Anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation

Abstract

The invention relates to the technical field of nonlinear optical measurement, in particular to an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation, which is used for measuring the anisotropic nonlinear refractive index of a material. The system comprises a pulse laser, a variable density attenuation sheet, a polaroid, a beam expanding system, an iris diaphragm, a beam splitter prism group, a spatial light modulator, a precise rotating table, a cylindrical mirror, a three-dimensional translation system, a sample bracket, a sample table, a detector, a computer analysis module and a rotation control system. The innovation of the invention lies in that a spherical lens machine in a traditional 4F system is replaced by a cylindrical mirror, directional focusing scanning is realized in combination with a rotating mechanism, nonlinear refractive indexes of a sample in all directions are quantitatively obtained by analyzing focal spot changes at different angles, selective excitation is only performed on the sample in a single direction, and the nonlinear refractive indexes of the sample in all directions are obtained. The problem of nonlinear response superposition in two orthogonal directions in a traditional spherical focusing system is eliminated.

Description

Technical Field

[0001] This invention relates to the field of nonlinear optical measurement technology, and in particular to an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation. Background Technology

[0002] Nonlinear refractive index is a key third-order nonlinear optical parameter characterizing the change of a material's refractive index with light intensity under a strong optical field. It is widely used in all-optical switches, optical limiters, optical communications, and laser mode locking. For materials with structural anisotropy, such as crystals, two-dimensional materials, and organically oriented thin films, their nonlinear refractive index usually shows significant differences with the crystal axis or molecular orientation. Therefore, accurately measuring the anisotropic nonlinear refractive index distribution is of great significance for material performance evaluation and device design.

[0003] In existing technologies, the most commonly used method for measuring nonlinear refractive index is the Z-scan technique based on a 4f coherent imaging system. This technique uses a pair of spherical lenses to form a 4f system, focusing a Gaussian beam onto the sample, recolliding it, and imaging it onto a detector. The nonlinear refractive index is inverted by recording the changes in the far-field spot as the sample moves near the focal point. Because the spherical lenses have the same focusing capability in both the x and y orthogonal directions, the beam forms an axisymmetric Gaussian focal point at the sample, causing nonlinear phase shifts in both directions to occur simultaneously and superimpose.

[0004] However, in the process of implementing the Z-scan technology based on the 4f spherical lens system, the inventors of this application discovered that the above technology has at least the following technical problems: Since the focused light field always maintains rotational symmetry, it is impossible to selectively excite a single orientation direction of the sample, resulting in the superposition of nonlinear responses in two orthogonal directions. Summary of the Invention

[0005] To overcome the above shortcomings, this invention provides an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation, aiming to improve the problem in the prior art that the focused light field always maintains rotational symmetry, which makes it impossible to selectively excite a single orientation direction of the sample, resulting in the superposition of nonlinear responses in two orthogonal directions.

[0006] In a first aspect, the present invention provides the following technical solution: an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation, the system comprising: a pulsed laser computer control system, a variable density attenuator computer control system, a polarizer computer control system, a beam expander computer control system, a variable aperture stop computer control system, a first beam splitter computer control system, a spatial light modulator computer control system, a second beam splitter computer control system, a first cylindrical mirror computer control system, a second cylindrical mirror computer control system, a sample stage computer control system, a three-dimensional translation system and a sample holder computer control system, a first photoelectric image sensor computer control system, a second photoelectric image sensor computer control system, a computer control system, a rotation control system computer control system, a first precision rotating stage computer control system, and a second precision rotating stage computer control system. The laser emitted by the pulsed laser computer control system sequentially passes through the variable density attenuator computer control system, the polarizer computer control system, the beam expander computer control system, and the variable aperture diaphragm computer control system before entering the first beam splitter computer control system. It then enters the spatial light modulator computer control system for wavefront modulation. The modulated light is reflected back to the first beam splitter computer control system and enters the second beam splitter computer control system. The second beam splitter computer control system splits the beam into a reference beam and a measurement beam. The reference beam enters the second photoelectric image sensor computer control system, while the measurement beam sequentially passes through the first cylindrical mirror computer control system for focusing, the sample placed on the sample stage computer control system, and the second cylindrical mirror computer control system for collimation before entering the first photoelectric image sensor computer control system. The first cylindrical mirror computer control system is mounted on the first precision rotary stage computer control system, and the second cylindrical mirror computer control system is mounted on the second precision rotary stage computer control system. The rotation axes of the first and second precision rotary stage computer control systems are coaxial with the main optical axis of the system and are synchronously driven by the same rotation control system computer control system. This allows the first and second cylindrical mirror computer control systems to rotate synchronously in the same direction at equal angles. Thus, while maintaining the focal position and optical axis alignment, only the direction of the focal line is changed, enabling unidirectional selective nonlinear excitation and anisotropic nonlinear refractive index measurement of the sample under test in different orientation directions.

