Method for detecting focal plane of focusing lens of high-energy laser

Abstract

The invention discloses a method for detecting a focal plane of a focusing lens of a high-energy laser, which aims to solve the problems of low precision, easy device damage, complex operation and the like of the existing detection system in a high-power laser scene, and comprises the following steps of: constructing an optical path system comprising a 1 / 2 wave plate, a polarization beam splitter prism, a 1 / 4 wave plate, an aperture diaphragm and a reflector, and utilizing a polarization state modulation principle to detect the focal plane of the focusing lens of the high-energy laser. And the reflected light is converted into S polarized light which is detected by a power meter detector. A power-position curve is drawn by moving the reflector and recording power and position data, and a critical point of power rapid attenuation is used as a focal plane measurement value. And a theoretical error introduced by the laser divergence angle is compensated by further combining a pre-generated error curve (obtained through simulation fitting), so that high-precision positioning of the focal plane is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of laser processing and manufacturing, and in particular to a method for detecting the focal plane of a high-energy laser focusing lens. Background Art

[0002] High-energy laser systems play an irreplaceable role in fields such as materials processing and nuclear fusion ignition. The accuracy of their focal position directly determines the efficiency of energy transmission and the effectiveness of their effects. Current mainstream methods for detecting the focal plane of high-energy laser focusing lenses (such as thermocouple array scanning and sapphire probe contact methods) have significant limitations: mechanical contact detection can easily cause surface ablation of optical components, especially under kilowatt-level lasers, where the probe life is less than 100 hours; off-axis imaging methods are limited by the damage threshold of the CCD target surface, making them unsuitable for high-power density scenarios; and traditional knife-edge methods lead to a sharp drop in positioning accuracy due to thermal deformation. These bottlenecks have long plagued in-situ detection of the focal plane of high-energy laser beams: complex operation, limited accuracy, and severe device loss. Summary of the Invention

[0003] In order to solve the problems of low precision, fragile devices, and complex operation in existing detection methods in high-power laser scenarios, the present invention provides a high-energy laser focusing lens focal plane detection method, which converts reflected light into S-polarized light detection through a polarization modulation optical path, combines power attenuation critical point positioning and divergence angle error compensation to achieve in-situ high-precision detection of the focal plane of the high-energy laser focusing lens.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention provides a method for detecting the focal plane of a high energy laser focusing lens, comprising: (1) Optical path modulation: A half-wave plate, a first polarization beam splitter prism, a second polarization beam splitter prism, a quarter-wave plate, and an aperture stop are sequentially arranged in the optical axis direction of the optical path in front of the focusing lens to be measured. A reflector with an axially adjustable position is arranged in the optical axis direction of the optical path behind the focusing lens to be measured. A power meter detector is arranged on the reflected optical path of the second polarization beam splitter prism. Two polarization beam splitters are used to increase the P polarization degree of the incident laser, improve measurement accuracy, and ensure that the light reaching the second beam splitter prism is P polarized light. (2) Polarization state modulation: The incident high-energy laser is first modulated into elliptically polarized light by a half-wave plate, then separated into P-polarized light by the first polarization beam splitter prism. The P-polarized light is converted into elliptically polarized light by the second polarization beam splitter prism and the quarter-wave plate. After passing through the aperture diaphragm to remove stray light, it is converged by the focusing lens to be measured. The reflector reflects the beam back along the original path. After passing through the quarter-wave plate again, its polarization state becomes S-polarized light. Finally, it is reflected by the second polarization beam splitter prism to the power meter detector, which records the power value of the reflected beam. (3) Data collection: Move the reflector along the optical axis of the optical path to change the distance between the reflector and the focusing lens to be measured, and synchronously record the power value of the light beam reflected by the second polarization beam splitter prism and the position of the reflector; (4) Focal plane calculation: A power-position curve is drawn based on the collected power value of the light beam reflected by the second polarization beam splitter prism and the reflector position data, and the starting position of rapid power decay on the curve is determined. The reflector position coordinates corresponding to the starting position of rapid power decay are recorded. This is the measured value of the focal plane position of the measured focusing lens.

