Free-form surface non-zero interference detection device and method based on thin film deformable mirror wavefront compensation

Abstract

The invention discloses a free-form surface non-zero interference detection device and method based on thin film deformable mirror wavefront compensation. The device comprises a laser source, a polarization regulation and control module, a film deformation mirror surface type control and monitoring module, an interference measurement module and a data processing and control module. The control and monitoring module performs closed-loop adjustment on the deformed mirror surface type to realize high-precision wavefront compensation; in the interference measurement module, reference light is generated by laser collimation, measurement light forms interference with the reference light after being modulated by a deformable mirror and a free-form surface to be measured, four frames of phase-shifting interferograms with 45-degree phase difference are extracted, and profile data after compensation are obtained by combining an algorithm; and carrying out point-to-point subtraction on the data and deformed mirror surface type data to obtain the surface type of the free-form surface to be detected, and completing high-precision detection. The problems that interference fringes are low in contrast ratio and cannot be distinguished due to aberration in free-form surface detection are effectively solved, the large-curvature and large-inclination free-form surface detection precision and efficiency are improved, and the method is suitable for high-precision free-form surface optical element detection scenes.

Description

Technical Field

[0001] This invention belongs to the field of optical surface shape measurement technology, specifically relating to a device and method for detecting non-zero position interferometry of freeform surfaces based on wavefront compensation of thin film deformable mirrors. Background Technology

[0002] In the field of optical surface measurement technology, high-precision detection of freeform surfaces relies heavily on wavefront compensation technology. Freeform surfaces have complex shapes and large slope variations. During the measurement process, interference factors such as system aberrations and environmental vibrations can easily lead to problems such as decreased contrast of interference fringes and phase ambiguity, which seriously affect measurement accuracy. Wavefront compensation technology has become a key factor determining the accuracy and stability of the entire measurement system.

[0003] Currently, the mainstream wavefront compensation techniques mainly include three categories: computational holography, spatial light modulator compensation, and deformable mirror compensation. Computational holography generates a fixed compensation wavefront by pre-setting a holographic pattern, but it is only suitable for low-dynamic compensation scenarios with static surfaces. Because its compensation mode is fixed, it cannot adapt to the randomly changing aberrations in freeform surface measurements in real time, resulting in insufficient flexibility. Spatial light modulators rely on small-unit pixel modulation to achieve wavefront correction, but their pixel structure easily introduces stray light, and their response speed is limited by the frame rate, making it difficult to cope with high-frequency dynamic aberrations. This significantly restricts their accuracy in large field-of-view freeform surface measurements.

[0004] Deformable mirrors are mechanical correction devices that directly act on the light wavefront. They have significant advantages in dynamic wavefront compensation for freeform surface measurements. Through the synergistic action of drive units, they can achieve continuous surface shape adjustment and avoid light field interference caused by small unit pixel structures. Their millisecond-level response speed can track high-frequency dynamic aberrations in real time. Their large dynamic deformation range can meet the requirements of large slope changes in freeform surfaces, making them particularly suitable for complex scenarios of high-frequency, long-stroke wavefront correction.

[0005] However, deformable mirrors face key technical bottlenecks in practical applications: On the one hand, during open-loop control, the actual deformable mirror surface shape deviates from the theoretical value due to factors such as driving errors and material stress release, leading to overcompensation or undercompensation of local surface shape. Moreover, it is impossible to monitor the dynamic surface shape changes of the deformable mirror in real time, and open-loop control cannot correct the compensation deviation in a timely manner. On the other hand, during closed-loop control, parameter optimization relies on trial-and-error experience, the actuator saturation problem is not effectively solved, and the robustness to measurement noise and model uncertainty is poor, making it difficult to adapt to the high-precision thin-film deformable mirror control problem in complex optical scenarios. On the other hand, traditional interferometry methods typically use mechanical phase shifting or frequency-modulated phase shifting when using phase-shifting interferometry. Mechanical phase shifting is easily affected by mechanical vibration, transmission gaps, and nonlinear hysteresis effects, making it difficult to achieve nanometer-level high-precision phase stepping. Especially in phase-shifting scenarios with a large dynamic range, accumulated mechanical errors will directly lead to phase extraction deviations. Frequency-modulated phase shifting, on the other hand, requires extremely high stability of the light source, and the linearity and repeatability of wavelength modulation are difficult to guarantee. It is also easily affected by changes in ambient temperature and air pressure, resulting in uneven phase shift step size, which affects the accuracy of phase calculation. It is difficult to achieve this, and the cumulative error can reduce the compensation effect, thus affecting the surface shape detection accuracy. Summary of the Invention

[0006] This invention aims to address the core problems of thin-film deformable mirror wavefront compensation technology in freeform surface interferometry, namely, insufficient wavefront compensation accuracy leading to local overcompensation or undercompensation, failing to accurately match the complex wavefront requirements of freeform surfaces; slow dynamic response speed and lag in compensation action affecting the timeliness of wavefront compensation; and difficulties in implementing phase-shifting interferometry, resulting in low phase-shifting accuracy and error accumulation, leading to phase calculation distortion. This invention proposes a device and method for detecting non-zero positions of freeform surfaces based on thin-film deformable mirror wavefront compensation.