[0007] Preferably, the first cylindrical mirror computer control system and the second cylindrical mirror computer control system have the same focal length and the same parameters, forming a symmetrical 4f imaging system. The optical distance between the two is equal to twice the focal length, so that the object-side focal points of the two are precisely coincided with the position of the sample under test in the sample stage computer control system. At any rotation angle, the focal point position remains fixed by synchronous rotation through the rotation control system and the computer control system.

[0008] Preferably, the computer control system of the rotation control system includes a main control unit, a dual-channel servo drive module, a high-resolution optical encoder, and a closed-loop feedback circuit. The rotation angle synchronization error between the computer control system of the first precision rotary table and the computer control system of the second precision rotary table is less than 0.002°. The computer control system sends angle commands to the computer control system of the rotation control system through an interface and receives angle feedback in real time, achieving automatic closed-loop angle scanning within the range of 0° to 180°.

[0009] Preferably, the computer control system of the spatial light modulator is a reflective pure phase spatial light modulator, in which the computer control system loads the phase pattern in real time and applies a programmable wavefront phase distribution to the incident beam to generate a linearly focused, elliptical focused, or arbitrarily asymmetric focused light field.

[0010] Preferably, the three-dimensional translation system and sample holder computer control system include an X, Y, Z three-axis electric translation stage and a high-precision sample rotation stage, which are uniformly controlled by the computer control system. The system can select either the rotation mode of the first cylindrical mirror computer control system or the second cylindrical mirror computer control system, or the rotation mode of the sample, to complete the angle scanning measurement.

[0011] Preferably, the computer control system is connected to the computer control systems of the pulsed laser, the spatial light modulator, the first photoelectric image sensor, the second photoelectric image sensor, the rotation control system, the three-dimensional translation system, and the sample holder, and executes the following fully automatic measurement process: Phase diagram of the computer control system for controlling laser power and spatial light modulator Synchronous drive of the first and second precision rotary stage computer control systems to change the focus line direction Simultaneously acquire and normalize reference and probe spot images at each set angle. Automatically fit the changes in focal spot half-width or light intensity distribution Calculate the nonlinear refractive index value corresponding to this angle. Generate a polar coordinate distribution plot and output the ratio of principal axis direction to anisotropy.

[0012] Preferably, the curvature axis directions of the first cylindrical mirror computer control system and the second cylindrical mirror computer control system are always kept strictly parallel or antiparallel through synchronous control of the rotation control system computer control system, ensuring that the 4f imaging relationship remains unchanged throughout the entire rotation process, with only the focus line direction rotating, without introducing additional astigmatism or off-axis errors.

[0013] Preferably, the sample stage computer control system integrates an electronically controlled temperature control module, which is connected in a closed loop with the computer control system.

[0014] Preferably, the system is built using a cage-type optical structure or a vibration-isolated optical platform, and the coaxial accuracy of all optical components and the computer control systems of the first and second precision rotary stages is better than 20μm, ensuring that the optical axis drift of the system is less than 5μm during the full-angle rotation of the computer control systems of the first and second cylindrical mirrors from 0° to 180°.