[0005] Using the above technical solution: 1. No additional light source is required. The polarization direction of the linearly polarized light can be rotated by adjusting the frontmost half-wave plate. This, in conjunction with the first polarizing beamsplitter prism at the rear, changes the ratio of the beam power transmitted / reflected by the first polarizing beamsplitter prism. (For example, rotating the half-wave plate so that the polarization direction of the incident laser is parallel to the transmission axis of the first polarizing beamsplitter prism maximizes the transmitted power and minimizes the reflected power.) By controlling the rotation angle of the half-wave plate, the power of the beam transmitted by the first polarizing beamsplitter prism can be continuously and repeatedly adjusted. This allows the input optical power entering the measurement system to be controlled, effectively preventing damage to optical components due to excessive power.

[0006] 2. Through optical path modulation, the incident light passes through a half-wave plate, then the first polarization beam splitter prism separates the P-polarized light, which is then converted to elliptically polarized light by the second polarization beam splitter prism and the quarter-wave plate. The aperture diaphragm filters out stray light, and the light is focused by the focusing lens to be measured. The light reflected by the reflector passes through a quarter-wave plate for a second time, converting it to S-polarized light, and then is reflected by the second polarization beam splitter prism to the power meter. The linearly polarized light P light is converted into linearly polarized light S light after passing through the 1 / 4 wave plate twice. Due to the characteristics of the polarization beam splitter prism (high transmission of P polarized light and high reflection of S polarized light), the linearly polarized light S light with an orthogonal polarization to the linearly polarized light P light will be reflected by the second polarization beam splitter prism and guided to the detection end. The detection end is located in the non-focusing area, avoiding the risk of damage to the CCD target surface. This fundamentally avoids the problem that the off-axis imaging method is difficult to apply to high power density scenarios due to the limitation of the CCD target surface damage threshold.

[0007] 3. In specific experiments, it was found that when the reflector is located outside the focal length range of the focusing lens under test, the light rays reflected by the reflector diverge significantly at the edges and cannot completely pass through the focusing lens again, causing the total power of the light spot received by the power meter detector to rapidly decay. Conversely, when the reflector is located within the focal length range of the focusing lens under test, the light rays reflected by the reflector can (all or most of) pass through the focusing lens again, so the total power of the light spot received by the power meter detector remains basically stable (for an ideal lens, the optical power theoretically remains unchanged; for non-ideal lenses with aberrations such as spherical aberration, the optical power only changes slightly). Based on this principle, by moving the reflector back and forth along the optical axis and synchronously recording the distance (position) between the reflector and the focusing lens and the optical power measured by the power meter detector at that position, a power-position relationship curve is plotted. The critical position on the curve where the power rapidly decays is identified. This position corresponds to the measurement position of the focus of the focusing lens under test.

[0008] Furthermore, the method further includes step (5) error compensation: pre-generating an error-focal length curve, substituting the measured value of the focal plane position of the measured focusing lens into the error-focal length curve to calculate the theoretical error value introduced by the laser divergence angle, and subtracting the theoretical error value from the measured value to obtain the final accurate measurement value of the focal plane position of the measured focusing lens.

[0009] Furthermore, the error-focal length curve is simulated and fitted using a non-sequential mode of Zemax optical design software.

[0010] Furthermore, the focal length measurement value is substituted into the error-focal length curve to directly obtain the corresponding theoretical error rate on the curve: theoretical error value = measurement value × theoretical error rate.

[0011] In specific applications, the theoretical error introduced by the laser beam divergence angle into the focal length measurement of the focusing lens improves measurement accuracy. In this invention, the beam divergence angle is determined according to the laser's technical specifications, and simulation is performed based on this beam divergence angle. A curve showing the error versus focal length (the error / focal length curve) is fitted. The measured value of the focal plane position of the focusing lens is then substituted into the error / focal length curve to calculate the theoretical error introduced by the laser divergence angle. Subtracting this theoretical error from the measured value accurately determines the actual focal plane position of the focusing lens, eliminating the theoretical error caused by the laser beam divergence angle and significantly improving measurement accuracy.