[0007] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0008] A free-form surface non-zero position interferometric detection device based on thin-film deformable mirror wavefront compensation includes: a laser, a first lens along the laser emission direction, a first half-wave plate, and a first polarizing beam splitter.

[0009] The first polarizing beam splitter splits the incident light into reflected light and transmitted light, wherein the reflected light is s-polarized and the transmitted light is p-polarized;

[0010] The light reflected by the first polarizing beam splitter is incident on the thin-film deformable mirror after passing through the first quarter-wave plate. After being reflected by the thin-film deformable mirror, it returns along the original path and passes through the first quarter-wave plate, the first polarizing beam splitter, the second lens, the third lens, the third half-wave plate, and the second polarizing beam splitter in sequence.

[0011] The second polarizing beam splitter splits the incident light into reflected light and transmitted light. The transmitted light is incident on the SH wavefront sensor as the feedback wavefront signal for the thin-film deformable mirror closed-loop control. The reflected light passes sequentially through the second quarter-wave plate, is incident on the freeform surface, is reflected by the freeform surface, returns along the original path, and sequentially passes through the second quarter-wave plate, the second polarizing beam splitter, the third polarizing beam splitter, and after being transmitted through the third polarizing beam splitter, it passes through the third quarter-wave plate and enters the polarizing camera as the measurement light.

[0012] The transmitted light from the first polarizing beam splitter passes sequentially through the second half-wave plate and the fourth polarizing beam splitter. The reflected light from the fourth polarizing beam splitter passes sequentially through the third polarizing beam splitter and the third quarter-wave plate, serving as reference light.

[0013] The SH wavefront sensor, polarization camera, and thin-film deformable mirror are respectively connected to the data processing and PC controller.

[0014] Furthermore, the detection device includes: a polarization control module, a thin film deformation mirror type control and monitoring module, an interferometric measurement module, and a data processing and PC controller module;

[0015] The polarization control module includes a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a fourth polarization beam splitter, a first half-wave plate, a second half-wave plate, a third half-wave plate, a first quarter-wave plate, a second quarter-wave plate, and a third quarter-wave plate, which are used to modulate and separate the polarization state of the laser.

[0016] The thin-film deformable mirror surface shape control and monitoring module includes a relay mirror system composed of a first polarizing beam splitter, a second polarizing beam splitter, a second lens, and a third lens, and an SH wavefront sensor. The reflected light from the thin-film deformable mirror transmitted through the first polarizing beam splitter enters the relay mirror system and then enters the second polarizing beam splitter. The transmitted light from the second polarizing beam splitter is incident on the SH wavefront sensor, which is used to detect the wavefront information of the thin-film deformable mirror in real time and transmit it to the data processing and PC controller. The data processing and PC controller uses the surface shape result of this monitoring to give a driving voltage to perform high-precision closed-loop control of the surface shape of the thin-film deformable mirror, thereby completing the control and monitoring of the surface shape of the thin-film deformable mirror.

[0017] The interferometric measurement module consists of: a second polarizing beam splitter receiving light from a relay mirror; the reflected light passing sequentially through a second quarter-wave plate, incident on a freeform surface, reflected by the freeform surface, returning along the original path, passing sequentially through the second quarter-wave plate, the second polarizing beam splitter, the third polarizing beam splitter, and after transmission through the third polarizing beam splitter, passing through the third quarter-wave plate to serve as the measurement light; the reference beam and the measurement beam passing through the third quarter-wave plate form left-handed and right-handed circularly polarized light, which interfere with each other and are collected by a polarization camera;

[0018] The data processing and PC controller module is connected to the SH wavefront sensor, polarization camera, and thin-film deformable mirror, respectively. It is used to receive wavefront information and interference fringe data, execute the optimal iterative control algorithm to generate control signals to drive the thin-film deformable mirror, and solve the surface shape of the thin-film deformable mirror.

[0019] Furthermore, in the polarization control module, the first half-wave plate, the second half-wave plate, and the third half-wave plate can be rotated and adjusted to change the polarization direction, thereby adjusting the intensity of the two beams in the polarization beam splitter and thus adjusting the fringe contrast; the first quarter-wave plate and the second quarter-wave plate are used to change the polarization direction, and the third quarter-wave plate converts linearly polarized light into circularly polarized light; the first polarization beam splitter, the second polarization beam splitter, the third polarization beam splitter, and the fourth polarization beam splitter divide the incident light into reflected light and transmitted light, wherein the polarization state of the reflected light is s-polarized and the polarization state of the transmitted light is p-polarized.

[0020] Furthermore, the surface shape control optical path generates a surface shape according to the aberrations to be compensated, measures the modulated wavefront of the thin film deformable mirror in real time through an SH wavefront sensor, applies an iterative learning control algorithm, and adjusts the surface shape of the thin film deformable mirror by generating control signals through data processing and PC controller.

[0021] Furthermore, the SH wavefront sensor includes a microlens array and a detector, used to calculate the wavefront slope by sub-spot centroid shift for wavefront reconstruction; the deformable mirror control signal is the thin-film deformable mirror channel voltage vector calculated by data processing and the PC controller.

[0022] Furthermore, the optical path of the interferometric measurement module and the optical path of the thin film deformable mirror control and monitoring module share the same laser and thin film deformable mirror. The optical path is separated by a polarizing beam splitter. The optical path difference between the two interfering beams is less than the laser coherence length, and the two beams interfere with the polarizing camera. The single-frame interferogram acquired by the polarizing camera can extract four phase-shifted interferograms with phase shifts of 0°, 45°, 90°, and 135°.