[0015] Secondly, the present invention provides the following technical solution: an anisotropic nonlinear refractive index measurement method, comprising the following steps: S1. System initialization: Calibrate the optical path in the low-power linear region and record the reference spot and probe spot. S2. Increase the laser power to the nonlinear response range; S3. Computer control system: The computer control system controls the rotation control system. The computer control system synchronously drives the first precision rotary table computer control system and the second precision rotary table computer control system to rotate the first cylindrical mirror computer control system and the second cylindrical mirror computer control system in steps according to the set angle. S4. Simultaneously acquire and normalize the reference spot and the detection spot images of the second photoelectric image sensor computer control system and the first photoelectric image sensor computer control system at each angle. S5. The computer control system automatically fits the changes in the focal spot half-width or light intensity distribution, and calculates the nonlinear refractive index value corresponding to the current focal line direction. S6. After completing the 0° to 180° scan, generate a nonlinear refractive index polar coordinate distribution map and output the principal axis direction and anisotropy ratio parameters.

[0016] The present invention has the following beneficial effects: 1. In this invention, by replacing the spherical lens in the traditional 4f system with a first cylindrical lens and a second cylindrical lens, and mounting them on a first precision rotary stage and a second precision rotary stage driven synchronously by the same rotation control system, the focusing line direction can be precisely adjusted within the range of 0° to 180°. Under the premise of keeping the focal position and optical axis completely unchanged, selective excitation is performed only on the sample in a single direction, eliminating the problem of superposition of nonlinear responses in two orthogonal directions in the traditional spherical focusing system.

[0017] 2. In this invention, a reflective pure phase spatial light modulator is introduced, and a programmable phase pattern is loaded by a computer in real time. This can generate a linearly focused, elliptical focused, or arbitrarily asymmetric focused light field. Combined with the rotation of the cylindrical mirror, the light field distribution is further optimized, aberrations are compensated, and the system’s detection sensitivity to weak anisotropic signals is greatly improved, achieving high-precision measurement.

[0018] 3. This invention employs a dual-channel reference detection structure, closed-loop synchronous control of the rotation control system, and fully automated computer process control, achieving fully automated operation of laser power, phase pattern, angle scanning, image acquisition, normalization, and data fitting. This greatly reduces human error, improves measurement repeatability and efficiency, and features a simple structure built entirely on a mature cage-type optical platform, facilitating rapid laboratory construction and functional expansion. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation proposed in this invention. Figure 2 The anisotropic nonlinear phase change curve of the anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation proposed in this invention is shown. Figure 3 This is a diagram showing the nonlinear refractive index distribution in different directions of an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation proposed in this invention.

[0020] Wherein, S01: pulsed laser, S02: variable density attenuator, S03: polarizer, S04: beam expander, S05: variable aperture stop, S06: first beam splitter, S07: spatial light modulator, S08: second beam splitter, S09: first cylindrical mirror, S10: three-dimensional translation system and sample holder, S11: sample stage, S12: second cylindrical mirror, S13: first photoelectric image sensor, S14: second photoelectric image sensor, S15: computer control system, S16: rotation control system, R01: first precision rotary stage, R02: second precision rotary stage. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] In an embodiment of the present invention, the excitation light generated by the pulsed laser S01 is adjusted in energy by a variable density attenuator S02 and then polarized by a polarizer S03. A uniform beam is selected by a beam expander S04 and a variable aperture S05 before entering the first beam splitter S06. After reflection and modulation by a spatial light modulator S07, the beam is split into two beams by a second beam splitter S08. One beam serves as a reference beam entering the reference optical path, while the other beam is focused by a first cylindrical mirror S09 and then illuminates the nonlinear material sample on the sample stage S11.