[0012] Furthermore, the fast axis direction of the quarter-wave plate is set to 45°, and the phase delay is set to 90°. By adjusting the fast axis direction of the quarter-wave plate located in front of the aperture stop, the light beam reflected back by the reflector is converted into S-polarized light after passing through the quarter-wave plate again, and then reflected by the second polarization beam splitter prism, and finally detected by the power meter detector. The power meter detector is located in the non-focus area to avoid the risk of damage to the CCD target surface.

[0013] Furthermore, the aperture of the aperture stop is 1 mm smaller than the diameter of the incident light spot, and the position parameter of the aperture stop is set to 3 cm in front of the measured focusing lens.

[0014] Furthermore, the first polarization beam splitting prism and the second polarization beam splitting prism have high transmittance for P-polarized light and high reflectance for S-polarized light.

[0015] Specifically, the models of the first polarization beam splitter prism and the second polarization beam splitter prism are PBS-JGS1-12.7 / 12.7 / 12.7.

[0016] Furthermore, the half wave plate is rotated, and cooperates with the first polarization beam splitter prism to control the power ratio of the transmitted laser beam.

[0017] By rotating and adjusting the frontmost 1 / 2 wave plate, the polarization direction of the linearly polarized light is rotated. This, in conjunction with the first polarization beam splitter prism at the rear, enables continuous and repeatable adjustment of the power of the light beam transmitted by the first polarization beam splitter prism, thereby controlling the input light power entering the measurement system.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. Without the need for an additional light source, the polarization direction of the linearly polarized light can be rotated by adjusting the front half-wave plate. This, in conjunction with the first polarizing beamsplitter prism at the rear, changes the power ratio of the beam transmitted / reflected by the first polarizing beamsplitter prism. This allows for continuous and repeatable adjustment of the power of the beam transmitted by the first polarizing beamsplitter prism, thereby controlling the input optical power to the measurement system and effectively preventing damage to optical components due to excessive power.

[0019] 2. By building an optical path system including a 1 / 2 wave plate, a polarization beam splitter prism, a 1 / 4 wave plate, an aperture stop and a reflector, and utilizing the principle of polarization state modulation, the reflected light is converted into S-polarized light and guided to the power meter detector for power detection due to the reflection of the second polarization beam splitter prism. The detection end is located in the non-focused area, avoiding the risk of CCD target surface damage. This fundamentally avoids the problem that the off-axis imaging method is difficult to apply to high power density scenarios due to the limitation of the CCD target surface damage threshold.

[0020] 3. During specific experiments, it was found that when the reflector was outside the focal length range of the measured focusing lens, the total optical power received by the power meter detector rapidly decayed as the reflector position moved away from the focal point. When the reflector was within the focal length range of the measured focusing lens, the total optical power received by the power meter detector did not significantly decay with changes in the reflector position. Based on this principle, by moving the reflector back and forth along the optical axis and synchronously recording the distance (position) between the reflector and the focusing lens and the optical power measured by the power meter detector at that position, a power-position relationship curve was plotted. The critical position on the curve where the power decayed rapidly was identified. This position corresponds to the measurement position of the focal point of the measured focusing lens.

