix data obtained from segmentation, which in turn reduces the resolution of the final crystal structure stress distribution map.

Claims

1. An apparatus for measuring the stress distribution in a KDP crystal structure, comprising: The system includes a laser, a convex lens, a first linear polarizer, a crystal sample to be tested, a vertical lifting stage, a horizontal displacement stage, a second linear polarizer, a CCD image sensor, a screen, and a computer. The laser, convex lens, first linear polarizer, crystal sample to be tested, second linear polarizer, and screen are arranged sequentially according to the optical path. The crystal sample to be tested is placed on the vertical lifting stage, which is then placed on the horizontal displacement stage. The crystal sample to be tested is a Z-cut, double-sided polished KDP crystal. The CCD image sensor and computer are connected.

2. The apparatus for measuring stress distribution in a KDP crystal structure according to claim 1, characterized in that, A 532nm laser with stable power was selected.

3. The apparatus for measuring the stress distribution of a KDP crystal structure according to claim 1, characterized in that, The focal length of the convex lens is 2-3 cm; the focal point of the convex lens is adjusted to be inside the crystal sample to be tested.

4. The apparatus for measuring the stress distribution of a KDP crystal structure according to claim 1, characterized in that, The polarization direction of the first linear polarizer is horizontal, and the polarization direction of the second linear polarizer is vertical. The polarization directions of the first and second linear polarizers are orthogonal. The second linear polarizer is close to the crystal sample to be tested to ensure that a complete wide-angle conical interference pattern can be collected on the screen.

5. The apparatus for measuring stress distribution in a KDP crystal structure according to claim 1, characterized in that, A sufficient distance L must be separated between the screen and the focal point of the convex lens. The size of L must be greater than the focal length of the convex lens to magnify the fringe spacing in the interference pattern and improve the measurement accuracy. The screen is equipped with a grid scale, with the smallest grid having a side length of 1 mm.

6. The apparatus for measuring stress distribution in a KDP crystal structure according to claim 1, characterized in that, The polishing precision of the crystal sample to be tested is 0-0.5λ.

7. The apparatus for measuring stress distribution in a KDP crystal structure according to claim 1, characterized in that, The CCD image sensor is used to acquire interference patterns. It is placed directly opposite the screen and focused on the screen.

8. A method for measuring the stress distribution of a KDP crystal structure using the apparatus according to any one of claims 1-7, comprising the steps of: (1) Place the crystal sample to be tested on the vertical lifting platform, turn on the recording mode of the laser and CCD image sensor, start the horizontal displacement stage, and collect the interference pattern video of one row of the crystal sample to be tested. (2) Return the horizontal displacement stage to its original position, start the vertical lifting stage to raise the crystal sample to be tested by a certain distance δy, turn on the recording mode of the laser and CCD image sensor, start the horizontal displacement stage, and collect the interference pattern video of one row of the crystal sample to be tested. (3) Repeat step (2) until all points of the crystal sample to be tested have been acquired and the interference pattern video has been collected; (4) Use video processing software to segment the interferometric pattern video acquired in step (3) into... Each interference pattern image is generated, and the test points corresponding to each image are evenly distributed along the row or column direction on the actual crystal sample to be tested. (5) Using image processing software, measure the distance d between the vertices of the hyperbola in the interference pattern obtained in step (4); then use the following formula to convert the distance d between the vertices of the hyperbola in each interference pattern into the absolute value of shear strain. S 21 ; ； in, β2=0.4389, β3=0.4639, p 66 =-0.064; L is the distance between the screen and the focal point of the convex lens; d represents the distance between the vertices of the hyperbola in the interference pattern image; (6) The absolute values ​​of shear strain corresponding to all interference pattern images S 21 Listed as one A two-dimensional matrix, using a plotting program, with this... The shear stress distribution of the crystal sample under test is plotted using a two-dimensional matrix.

9. The method for measuring the stress distribution of a KDP crystal structure according to claim 8, characterized in that, In step (1), after the crystal sample to be tested is placed on the vertical lifting platform, it must be left for 5 minutes before the laser is turned on to start the measurement.

10. The method for measuring the stress distribution of a KDP crystal structure according to claim 8, characterized in that, In step (5), the method for measuring the distance d between the vertices of the hyperbola in the interference pattern image is as follows: measure the number of pixels spanned by the distance between the vertices of the hyperbola in the interference pattern image and the number of pixels spanned by the smallest unit square on the screen, and obtain the distance d between the vertices of the hyperbola by comparison.

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