[0023] The rotating cylindrical mirror assembly S10 is mounted on an electrically driven rotating stage, and the optical field direction is scanned by controlling the angle. The sample-emitted light is collimated by the second cylindrical mirror S12 and enters the detection optical path. The first photoelectric image sensor S13 acquires the image of the detection spot, while the second photoelectric image sensor S14 acquires the reference spot to achieve light intensity normalization correction. The computer S15 performs data analysis and nonlinear refractive index inversion on the acquired images, and outputs nonlinear refractive index parameters in different directions.

[0024] This invention provides an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation. The system utilizes a cage-like structure to construct the device, which includes a laser optical path channel, a reference optical path channel, and a probe optical path channel. Main components: S01–S07 (pulsed laser, variable density attenuator, polarizer, beam expander, variable aperture stop, first beam splitter, spatial light modulator) The pulsed laser S01 is fixed to one end of the optical platform and securely connected to the platform via a mechanical support. The laser output end is connected to a neutral density attenuator S02. The neutral density attenuator S02 can continuously adjust the transmitted light energy through a rotation mechanism to prevent damage to subsequent components due to high energy. The polarizer S03 is used to control the polarization direction of the incident light, and the beam expander system S04 consists of two sets of lenses to achieve the expansion and collimation of the laser beam.

[0025] The expanded beam is limited by a variable aperture S05 and enters the first beam splitter S06. S06 splits the laser into two paths: one beam is transmitted into the spatial light modulator S07; the other beam is reflected as a backup monitoring beam. The spatial light modulator S07 is a pure phase modulator, capable of modulating the incident light wavefront by 0–2π phase. Its modulation information is controlled by a computer S15, enabling dynamic and programmable adjustment of the beam phase distribution.

[0026] The light modulated by the SLM returns to the first beam splitter S06, where it is combined with the reflected light and continues to propagate downwards to the second beam splitter S08. S08 splits the beam again into a reference beam and a measurement beam: the reference beam enters the photoelectric sensor S14 to monitor the laser output energy in real time and feeds it back to the computer S15 for intensity correction; the measurement beam propagates along the main optical path and is focused sequentially by the first cylindrical mirror S09 onto the nonlinear material sample to be tested on the sample stage S11.

[0027] The first cylindrical mirror S09 is mounted on a cage-type mounting plate, which is fixed directly below the cage-type cube on which the second beam splitter S08 is mounted. The two are connected by a cage-type assembly support rod. By adjusting the length of the support rod, the focal length of the microscope objective can be precisely adjusted so that its front focus falls on the sample stage.

[0028] The sample holder S10 and sample stage S11 are located at the center of the system. The sample holder S10 can rotate around the vertical axis to achieve measurements at different crystal orientation angles. The sample stage S11 is mounted on a three-dimensional translation stage and is electrically controlled to achieve micro-displacement adjustment in the x, y, and z directions to ensure that the optical axis is aligned with the center region of the sample.

[0029] After the pulsed laser S01 emits laser light, it is expanded and collimated by the beam expansion system S04, and then attenuated by the neutral density attenuator. The attenuation value of the polarizer ensures that other components are not damaged by the beam energy. The two-dimensional object defined after entering the polarizer is in the normal direction. The linearly polarized monochromatic plane wave at the incident point is: Formula 1 In Equation 1: ω is the angular frequency of the light wave; k is the wave vector; E0 is the amplitude of the light field; τ is the laser pulse width. In Equations 1 to 7, x, y, and z are spatial coordinates, the z-axis is the optical axis, t is the time parameter, and i is the imaginary unit.

[0030] The light beam enters the spatial light modulator S07, allowing arbitrary adjustment of the phase distribution of the incident beam between 0 and 2π. The modulated excitation light can be represented as: Equation 2 In Equation 2: Ra and Lp represent the spot radius and the phase-change spot radius of the central part, respectively; is the phase delay of the phase-change spot in the central part; circ() is a circular function with a value of 1 inside the circle and a value of 0 outside the circle. This indicates the phase distribution.