[0021] 4. In the present invention, the beam divergence angle is determined according to the technical specifications of the laser, and simulation is performed based on the beam divergence angle. A curve showing the error varying with focal length (error / focal length curve) is obtained by fitting. The measured value of the focal plane position of the measured focusing lens is then substituted into the error / focal length curve, and the theoretical error value introduced by the laser divergence angle is calculated. The theoretical error value is subtracted from the measured value to accurately determine the actual focal plane position of the focusing lens, thereby eliminating the theoretical error caused by the laser beam divergence angle and significantly improving the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 Schematic diagram of the structure of the focal plane detection system of the high-energy laser focusing lens in the embodiment; Figure 2 This is a light path diagram of the focal plane detection system of the high-energy laser focusing lens in the embodiment; Figure 3 is a schematic diagram of an embodiment in which the reflector is located outside the focal length range of the focusing lens to be measured; Figure 4 A schematic diagram of an embodiment in which the reflector is located within the focal length range of the focusing lens to be measured; Figure 5 The power-position curve diagram obtained by experimental detection in the embodiment; Figure 6 Schematic diagram (part) of a simulation model established using Zemax software in the embodiments; Figure 7 : is an error curve diagram of lenses with different focal lengths under the condition that the laser divergence angle is 1.12 mrad in the embodiment; Among them, the specific drawings are marked as follows: Laser 1, lifting device 2, 1 / 2 wave plate 3, wave plate rotating frame 4, first polarization beam splitter prism 5, second polarization beam splitter prism 6, 1 / 4 wave plate 7, aperture stop 8, focusing lens 9, reflector 10, reflector bracket 11, slider 12, linear guide assembly 13, power meter detector 14. DETAILED DESCRIPTION

[0024] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] Example 1 This embodiment discloses a high energy laser focusing lens focal plane detection system, such as Figure 1 As shown in Figure 1, the core optical components of the detection system include a half-wave plate 3, two polarization beam splitting prisms (a first polarization beam splitting prism 5 and a second polarization beam splitting prism 6), a quarter-wave plate 7, an aperture stop 8, a focusing lens 9, and a plane mirror 10. The experimental platform is shown in Figure 1. Figure 1 As shown. A half-wave plate 3, a first polarization beam splitter prism 5, a second polarization beam splitter prism 6, a quarter-wave plate 7, and an aperture stop 8 are sequentially arranged in the optical axis direction in front of the focusing lens 9 to be measured. A reflector 10 with an adjustable axial position is arranged in the optical axis direction behind the focusing lens 9 to be measured. A power meter detector 14 is arranged in the reflected light path of the second polarization beam splitter prism 6.

[0026] The half-wave plate 3 is mounted on a wave plate rotating frame 4. The frame controls its rotation about the optical axis of the optical path, cooperating with the first polarizing beam splitter prism 5 to control the power ratio of the transmitted laser beam. By rotating and adjusting the frontmost half-wave plate 3, the polarization direction of the linearly polarized light is rotated. This, in conjunction with the rearward first polarizing beam splitter prism 5, enables continuous and repeatable adjustment of the power of the light beam transmitted by the first polarizing beam splitter prism 5, thereby controlling the input optical power entering the measurement system.

[0027] The reflector 10 is mounted on a reflector bracket 11. The bottom of the reflector bracket 11 is mounted on a linear guide assembly 13 via a slider 12. The linear guide assembly 13 is arranged along the optical axis of the optical path. During operation, the slider 12 moves linearly along the linear guide assembly 13, thereby driving the reflector 10 along the optical axis of the optical path, changing the distance between the reflector 10 and the focusing lens 9 to be measured.

[0028] The laser 1 is mounted on the lifting device 2, and the high-intensity laser beam emitted by the laser 1 is adjusted to keep the same optical axis center as the subsequent modulation optical path.

[0029] like Figure 2 As shown, the high-energy laser emitted by the laser 1 is first modulated into elliptically polarized light by the 1 / 2 wave plate 3, and then transmitted through the first polarization beam splitter prism 5 to separate the P polarized light. Subsequently, the transmitted P polarized light is converted into elliptically polarized light by the second polarization beam splitter prism 6 and the 1 / 4 wave plate 7, changing the polarization characteristics of the laser. After the stray light in the reflected light is filtered out by the aperture diaphragm 8, the laser beam is converged by the focusing lens 9 to be measured and propagated to the focal position. The reflector 10 placed after the focusing lens 9 is used to reflect the converged laser beam back along the original path. Going back, the reflector 10 reflects the light beam back along the original path. The light beam reflected by the reflector 10 passes through the focusing lens 9, the aperture diaphragm 8 and the 1 / 4 wave plate 7 in sequence (after passing through the 1 / 4 wave plate 7 again, the elliptically polarized light becomes S polarized light). Due to the characteristics of the polarization beam splitter prism (high transmittance for P polarized light and high reflection for S polarized light), the linear polarized light S light that is orthogonally polarized to the linear polarized light P light will be reflected by the second polarization beam splitter prism 6 and become S parallel light to guide to the power meter detector 14, and the power meter detector 14 records the power value of the reflected light beam.

