Measuring device of propagation correction type Shack-Hartmann wavefront sensor

Abstract

The invention belongs to the technical field of optical mirror surface shape measurement, and particularly relates to a measuring device of a propagation correction type Shack-Hartmann wavefront sensor. The measuring device comprises a collimation light source used for generating plane detection light; the beam splitter prism is used for transmitting the detection light and reflecting the signal light; the micro-lens array is used for dividing the reflected signal light into a plurality of sub light beams; the photoelectric detector is used for receiving a focused light spot image; the clamp is used for fixing and controlling a sample; the data processing module is used for reconstructing a reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor and recovering the real surface shape of the mirror surface according to a wavefront propagation correction model and an equiphase reflection conjugate model; according to the invention, the precision and stability of surface shape reconstruction of the optical mirror surface are greatly improved, and the stability and resolution capability of the Shack-Hartmann wavefront sensor are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical mirror surface shape measurement, and in particular relates to a measuring device of a propagation-corrected Shack-Hartmann wavefront sensor. Background Art

[0002] In the field of optical mirror surface measurement technology, traditional contact measurement methods have wide applicability but are inefficient and carry the risk of damaging the optical surface. Phase measurement deflectometry offers the advantages of a large field of view and high efficiency, but typically only achieves sub-micron measurement accuracy. Interferometers can achieve high-precision measurements but have stringent requirements for the experimental environment, requiring the suppression of environmental disturbances such as vibration and airflow. Although dynamic interferometry technology has improved its ability to resist environmental interference to a certain extent, its measurement accuracy and lateral resolution have declined.

[0003] As a non-interferometric measurement method, the Shack-Hartmann wavefront sensor has attracted widespread attention due to its non-contact, high efficiency, excellent stability, high accuracy, and low cost. Recent advances in centroid calculation algorithms and wavefront reconstruction techniques have further improved the method's measurement accuracy and dynamic range. Consequently, the Shack-Hartmann wavefront sensor has become widely used in optical mirror surface shape measurement.

[0004] However, Shack-Hartmann wavefront sensors still face numerous challenges. Modulation of the wavefront by the reflector disrupts the consistency of the wavefront slope, producing spatially varying wavefront distortions that alter the original wavefront shape. These distortions are particularly detrimental to accuracy when measuring highly deflected mirror surfaces or when propagating over long distances.

[0005] The 4f relay system can optically compensate for the distortion caused by wavefront propagation, but appropriate calibration techniques are required to correct for the introduced aberrations. Another approach is reverse ray tracing, which can provide a more accurate estimate of the mirror surface without introducing aberrations. The paper "Robust and accurate measurement of optical freeform surfaces with wavefront deformation correction" (H.Lyu,L.Kong,S.Wang,et al.,"Robust and accurate measurement of optical freeform surfaces with wavefrontdeformation correction,"Optics Express 30,7831-7844(2022).) proposes a multi-sensor fusion measurement system that uses a chromatic aberration confocal sensor to track the optical path; the paper "Method for testingfreeform surfaces based on a Shack-Hartmann sensor with plane wavefrontscanning and stitching" (J.Wang,X.Wang,L.Peng,et al.,"Method for testingfreeform surfaces based on a Shack-Hartmann sensor with plane wavefrontscanning and stitching,"Optics Express31,36702-36724(2023).) proposes a wavefront tracking method based on the reflection law combined with sub-aperture stitching technology to improve measurement accuracy. While these methods can reduce wavefront errors during free-space propagation to some extent, they still fail to fully reveal the complex relationship between the incident and reflected wavefronts, nor do they consider the effects of different propagation media. Furthermore, most methods still rely on the paraxial ray approximation, which results in complex calculations. Summary of the Invention

[0006] The object of the present invention is to provide a measuring device for a propagation-corrected Shack-Hartmann wavefront sensor, so as to reduce the interference of wavefront propagation distortion and traditional reflection mapping model on optical mirror detection.