[0031] After passing through the first beam splitter S06, one beam enters the main optical path. After passing through the second cubic beam splitter S08, the laser beam is split into two beams: one is a reference beam entering the reference optical path channel, and the other is a probe beam. The probe beam sequentially passes through the first cylindrical mirror S09, the three-dimensional translation system and sample holder S10, the sample stage S11, and the second cylindrical mirror S12 before entering the probe optical path to acquire images from the first photoelectric sensor S13. A 4f system is formed by two identical cylindrical mirrors. By adjusting the distance between the two cylindrical mirrors and setting their object-side focal points to overlap and lie within the nonlinear material to be measured, the electric field distribution on the spectral plane of the 4f system is as follows: Formula 3 In Equation 3: FT is the symbol for Fourier transform; f1 is the focal length of the objective lens; λ is the wavelength of the incident laser. This represents the complex amplitude distribution of the light field on the spectral plane (i.e., the focal plane of the lens). This represents the complex amplitude distribution of the incident light field on the object plane; The kernel function represents the Fourier transform; u and v are the spatial coordinates at the focal plane. The change in local refractive index is caused by the phase rotation Kerr effect. With strength Proportional: ; It's an angle. The dependent anisotropic nonlinear coefficients (given in tensor form later). The corresponding phase delay: ; Parameter Examples and Numerical Explanations For anisotropic lossless Kerr media, its nonlinear refractive index Angle with the polarization direction of the incident light There is a periodic relationship. The nonlinear coefficients along the principal axis are defined as follows: , Then the equivalent nonlinear refractive index in any direction can be expressed as: Formula 4 Simplifying it to biangle form, we get: Formula 5 The first term represents the average nonlinear response, and the second term is the anisotropic correction term, reflecting the principal axis symmetry of the material.

[0032] In the experiment, to account for principal axis offset or non-ideal rotation effects, a second harmonic expansion can be further adopted: Formula 6 in, For average components, , These correspond to the double-angle amplitude and phase offset, respectively, reflecting the rotational asymmetry of the sample. By fitting the experimental data, the principal axis orientation angle and anisotropy intensity can be obtained: Formula 7 Combining the above equations allows for anisotropic nonlinear refractive index measurement of the sample. The probe light source is a pulsed laser with a pulse width of 21 ps (FWHM) and a wavelength of 532 nm. The spatial light modulator is a pure phase-type spatial light modulator with a phase modulation range of 0-2π and a pixel resolution of 1920*1080.

[0033] Two cylindrical mirrors are made of fused silica with a focal length of 75 mm and a radius of curvature of 150 mm. They are arranged symmetrically along the optical axis to form a 4f system, enabling uniaxial focusing and recollimation of the light field. By controlling the rotation angle θ of the first cylindrical mirror with a stepper motor, the focusing direction of the incident light on the sample surface can be continuously adjusted, thereby enabling quantitative measurement of the nonlinear refractive index of the material under different crystal orientations.

[0034] In a preferred embodiment of the present invention, a numerical simulation of an anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation was performed using MATLAB. The simulation used λ=800nm, sample thickness L=1mm, and incident light intensity I0=10. 9 W / m 2 Assume the nonlinear refractive index of the sample along the x and y directions are respectively and m 2 Simulation results show that when the rotation angle θ of the cylindrical mirror increases from 0° to 180°, the nonlinear phase change Δφ(θ) exhibits a typical biaxial elliptical distribution, with the phase change being the largest in the direction of the principal crystal axis. This indicates that the system can accurately distinguish the nonlinear refractive response of the material in different directions.