[0023] A method for detecting non-zero positions of free-form surfaces using wavefront compensation based on thin-film deformable mirrors, employing the aforementioned device, includes the following steps:

[0024] Step 1, Optical Path Initialization: Adjust the polarization control module. The laser beam is collimated by the first lens. The collimated beam is separated into s-polarized light and p-polarized light by the first polarization beam splitter. Adjusting the first half-wave plate can adjust the intensity ratio of p-polarized light to s-polarized light. The p-polarized light is modulated by the first quarter-wave plate and then incident on the thin film deformable mirror. The s-polarized light is transmitted to form a reference beam. Adjusting the first half-wave plate and the second half-wave plate ensures that the intensity of the reference light and the measurement light for interference are equal.

[0025] Step 2, Closed-loop control of surface shape: Select the aberrations to be compensated, control the thin film deformable mirror to generate a surface shape to compensate for the aberrations, the light reflected by the thin film deformable mirror passes sequentially through the first quarter-wave plate, the first polarizing beam splitter, the second lens, the third lens, the third half-wave plate, and the second polarizing beam splitter to be incident on the SH wavefront sensor, the SH wavefront sensor detects the wavefront information and transmits it to the data processing and PC controller, the data processing and PC controller calculates the surface shape error and generates a control signal to drive the surface shape of the thin film deformable mirror to continuously approach the target surface shape;

[0026] Step 3, Interference Detection: The second polarizing beam splitter receives the light from the relay mirror. The reflected light passes sequentially through the second quarter-wave plate and is incident on the freeform surface. After being reflected by the freeform surface, it returns along the original path and passes sequentially through the second quarter-wave plate, the second polarizing beam splitter, the third polarizing beam splitter, and after being transmitted through the third polarizing beam splitter, it passes through the third quarter-wave plate to serve as the measurement light. Compared to the reference light, the phase plane of the measurement light is modulated by the thin-film deformable mirror and the freeform surface. The reference light and the measurement light form interference fringes on the polarizing camera target surface after passing through the third quarter-wave plate. The camera acquires the fringe image in real time and transmits it to the data processing and PC controller.

[0027] Step 4: Surface Shape Calculation Results: Data processing and PC controller end perform phase shift extraction on the interference fringes to obtain 4 frames of interference patterns with a 45° phase shift. Substitute these into the four-step phase shift algorithm, perform phase unwrapping processing, and use Zernike fitting to fit the phase surface to obtain high-precision compensated surface shape data. Subtract the surface shape data of the thin film deformable mirror measured by the SH wavefront sensor point-to-point to obtain the freeform surface shape data, thereby realizing the detection of non-zero position interference of freeform surfaces based on thin film deformable mirror wavefront compensation.

[0028] Furthermore, in the second step, the control law of the iterative learning control algorithm is expressed by the following expression:

[0029] Control rate:

[0030]

[0031] in , The input voltage for each channel in the next iteration of the thin film deformable mirror; The input voltages for each channel of the thin-film deformable mirror in this iteration; γ represents the deviation between the actual and ideal surface shape of the deformable thin film mirror in this iteration; γ and β are controller parameters that affect the iteration speed, surface shape generation error, and voltage saturation of the deformable thin film mirror; and the M matrix is ​​the influence function matrix of the deformable thin film mirror, which can be obtained from system identification.

[0032] Furthermore, in the fourth step, the phase information of the polarization interference fringes is calculated using the following formula:

[0033]

[0034] in The intensity of interference fringes with a 45° phase shift was extracted from a single-frame interferogram acquired by a polarization camera.

[0035] The beneficial effects of this invention are:

[0036] This invention utilizes the shape of a thin-film deformable mirror to modulate the wavefront of a freeform surface, and achieves freeform surface shape interference detection and control and monitoring of the thin-film deformable mirror shape through closed-loop control of the thin-film deformable mirror; and uses polarization interference to solve the problems of difficult phase-shifting interference and low phase-shifting accuracy, thus solving the bottleneck of detecting freeform surface shapes with large curvature and large tilt. Attached Figure Description

[0037] Figure 1 The overall system optical path diagram demonstrates the construction and collaborative working principle of the deformable mirror-type control optical path and the interferometric detection optical path.

[0038] Figure 2 This is a flowchart for the control and monitoring of the deformable mirror surface, and it illustrates the closed-loop control principle of the deformable mirror.

[0039] Figure 3 This is a flowchart of the interference detection process.

[0040] Figure 4 This is a flowchart of the freeform surface inspection process and the inspection results. Detailed Implementation

[0041] This invention addresses the shortcomings of existing wavefront compensation techniques in freeform surface detection by proposing a non-zero position interferometric detection device and method for freeform surfaces based on thin-film deformable mirror wavefront compensation. By constructing a multi-optical-path collaborative system and operating it according to a specific process, the device achieves deformable mirror wavefront compensation generation and freeform surface interferometric measurement.

[0042] The device of the present invention, such as Figure 1 As shown, it specifically includes the following components:

[0043] Laser 1, first lens 2 along the emission direction of laser 1, first half-wave plate 3, first polarizing beam splitter 6.