[0030] The principle of polarization modulation is as follows: After the incident light passes through the 1 / 2 wave plate 3 and the first polarization beam splitter prism 5, the incident light becomes linearly polarized light P propagating along the z axis (optical axis), and its Jones vector is .

[0031] The P-polarized light is converted into S-polarized light after passing through the quarter-wave plate 7 twice, whose fast axis is at an angle of 45° to the horizontal direction. The Jones matrix of the output light is: .

[0032] Due to the characteristics of the polarization beam splitter prism (high transmission of P-polarized light and high reflection of S-polarized light), the S light with a polarization orthogonal to the P light will be reflected by the second polarization beam splitter prism 6 and directed to the power meter detector 14.

[0033] like Figure 3 As shown in FIG, when the reflector 10 is outside the focal length range of the focusing lens 9 to be measured, the edge light rays diverge significantly after being reflected by the reflector 10 and cannot all pass through the focusing lens 9 again, resulting in a rapid attenuation of the total power of the light spot received by the detector. Figure 4As shown in the figure, when the reflector 10 is within the focal length range of the measured focusing lens 9, the light reflected by the reflector 10 can (all or most of) pass through the focusing lens 9 again, so the total power of the light spot received by the detector remains basically stable (for an ideal lens, the light power theoretically remains unchanged; for a non-ideal lens with aberrations such as spherical aberration, the light power only changes slightly).

[0034] Based on the above principle, by moving the reflector 10 back and forth along the optical axis, and synchronously recording the distance (position) between the reflector 10 and the measured focusing lens 9 and the optical power of the detector corresponding to the position. Figure 5 The power-position relationship curve shown identifies the critical position on the curve where the power rapidly decays, which corresponds to the measurement position of the focus of the focusing lens 9 to be measured.

[0035] In specific applications, the theoretical error in the focal length measurement of focusing lens 9 is introduced by the divergence angle of the laser beam 1. To ensure measurement accuracy, the present invention determines the beam divergence angle of laser 1 according to its technical specifications and performs simulation based on this beam divergence angle. A curve showing the error versus focal length (error / focal length curve) is fitted. The measured value of the focal plane position of focusing lens 9 is then substituted into the error / focal length curve to calculate the theoretical error introduced by the divergence angle of laser 1. Subtracting this theoretical error from the measured value accurately determines the actual focal plane position of focusing lens 9, eliminating the theoretical error caused by the divergence angle of laser 1 and significantly improving measurement accuracy.

[0036] The specific process of obtaining the error / focal length curve is as follows: 1. Optical Component Modeling This system uses Zemax optical design software's non-sequential mode for simulation. Compared to sequential mode, non-sequential mode can accurately track the propagation paths of light on and within optical components, accounting for complex optical phenomena such as light scattering and polarization state changes. This mode is ideal for simulating the propagation characteristics of high-energy laser beams in polarization-modulated detection systems.