[0035] Figure 2 The anisotropic nonlinear phase change curves reflect the nonlinear refractive index response of the material under different cylindrical mirror rotation angles. The elliptical distribution of the curves clearly shows the biaxial anisotropy of the sample. Furthermore, as... Figure 3 The figure shows the nonlinear refractive index distribution curves of the sample in different directions θ. In the experiment, a pulsed laser with a center wavelength of 800 nm was used as the incident light source, which was focused onto the center of the sample after beam expansion and spatial light modulation. The sample is a nonlinear crystal with anisotropic optical response, and its third-order nonlinear refractive index coefficients in the x-axis and y-axis directions are as follows: By changing the detection angle θ (0°, 30°, 60°, and 90° respectively), the equivalent nonlinear refractive index change in each direction was measured. ,from Figure 3As can be seen, the refractive index distribution in all directions exhibits a Gaussian shape, reaching its maximum value at the center, indicating the modulation effect of the laser intensity distribution in the sample. However, as the angle θ increases, the peak value of the curve gradually decreases, indicating that the equivalent nonlinear refractive index weakens with the change of angle, reflecting the anisotropic nonlinear response characteristics of the material.

[0036] The refractive index changes most significantly at the θ=0° direction (red line), approximately [value missing]. ; In the directions θ=30° (blue line) and 60° (green line), The peak value gradually decreases; In the direction of θ=90° (purple line), Minimum, approximately .

[0037] This result indicates that the material exhibits the strongest nonlinear optical effect along the principal axis, while its response is weaker in the perpendicular direction. By comparing the refractive index changes in different directions, the ratio of the material's nonlinear anisotropy parameters can be quantitatively calculated.

[0038] In summary, this experiment verifies that the anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation can accurately distinguish the nonlinear response of the sample in different directions, providing an effective means for the characteristic analysis of anisotropic nonlinear optical materials.

[0039] This invention proposes an anisotropic nonlinear refractive index measurement device and method based on spatial light modulation and a 4f system structure. The system uses a pulsed laser as the light source and a 4f system constructed with cylindrical lenses to achieve precise control of the spatial frequency distribution and phase information of the incident light on the spectral plane. By loading a programmable phase diagram onto the spatial light modulator (SLM), dynamic adjustment of the incident light polarization direction and wavefront distribution is achieved, enabling nonlinear response measurements in different directions within the same device. The advantages of this embodiment are: detailed frequency and time domain models make the inversion more physically self-consistent; polarization / intensity dual constraints improve parameter discriminability; multi-angle scanning and regularized inversion can stably solve for anisotropic tensor components in noisy environments; and the wavefront programmability provided by the SLM optimizes signal-to-noise ratio and extends to spatially resolved phase modulation strategies.

[0040] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation, characterized in that, The system includes: a pulsed laser, a variable density attenuator, a polarizer, a beam expander, a variable aperture stop, a first beam splitter, a spatial light modulator, a second beam splitter, a first cylindrical mirror, a second cylindrical mirror, a sample stage, a three-dimensional translation system and a sample holder, a first photoelectric image sensor, a second photoelectric image sensor, a computer control system, a rotation control system, a first precision rotary stage and a second precision rotary stage; The laser beam emitted by the pulsed laser passes sequentially through a variable density attenuator, a polarizer, a beam expander, and a variable aperture stop before entering the first beam splitter. It then enters the spatial light modulator for wavefront modulation. The modulated light is reflected back to the first beam splitter and enters the second beam splitter. The second beam splitter splits the beam into a reference beam and a measurement beam. The reference beam enters the second photoelectric image sensor, while the measurement beam passes sequentially through a first cylindrical mirror for focusing, the sample placed on the sample stage, and a second cylindrical mirror for collimation before entering the first photoelectric image sensor. The first cylindrical mirror is mounted on the first precision rotating stage, and the second cylindrical mirror is mounted on the second precision rotating stage. The rotation axes of the first and second precision rotating stages are coaxial with the main optical axis of the system and are synchronously driven by the same rotation control system, so that the first and second cylindrical mirrors rotate synchronously in the same direction and at the same angle. Thus, while keeping the focal position and optical axis alignment unchanged, only the direction of the focal line is changed, thereby realizing the unidirectional selective nonlinear excitation and anisotropic nonlinear refractive index measurement of the sample under test in different orientation directions.

2. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The first cylindrical mirror and the second cylindrical mirror have the same focal length and the same parameters, forming a symmetrical 4f imaging system. The optical distance between them is equal to twice the focal length, so that the object-side focal points of the two mirrors are precisely coincident at the position of the sample on the sample stage. At any rotation angle, the focal point position remains fixed by synchronous rotation through the rotation control system.

3. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The rotation control system includes a main control unit, a dual-channel servo drive module, a high-resolution optical encoder, and a closed-loop feedback circuit. The synchronization error of the rotation angle between the first precision rotary table and the second precision rotary table is less than 0.002°. The computer control system sends angle commands to the rotation control system through an interface and receives angle feedback in real time, achieving automatic closed-loop angle scanning within the range of 0° to 180°.

4. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The spatial light modulator is a reflective pure phase spatial light modulator. The phase pattern is loaded in real time by the computer control system, and a programmable wavefront phase distribution is applied to the incident beam to generate a linearly focused, elliptical focused, or arbitrarily asymmetric focused light field.

5. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The three-dimensional translation system and sample holder include an X, Y, and Z three-axis electric translation stage and a high-precision sample rotation stage, which are uniformly controlled by a computer control system. It can select any mode, such as rotating the first cylindrical mirror and the second cylindrical mirror or rotating the sample, to complete the angle scanning measurement.

6. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The computer control system is connected to the pulsed laser, spatial light modulator, first photoelectric image sensor, second photoelectric image sensor, rotation control system, three-dimensional translation system, and sample holder, and executes the following fully automated measurement process: Controlling laser power and spatial light modulator phase pattern Synchronously drive the first and second precision rotary stages to change the direction of the focusing line. Simultaneously acquire and normalize reference and probe spot images at each set angle. Automatically fit the changes in focal spot half-width or light intensity distribution Calculate the nonlinear refractive index value corresponding to this angle. Generate a polar coordinate distribution plot and output the ratio of principal axis direction to anisotropy.

7. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The curvature axes of the first and second cylindrical mirrors are synchronously controlled by the rotation control system to always maintain strict parallelism or antiparallelism, ensuring that the 4f imaging relationship remains unchanged throughout the entire rotation process, with only the focus line direction rotating, without introducing additional astigmatism or off-axis errors.

8. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The sample stage integrates an electronically controlled temperature control module, which is connected in a closed loop to the computer control system.

9. The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation according to claim 1, characterized in that, The system is built using a cage-type optical structure or a vibration-isolated optical platform. The coaxial accuracy of all optical components and the first and second precision rotary stages is better than 20μm, ensuring that the optical axis drift of the system is less than 5μm during the full-angle rotation of the first and second cylindrical mirrors from 0° to 180°.

10. A method for measuring anisotropic nonlinear refractive index, characterized in that, The anisotropic nonlinear refractive index measurement system based on cylindrical mirror rotation, as described in any one of claims 1-9, comprises the following steps: S1. System initialization: Calibrate the optical path in the low-power linear region and record the reference spot and probe spot. S2. Increase the laser power to the nonlinear response range; S3. The computer control system controls the rotation control system to synchronously drive the first precision rotary table and the second precision rotary table, and rotate the first cylindrical mirror and the second cylindrical mirror step by step according to the set angle. S4. Simultaneously acquire and normalize the reference spot image of the second photoelectric image sensor and the detection spot image of the first photoelectric image sensor at each angle; S5. The computer control system automatically fits the changes in the focal spot half-width or light intensity distribution, and calculates the nonlinear refractive index value corresponding to the current focal line direction. S6. After completing the 0° to 180° scan, generate a nonlinear refractive index polar coordinate distribution map and output the principal axis direction and anisotropy ratio parameters.

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