[0044] The first polarizing beam splitter 6 splits the incident light into reflected light and transmitted light, wherein the reflected light is s-polarized and the transmitted light is p-polarized. The reflected light passes through the first quarter-wave plate 4 and is incident on the thin-film deformable mirror 5. After being reflected by the thin-film deformable mirror 5, it returns along the original path, passing sequentially through the first quarter-wave plate 4, the first polarizing beam splitter 6, the second lens 7, the third lens 8, the third half-wave plate 9, and the second polarizing beam splitter 12. The transmitted light passes sequentially through the second half-wave plate 18 and the fourth polarizing beam splitter 19. The light reflected by the fourth polarizing beam splitter 19 passes sequentially through the third polarizing beam splitter 15 and the third quarter-wave plate 16, serving as reference light.

[0045] The second polarizing beam splitter 12 splits the incident light into reflected light and transmitted light. The transmitted light is incident on the SH wavefront sensor 14 and serves as the feedback wavefront signal for the closed-loop control of the thin-film deformable mirror 5-sided shape. The reflected light passes sequentially through the second quarter-wave plate 10, is incident on the freeform surface 11, is reflected by the freeform surface 11, returns along the original path, and sequentially passes through the second quarter-wave plate 10, the second polarizing beam splitter 12, the third polarizing beam splitter 15, and after being transmitted through the third polarizing beam splitter 15, passes through the third quarter-wave plate 16 and enters the polarizing camera 17 as the measurement light.

[0046] The SH wavefront sensor 14, polarization camera 17, and thin film deformable mirror 5 are respectively connected to the data processing and PC controller 13.

[0047] The device of the present invention is specifically divided into the following functional modules:

[0048] The polarization control module includes a first polarization beam splitter 6, a second polarization beam splitter 12, a third polarization beam splitter 15, a fourth polarization beam splitter 19, a first half-wave plate 3, a second half-wave plate 18, a third half-wave plate 9, a first quarter-wave plate 4, a second quarter-wave plate 10, and a third quarter-wave plate 16, which are used to modulate and separate the polarization state of the laser.

[0049] Thin-film deformable mirror surface shape control and monitoring module: includes a relay mirror system consisting of a first polarizing beam splitter 6, a second polarizing beam splitter 12, a second lens 7, and a third lens 8, and an SH wavefront sensor 14. The reflected light from the thin-film deformable mirror 5 transmitted by the first polarizing beam splitter 6 enters the relay mirror system and then enters the second polarizing beam splitter 12. The transmitted light from the second polarizing beam splitter 12 is incident on the SH wavefront sensor 14, which is used to detect the wavefront information of the thin-film deformable mirror in real time and perform high-precision closed-loop control of the thin-film deformable mirror surface shape.

[0050] Interferometric measurement module: The second polarizing beam splitter 12 receives the light from the relay mirror. The reflected light passes sequentially through the second quarter-wave plate 10 and is incident on the freeform surface 11. After being reflected by the freeform surface 11, it returns along the original path and passes sequentially through the second quarter-wave plate 10, the second polarizing beam splitter 12, the third polarizing beam splitter 15, and after being transmitted through the third polarizing beam splitter 15, it passes through the third quarter-wave plate 16 as the measurement light. The reference beam and the measurement beam pass through the third quarter-wave plate 16 to form left-handed and right-handed circularly polarized light, which interfere with each other and are collected by the polarization camera.

[0051] Data processing and PC controller module: The data processing and PC controller 13 is connected to the SH wavefront sensor 14, the polarization camera 17 and the thin film deformable mirror 5 respectively, and is used to receive wavefront information and interference fringe data, generate control signals to drive the thin film deformable mirror and solve the surface shape of the thin film deformable mirror.

[0052] The method of the present invention is as follows Figure 2 As shown, the main steps include:

[0053] Step 1, Optical Path Initialization: Adjust the polarization control module. The laser beam is collimated by the first lens. The collimated beam is separated into s-polarized light and p-polarized light by the first polarization beam splitter. Adjusting the first half-wave plate can adjust the intensity ratio of p-polarized light to s-polarized light. The p-polarized light is modulated by the first quarter-wave plate and then incident on the thin film deformable mirror. The s-polarized light is transmitted to form a reference beam. Adjusting the first half-wave plate and the second half-wave plate ensures that the intensity of the reference light and the measurement light for interference are equal.

[0054] Step 2, Closed-loop control of surface shape: Select the aberrations to be compensated, control the thin-film deformable mirror to generate a surface shape to compensate for the aberrations. The light reflected from the thin-film deformable mirror passes sequentially through the first quarter-wave plate, the first polarizing beam splitter, the second lens, the third lens, the third half-wave plate, and the second polarizing beam splitter to be incident on the SH wavefront sensor. The SH wavefront sensor detects the wavefront information, calculates and fits the Zernike coefficients, and transmits them to the data processing and PC controller. The data processing and PC controller solves the fitted Zernike surface shape error and generates a control signal according to the iterative learning control algorithm to drive the surface shape of the thin-film deformable mirror to approach the target surface shape.