[0037] Based on the actual application scenario, the laser wavelength is set to 1064nm, which is widely used in the field of solid-state lasers and has good atmospheric transmission performance. The Gaussian distribution is used to describe the intensity distribution of the laser beam, which conforms to the intensity distribution characteristics of high-energy laser beams under far-field conditions. The mathematical expression is: ; in, is the light intensity at a distance r from the center of the beam, is the center intensity of the beam, is the beam waist radius. At the same time, the initial polarization state is defined as P-polarized light. Using the polarization component library of Zemax software, the polarization splitting characteristics of the polarization beam splitter prism are defined. According to Fresnel's law and thin-film optics theory, a specific film layer structure is set to achieve high transmittance of P-polarized light and high reflectivity of S-polarized light. Using the Jones matrix, a quarter-wave plate 7 is set in Zemax, with the fast axis direction of the wave plate set to 45° and the phase delay set to 90°. The light beam passes through the quarter-wave plate 7 twice, forward and backward, to achieve mutual conversion between P-polarized light and S-polarized light. The aperture and position parameters of the aperture are precisely set (the aperture of aperture 8 is 1 mm smaller than the incident light spot diameter and is located 3 cm in front of the focusing lens 9 to be tested) to simulate its filtering effect on stray light, allowing only light within a specific aperture range to pass. According to the actual design parameters, the properties of the focusing lens 9, such as the curvature radius of 9.786, the aperture of 25.4mm, the material of N-BK7, and the focal length of 100mm, are input into Zemax to construct the geometric and optical model of the focusing lens 9 to simulate the converging effect of the laser beam. The reflectivity of the reflector 10 is set to close to 100% to minimize energy loss. Finally, a power meter detector 14 is set above the second polarization beam splitter prism 6 to detect changes in the system light field. The simulation model of the system design is shown in the figure below. Figure 6 shown.

[0038] 2. Create an error / focal length curve In geometric optics, the ideal lens image satisfies the Gaussian formula , (u is the object distance, v is the image distance, and f is the focal length). However, actual laser light sources have a divergence angle. When a beam enters a lens at this divergence angle, the light is not strictly parallel to the optical axis, resulting in a deviation between the actual focus position and the ideal focal plane. This error stems from the interference of non-parallel incident light on the focused image.

[0039] Assuming the laser divergence angle is ϕ, after passing through an ideal lens with an aperture D and a focal length f, the deviation between the actual image distance v and the ideal image distance f can be derived through the geometric relationship: ; ; Theoretical error rate = ; Theoretical error rate = ; Under an ideal lens with a focal length of 100 mm, the relative deviation introduced when the divergence angle of laser 1 is 1.12 mrad should be about 4%. The error / focal length curve at ϕ=1.12 mrad Figure 7 As shown in the figure, according to Figure 5 Get the measured value of the focus of the focusing lens 9, and put the focal length measurement value into Figure 7 The theoretical error rate is obtained, and the theoretical error value = the measured value × the theoretical error rate. Then the theoretical error value is subtracted from the focal length measurement value to obtain the final accurate measurement value of the focal plane position of the measured focusing lens 9.

[0040] Example 2 This embodiment discloses a specific implementation method of the high-energy laser focusing lens focal plane detection system. The specific implementation steps are as follows: (1) Install in order along the direction of light transmission: 1 / 2 wave plate 3 → first polarization beam splitter prism 5 → second polarization beam splitter prism 6 → 1 / 4 wave plate 7 → aperture stop 8 → focusing lens 9 (nominal back focus: 99 mm) → reflector 10. Place a power meter detector 14 at the end of the reflected light path of the second polarization beam splitter prism 6.

[0041] (2) Adjust the angle of the 1 / 2 wave plate 3 to control the laser power incident on the system.

[0042] (3) Adjust the fast axis direction of the quarter wave plate 7 to 45° to ensure that the reflected light is converted from P-polarized light to S-polarized light after passing through the quarter wave plate 7 twice, and is finally reflected by the second polarization beam splitter prism 6 to the power meter detector 14.

[0043] (4) Data acquisition: Move the reflector 10 along the optical axis to change the distance between it and the focusing lens 9; and simultaneously record the position of the reflector 10 and the reflected light power value tested by the power meter detector 14.

[0044] (5) Draw the power-position curve (e.g. Figure 5 As shown in the figure, in the power-position curve, the critical point where the power rapidly decays is 104.7 mm.

[0045] (6) Error compensation and precise positioning: According to the pre-acquired error / focal length curve (such as Figure 7 As shown in Figure 2), the measured value in step (5) is substituted into the error curve to obtain the theoretical error value due to the divergence angle of laser 1 = 104.7*4.19% = 4.3869 mm.

[0046] (7) Final focal length = measured value − theoretical error = 104.7 - 4.3869 = 100.3131 mm. The error in the measurement result = (100.3131 - 99) / 99 * 100% = 1.33%. This error is mainly due to the spherical aberration of the lens being measured, which is a non-ideal lens.

[0047] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the focal plane of a high-energy laser focusing lens, characterized in that: include (1) Optical path modulation: A half-wave plate, a first polarization beam splitter prism, a second polarization beam splitter prism, a quarter-wave plate, and an aperture stop are sequentially arranged in front of the focusing lens to be measured along the optical axis of the optical path; a reflector with adjustable axial position is arranged behind the focusing lens to be measured along the optical axis of the optical path; and a power meter detector is arranged on the reflected optical path of the second polarization beam splitter prism; (2) Polarization state modulation: The incident high-energy laser is first modulated into elliptically polarized light by a half-wave plate, then separated into P-polarized light by the first polarization beam splitter prism. The P-polarized light is converted into elliptically polarized light by the second polarization beam splitter prism and the quarter-wave plate. After passing through the aperture diaphragm to remove stray light, it is converged by the focusing lens to be measured. The reflector reflects the beam back along the original path. After passing through the quarter-wave plate again, its polarization state becomes S-polarized light. Finally, it is reflected by the second polarization beam splitter prism to the power meter detector, which records the power value of the reflected beam. (3) Data collection: Move the reflector along the optical axis of the optical path to change the distance between the reflector and the focusing lens to be measured, and synchronously record the power value of the light beam reflected by the second polarization beam splitter prism and the position of the reflector; (4) Focal plane calculation: A power-position curve is drawn based on the collected power value of the light beam reflected by the second polarization beam splitter prism and the reflector position data, and the starting position of rapid power decay on the curve is determined. The reflector position coordinates corresponding to the starting position of rapid power decay are recorded. This is the measured value of the focal plane position of the measured focusing lens.

2. The method for detecting the focal plane of a high energy laser focusing lens according to claim 1, wherein: The method further includes step (5) error compensation: pre-generating an error-focal length curve, substituting the measured value of the focal plane position of the focusing lens to be measured into the error-focal length curve to calculate the theoretical error value introduced by the laser divergence angle, and subtracting the theoretical error value from the measured value to obtain the final accurate measurement value of the focal plane position of the focusing lens to be measured.

3. The method for detecting the focal plane of a high energy laser focusing lens according to claim 2, wherein: The error-focal length curve is simulated and fitted using the non-sequential mode of Zemax optical design software.

4. The method for detecting the focal plane of a high energy laser focusing lens according to claim 3, wherein: Substituting the focal length measurement value into the error-focal length curve directly obtains the corresponding theoretical error rate on the curve: theoretical error value = measurement value × theoretical error rate.

5. The method for detecting the focal plane of a high energy laser focusing lens according to claim 1, wherein: The fast axis direction of the quarter wave plate is set to 45°, and the phase delay is set to 90°.

6. The method for detecting the focal plane of a high energy laser focusing lens according to claim 1, wherein: The aperture diaphragm is 1 mm smaller than the incident light spot diameter and is located 3 cm in front of the focusing lens to be measured.

7. The method for detecting the focal plane of a high energy laser focusing lens according to claim 1, wherein: The first polarization beam splitting prism and the second polarization beam splitting prism have high transmittance for P-polarized light and high reflectance for S-polarized light.

8. The method for detecting the focal plane of a high energy laser focusing lens according to claim 1, wherein: The rotating half wave plate cooperates with the first polarization beam splitter prism to control the power ratio of the transmitted laser beam.

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