[0055] Step 3, Interference Detection: The second polarizing beam splitter receives the light from the relay mirror. The reflected light passes sequentially through the second quarter-wave plate and is incident on the freeform surface. After being reflected by the freeform surface, it returns along the original path and passes sequentially through the second quarter-wave plate, the second polarizing beam splitter, the third polarizing beam splitter, and after being transmitted through the third polarizing beam splitter, it passes through the third quarter-wave plate to serve as the measurement light. Compared to the reference light, the phase plane of the measurement light is modulated by the thin-film deformable mirror and the freeform surface. The reference light and the measurement light form interference fringes on the polarizing camera target surface after passing through the third quarter-wave plate. The camera acquires the fringe image in real time and transmits it to the data processing and PC controller.

[0056] Step 4: Surface Shape Calculation Results: Data processing and PC controller end perform phase shift extraction on the interference fringes to obtain 4 frames of interference patterns with a 45° phase shift. Substitute these into the four-step phase shift algorithm, perform phase unwrapping processing, and use Zernike fitting to fit the phase surface to obtain high-precision compensated surface shape data. Subtract the surface shape data of the thin film deformable mirror measured by the SH wavefront sensor point-to-point to obtain the freeform surface shape data, thereby realizing the detection of non-zero position interference of freeform surfaces based on thin film deformable mirror wavefront compensation.

[0057] To make the technical solution of the present invention clearer, the following describes in detail a free-form surface non-zero position interferometry detection based on thin-film deformable mirror wavefront compensation, in conjunction with specific component parameters and operation procedures.

[0058] System composition and parameter selection:

[0059] Laser source: A semiconductor laser with a wavelength of 632.8nm and an output power of 5mW is selected to ensure that the monochromaticity and coherence meet the requirements of interferometric measurement.

[0060] Polarization control module: Polarization beam splitter: splitting ratio 1:1, polarization extinction ratio >1000:1, suitable for wavelength range of 400-700nm;

[0061] Half-wave plate: 25.4 mm in diameter, 632.8 nm in wavelength, used to adjust the intensity ratio of s / p polarized light;

[0062] Quarter-wave plate: 25.4 mm in diameter, 632.8 nm in wavelength, converts linearly polarized light into circularly polarized light.

[0063] Thin film deformable mirror: It adopts a 32-element thin film deformable mirror with an effective aperture of 11mm, a maximum deformation of ±10μm, and the electrodes can receive a maximum voltage of 250V, which is suitable for the large slope change requirements of free-form surfaces.

[0064] Surface shape control optical path:

[0065] Relay lens system: Achromatic lens 2 has a focal length of 250mm, lens 3 has a focal length of 100mm and a diameter of 25.4mm, ensuring that the beam can be reduced to the focal plane size of the SH wavefront sensor and that there is no aberration transmission;

[0066] SH wavefront sensor: Maximum of 73*45 effective microlenses, lens spacing of 150um, maximum detector camera resolution of 1920*1200 pixels, sensing area of ​​7.20mm*5.40mm, real-time detection of wavefront slope.

[0067] Interferometric optical path:

[0068] Freeform surface: A reflector with certain surface features is used as the freeform surface to be tested.

[0069] Polarization camera: 2448*2048 pixels resolution, 3.45μm pixel size, built-in polarizer array, can extract polarization components in four directions (0°, 45°, 90°, 135°) in a single frame, with a maximum frame rate of 75fps.

[0070] Data processing and control unit: An industrial PC equipped with an Intel i7 processor and 16GB of memory. It connects to the S-HWFS and polarization camera via a USB 3.0 interface and to the deformable mirror via a network cable. The control software is developed using LabVIEW and supports real-time data acquisition and closed-loop control.

[0071] The steps for optical path setup and initialization are shown in the optical path diagram below. Figure 1 As shown.

[0072] Mechanical installation: Fix all optical components on the vibration isolation optical platform, adjust the height of the laser source to make the beam parallel to the platform, and ensure that the centers of all components are at the same height.

[0073] Polarization state adjustment:

[0074] The laser beam is collimated into a parallel beam by lens 1 and incident on the first polarizing beam splitter, which separates the s-polarized light and the p-polarized light.

[0075] The first half-wave plate is rotated, and the intensity of the two beams is monitored by a power meter. The intensity ratio of the transmitted light to the reflected light is adjusted to 1:1 to meet the interference contrast requirement. The s-polarized light is incident on the thin film deformable mirror after passing through the first quarter-wave plate. At this time, the beam is converted into circularly polarized light. The p-polarized light is used as a reference light. The polarization direction is adjusted by the second half-wave plate to ensure that it matches the polarization state of the light to be measured later.

[0076] Rotating the third quarter-wave plate adjusts the angle between the optical axis of the quarter-wave plate and the y-axis to 45°. After the polarization state of the interference light is modulated by the quarter-wave plate, the light intensity received by the four polarization channels of the polarization camera can respectively contain... Phase information, Given the original phase difference, four phase-shifted fringes can be extracted from one frame of image acquired by the polarization camera.

[0077] Wavefront Deformation Mirror Shape Control and Monitoring:

[0078] Surface shape closed-loop control, wavefront deformation mirror surface shape control generation and monitoring block diagram as follows: Figure 2 As shown.

[0079] Step 1: Generate 200 sets of random relative voltages, with a voltage range of ±30% of the operating voltage (0.5V). Measure the corresponding wavefronts and calibrate the operating point wavefront. The linearized influence matrix M is identified.

[0080] Step 2: Set the desired compensation wavefront (e.g., astigmatism, defocus), and calculate the relative wavefront with respect to the operating voltage wavefront. , Relative to the wavefront, In order to compensate for the wavefront, The initial wavefront, the wavefront when the working voltage is 0.5;

[0081] Step 3: Initialization , Set parameters for the initial operating voltage. Substitute into the iterative learning control law: in The input voltage of the wavefront deformable mirror is calculated iteratively. After each iteration, the wavefront data is collected after the voltage is applied until the wavefront error converges, thereby realizing closed-loop control and surface shape monitoring of the wavefront deformable mirror.

[0082] Interferometry and surface model calculation:

[0083] Interference fringe acquisition: The polarization camera acquires the interference fringes of the reference light and the measurement light in real time. A single frame image contains four polarization components, which are extracted into four interferograms with a 45° phase shift by the algorithm.

[0084] Phase calculation: Substitute into the four-step phase shifting algorithm formula:

[0085]

[0086] in The intensity of interference fringes with a 45° phase shift was extracted from a single-frame interferogram acquired by a polarization camera.

[0087] The wrapped phase is obtained, and the 2π jump is removed by the phase unwrapping algorithm. Then, the data is fitted by Zernike polynomials (first 36 terms) to obtain the compensated face shape data.

[0088] Accuracy verification and optimization:

[0089] The surface profile data of the freeform surface to be measured is obtained by subtracting the surface profile data of the surface measured by the SH wavefront sensor point by point after the interferometric measurement compensation.

[0090] The above implementation scheme enables interference detection of freeform surfaces with arbitrary shapes.

[0091] Conduct the experiment according to the implementation plan. Figure 1 This is the experimental optical path diagram. Figure 2 This is a control block diagram for deformable mirror surfaces. Figure 3 This is a flowchart of interferometry and data processing. Figure 4 To process the interferogram data and the processing results, the surface shape data after interferometric compensation is subtracted point-to-point from the deformed mirror surface detection data to obtain the surface shape data of the freeform surface to be measured.

[0092] This invention utilizes a closed-loop control system combining a surface shape control optical path and an SH wavefront sensor: the SH wavefront sensor acquires the actual wavefront of the deformable mirror in real time, and the control unit generates a correction signal based on the feedback data to dynamically adjust the surface shape of the deformable mirror, thus achieving real-time closed-loop measurement feedback correction. It employs an iterative learning control method for wavefront correction of thin-film deformable mirrors, which balances stability, convergence speed, correction accuracy, and actuator saturation control. This solves the problems of complex parameter tuning, slow convergence, large steady-state error, and actuator saturation runaway in existing control algorithms, achieving fast, stable, and high-precision wavefront compensation for thin-film deformable mirrors.

[0093] This invention employs various polarization devices to form a polarization module, generating a single-frame polarization interference detection. It uses the single-frame polarization interference fringes to extract four-frame phase-shifted interferograms with a 45° phase difference. Through polarization control technology, multi-step precise phase shifting is achieved in a single-frame image, avoiding inherent errors from mechanical or wavelength phase shifting. Combined with a four-step phase shifting algorithm, the phase calculation accuracy is significantly improved, fundamentally reducing the impact of phase shifting errors on compensation and detection accuracy.

[0094] This invention utilizes the dynamic wavefront compensation capability of a thin-film deformable mirror and the synergy of dual optical paths: the surface shape control optical path compensates the measured wavefront in real time to optimize the quality of interference fringes; the interferometry optical path, through precise phase calculation and surface shape reconstruction, combines the compensated surface shape data with the deformable mirror surface shape data for point-to-point subtraction to directly obtain the freeform surface shape, effectively eliminating the influence of system aberrations and environmental interference, and solving the bottleneck of detection accuracy for freeform surfaces with large curvature and large tilt.

[0095] In summary, this invention provides a device and method for detecting non-zero position interferometry on freeform surfaces based on wavefront compensation of a thin-film deformable mirror, relating to the field of optical detection technology. The device includes a laser source, a polarization control module, a thin-film deformable mirror surface type control and monitoring module, an interferometry module, and a data processing and control module. The thin-film deformable mirror surface type control and monitoring module works collaboratively with the interferometry module. In the thin-film deformable mirror surface shape control and monitoring module, an iterative learning control algorithm is used to change the surface shape of the thin-film deformable mirror to compensate for the measurement wavefront, based on the degree of interference fringe distortion of the freeform surface. The wavefront sensor collects the wavefront of the thin-film deformable mirror in real time and feeds it back to the control unit. The control unit generates a control signal to perform closed-loop control of the surface shape of the thin-film deformable mirror to achieve high-precision wavefront compensation. In the interferometry module, the reference wavefront is directly generated by a processed laser source. The measurement light is formed after being modulated by the thin-film deformable mirror and the freeform surface under test. The measurement light and the reference light interfere with each other. A polarization camera collects a single frame of interference fringes. Four phase-shifted interferograms with a 45° phase difference are extracted from the single frame of interference fringes. The surface shape is reconstructed by combining a four-step phase-shifting algorithm and phase solution technology to obtain the compensated surface shape data. The compensated surface shape data is subtracted point-to-point from the surface shape data of the thin-film deformable mirror to obtain the surface shape of the freeform surface under test, thereby achieving high-precision detection. This invention utilizes the dynamic wavefront compensation capability of a thin-film deformable mirror to effectively solve the problem of low contrast and indistinguishability of interference fringes caused by surface tilt, defocus, or higher-order aberrations in freeform surface inspection. It improves the inspection accuracy and efficiency of freeform surfaces with large curvature and large tilt, and is suitable for inspection scenarios of high-precision freeform surface optical components.

[0096] The above embodiments are merely typical implementations of the present invention and are not intended to limit the present invention. Any equivalent substitutions or improvements made within the scope of the claims of the present invention are within the protection scope of the present invention.

Claims

1. A freeform surface non-zero position interferometric detection device based on thin-film deformable mirror wavefront compensation, characterized in that, include: Laser (1), first lens (2) along the emission direction of the laser (1), first half-wave plate (3), first polarizing beam splitter (6); The first polarizing beam splitter (6) splits the incident light into reflected light and transmitted light, wherein the reflected light is s-polarized and the transmitted light is p-polarized; the reflected light from the first polarizing beam splitter (6) passes through the first quarter-wave plate (4) and is incident on the thin film deformable mirror (5). After being reflected by the thin film deformable mirror (5), it returns along the original path and passes through the first quarter-wave plate (4), the first polarizing beam splitter (6), the second lens (7), the third lens (8), the third half-wave plate (9), and the second polarizing beam splitter (12) in sequence; the transmitted light from the first polarizing beam splitter (6) passes through the second half-wave plate (18) and the fourth polarizing beam splitter (19) in sequence, and the reflected light from the fourth polarizing beam splitter (19) passes through the third polarizing beam splitter (15) and the third quarter-wave plate (16) in sequence, serving as reference light; The second polarizing beam splitter (12) splits the incident light into reflected light and transmitted light; the transmitted light from the second polarizing beam splitter (12) is incident on the SH wavefront sensor (14) as the feedback wavefront signal for the closed-loop control of the surface shape of the thin film deformable mirror (5); the reflected light from the second polarizing beam splitter (12) passes through the second quarter-wave plate (10) in sequence, is incident on the freeform surface (11), is reflected by the freeform surface (11), returns along the original path, passes through the second quarter-wave plate (10), the second polarizing beam splitter (12), the third polarizing beam splitter (15) in sequence, is transmitted through the third polarizing beam splitter (15) and passes through the third quarter-wave plate (16) into the polarizing camera (17) as the measurement light; The SH wavefront sensor (14), polarization camera (17) and thin film deformable mirror (5) are respectively connected to the data processing and PC controller (13).

2. The apparatus according to claim 1, characterized in that, include: Polarization control module, thin film deformation mirror type control and monitoring module, interferometry module, data processing and PC controller module; The polarization control module includes a first polarization beam splitter (6), a second polarization beam splitter (12), a third polarization beam splitter (15), a fourth polarization beam splitter (19), a first half-wave plate (3), a second half-wave plate (18), a third half-wave plate (9), a first quarter-wave plate (4), a second quarter-wave plate (10), and a third quarter-wave plate (16) for modulating and separating the polarization state of the laser. The thin-film deformable mirror surface shape control and monitoring module includes a relay mirror system composed of a first polarizing beam splitter (6), a second polarizing beam splitter (12), a second lens (7), and a third lens (8), and an SH wavefront sensor (14). The light reflected from the thin-film deformable mirror (5) transmitted by the first polarizing beam splitter (6) enters the relay mirror system and then enters the second polarizing beam splitter (12). The light transmitted by the second polarizing beam splitter (12) is incident on the SH wavefront sensor (14) to detect the wavefront information of the thin-film deformable mirror (5) in real time and transmit it to the data processing and PC controller (13) for high-precision closed-loop control of the surface shape of the thin-film deformable mirror (5). The interferometric measurement module: The second polarizing beam splitter (12) receives the light from the relay mirror. The reflected light passes through the second quarter-wave plate (10) in sequence and is incident on the freeform surface (11). After being reflected by the freeform surface (11), it returns along the original path and passes through the second quarter-wave plate (10), the second polarizing beam splitter (12), the third polarizing beam splitter (15) in sequence. After being transmitted through the third polarizing beam splitter (15), it passes through the third quarter-wave plate (16) as the measurement light. The reference beam and the measurement beam pass through the third quarter-wave plate (16) to form left-handed and right-handed circularly polarized light, which interfere with each other and are collected by the polarization camera (17). The data processing and PC controller module: The data processing and PC controller (13) is connected to the SH wavefront sensor (14), polarization camera (17) and thin film deformable mirror (5) respectively. It is used to receive wavefront information and interference fringe data, execute the optimal iterative control algorithm to generate control signals to drive the thin film deformable mirror (5), and solve the surface shape of the thin film deformable mirror (5) and process the interference fringes.

3. The apparatus according to claim 1, characterized in that, In the polarization control module, the first half-wave plate (3), the second half-wave plate (18), and the third half-wave plate (9) can be rotated to change the polarization direction and adjust the intensity of the two beams of light in the polarization beam splitter, thereby adjusting the fringe contrast; the first quarter-wave plate (4) and the second quarter-wave plate (10) are used to change the polarization direction, and the third quarter-wave plate (16) converts linearly polarized light into circularly polarized light; the first polarization beam splitter (6), the second polarization beam splitter (12), the third polarization beam splitter (15), and the fourth polarization beam splitter (19) divide the incident light into reflected light and transmitted light, wherein the polarization state of the reflected light is s-polarized and the polarization state of the transmitted light is p-polarized.

4. The apparatus according to claim 1, characterized in that, The thin film deformable mirror surface shape control and monitoring module generates the surface shape according to the aberration to be compensated, measures the modulated wavefront of the thin film deformable mirror (5) in real time through the SH wavefront sensor (14), applies the iterative learning control algorithm, and adjusts the surface shape of the thin film deformable mirror (5) by generating control signals through data processing and PC controller (13), thereby completing the control and monitoring of the deformable mirror surface shape.

5. The apparatus according to claim 4, characterized in that, The SH wavefront sensor (14) includes a microlens array and a detector, used to calculate the wavefront slope by sub-spot centroid offset for wavefront reconstruction; the deformable mirror control signal is the channel voltage vector of the thin film deformable mirror (5) obtained by data processing and calculation at the PC controller (13).

6. The apparatus according to claim 1, characterized in that, The interferometric measurement module and the thin film deformable mirror control and monitoring module share the same laser (1) and thin film deformable mirror (5) in their optical paths. The optical paths are separated by a polarizing beam splitter. The optical path difference between the two interfering beams is less than the laser coherence length. The two beams interfere with each other at the polarizing camera (17). The single-frame interferogram acquired by the polarizing camera (17) can extract four phase-shifted interferograms with phase shifts of 0°, 45°, 90°, and 135°.

7. A method for detecting non-zero positions of free-form surfaces based on wavefront compensation using a thin-film deformable mirror, employing the apparatus described in any one of claims 1-6, characterized in that, Includes the following steps: Step 1, Optical path initialization: Adjust the polarization control module. The laser beam is collimated by the first lens (2). The collimated beam is separated into s-polarized light and p-polarized light by the first polarization beam splitter (6). Adjusting the first half-wave plate (3) can adjust the intensity ratio of p-polarized light to s-polarized light. The p-polarized light is modulated by the first quarter-wave plate (4) and then incident on the thin film deformable mirror (5). The s-polarized light is transmitted to form a reference beam. Adjust the first half-wave plate (3) and the second half-wave plate (18) to ensure that the intensity of the reference light and the measurement light for interference are equal. Step 2, closed-loop control of surface shape: Select the aberration to be compensated, control the thin film deformable mirror (5) to generate the aberration-compensated surface shape, the light reflected by the thin film deformable mirror passes through the first quarter-wave plate (4), the first polarizing beam splitter (6), the second lens (7), the third lens (8), the third half-wave plate (9), and the second polarizing beam splitter (12) in sequence and is incident on the SH wavefront sensor (14), the SH wavefront sensor (14) detects the wavefront information and transmits it to the data processing and PC controller (13), the data processing and PC controller (13) calculates the surface shape error and generates a control signal to drive the surface shape of the thin film deformable mirror (5) to continuously approach the target surface shape; Step 3, Interference Detection: The second polarizing beam splitter (12) receives the light from the relay mirror. The reflected light passes through the second quarter-wave plate (10) in sequence and is incident on the freeform surface (11). After being reflected by the freeform surface (11), it returns along the original path and passes through the second quarter-wave plate (10), the second polarizing beam splitter (12), the third polarizing beam splitter (15) in sequence. After being transmitted through the third polarizing beam splitter (15), it passes through the third quarter-wave plate (16) as the measurement light. Compared with the reference light, the phase surface of the measurement light is modulated by the thin film deformable mirror (5) and the freeform surface (11). The reference light and the measurement light form interference fringes on the target surface of the polarizing camera (17) after passing through the third quarter-wave plate (16). The camera collects the fringe image in real time and transmits it to the data processing and PC controller (13). Step 4: Surface shape calculation results: The data processing and PC controller (13) perform phase shift extraction on the interference fringes to obtain 4 frames of interference patterns with a 45° phase shift. Substitute them into the four-step phase shift algorithm, perform phase unwrapping processing, and use Zernike to fit the phase surface to obtain high-precision compensated surface shape data. Subtract the surface shape data of the thin film deformable mirror (5) measured by the SH wavefront sensor (14) point by point to obtain the surface shape data of the freeform surface (11), thereby realizing the detection of non-zero position interference of freeform surface based on wavefront compensation of thin film deformable mirror.

8. The method according to claim 7, characterized in that, In the second step, the control law of the iterative learning control algorithm is expressed by the following expression: Control Law: ； in , The input voltage for each channel of the thin film deformable mirror (5) in the next iteration; It is the identity matrix; The input voltage of each channel of the thin film deformable mirror (5) in this iteration; γ represents the deviation between the actual and ideal surface shape of the deformable thin film mirror (5) in this iteration; γ and β are controller parameters that affect the iteration speed, surface shape generation error, and voltage saturation of the deformable thin film mirror (5); the M matrix is ​​the influence function matrix of the deformable thin film mirror (5), which can be obtained from the system identification. This is for the transpose of the influence function matrix.

9. The method according to claim 7, characterized in that, In the fourth step, the phase information of the polarization interference fringes is calculated using the following formula: ； in The intensity of interference fringes with a 45° phase shift was extracted from a single-frame interferogram acquired by a polarization camera. x and y are the coordinates of the corresponding pixel points in the interferogram.

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