[0007] The measuring device of the propagation-corrected Shack-Hartmann wavefront sensor provided by the present invention has a structure as shown in FIG. Figure 1As shown, it specifically includes: a collimated light source, a beam splitter prism, a microlens array, a photodetector, a one-dimensional motion sample fixture, a slide, a stepper motor drive module, and a data processing module; wherein:

[0008] A collimated light source is used to generate a plane probe beam, or a plane wavefront beam, which is incident directly horizontally onto a beam splitter prism. The beam splitter prism is precisely positioned between the collimated light source and the optical surface to be measured, with its incident plane at a 45° angle to the horizontal. After penetrating the inclined surface of the beam splitter prism, the plane wavefront beam strikes the optical surface to be measured at a perpendicular angle. The optical surface to be measured reflects the wavefront beam back to the inclined surface of the beam splitter prism. After reflection from the inclined surface, the beam is redirected toward the microlens array, perpendicular to the original incident beam path.

[0009] The microlens array is parallel to the reflected light path of the beam splitter prism and is installed on the outgoing light path of the beam splitter prism to decompose the reflected wavefront into multiple sub-wavefronts and focus them;

[0010] The photodetector is firmly fixed on the focal plane of the microlens array and is used to collect the focused light spot array image;

[0011] The fixture is used to support and fix the optical mirror to be tested. It is mounted on the slide. The stepper motor drive module provides linear motion guidance for the slide, and moves precisely along the normal direction of the mirror to be tested (that is, the direction perpendicular to the mirror surface), thereby achieving precise adjustment of the position of the optical mirror to be tested.

[0012] A data processing module is used to reconstruct the reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor and restore the true surface shape of the mirror based on the wavefront propagation correction model and the equal phase reflection conjugate model; the data processing module is deployed in a computer in the form of software;

[0013] The computer is connected to the photoelectric detector and stepper motor drive module through cables. On the one hand, it receives the light spot position data collected by the photoelectric detector, and on the other hand, it controls the displacement operation of the optical mirror, while performing key tasks such as wavefront reconstruction and surface shape calculation.

[0014] The optical path of the measuring device can be summarized as follows: collimated light source → beam splitter (transmission) → mirror to be measured (reflection) → beam splitter (reflection) → microlens array → photodetector.

[0015] This invention combines wavefront backtracking with the equal-phase reflection conjugate model to establish a propagation correction model (propagation correction model = wavefront backtracking and equal-phase reflection conjugate model), effectively reducing wavefront propagation errors. This invention not only improves the measurement accuracy and stability of optical mirrors, but also enables high-precision wavefront reconstruction over long propagation distances.

[0016] Further:

[0017] The collimated light source includes a semiconductor laser, a single-mode optical fiber, a fiber coupler, and a focus-adjustable collimating lens assembly. The semiconductor laser is directly coupled to the input end of the single-mode optical fiber to introduce laser energy into the optical fiber. The output end is connected to the input end of the fiber coupler, which transmits the light beam to the focus-adjustable collimating lens assembly to calibrate the divergent light beam into a plane wavefront beam.

[0018] The beam splitter prism is a 45°-45° optical glass prism having an anti-reflection coating and a partial reflection coating structure to maintain the quality of the incident wavefront.

[0019] The microlens array is a regularly arranged plano-convex lens array, the aperture of the microlens unit is no more than 300 μm, and the focal length is less than 20 mm.

[0020] The photodetector is a high-resolution CMOS image sensor with a pixel size of less than 2 μm and an effective pixel array larger than 3000×3000.

[0021] The fixture is a multifunctional mechanical adjustment device that integrates x-axis and y-axis translation and tilt adjustment functions, with a maximum tilt range of ±5°; it can accurately adjust the position and posture of the mirror to be measured along multiple dimensions.

[0022] The slide is a mechanical transmission actuator with a high-precision linear guide at the bottom, which is precisely matched with a ball slider or roller slider to provide stable support. The linear guide is longer than 200mm and the motion resolution is better than 1μ.

[0023] The stepper motor driving module 7 drives the slide to move along the linear guide rail.

[0024] Further:

[0025] The collimated light source is used to adjust the divergence angle and uniformity of the output light beam to form a collimated plane wavefront with high uniformity and low wavefront error, which is used to illuminate the mirror to be measured;

[0026] The beam splitter prism guides the reflected wavefront toward the microlens array and minimizes wavefront distortion caused by chromatic aberration and multiple reflections;

[0027] The microlens array features a high fill rate and low optical aberration, ensuring high-precision spatial sampling of the reflected wavefront. Specifically, the microlens array utilizes circular, hexagonal, or square apertures, arranged in an array, to decompose incident light into a corresponding number of sub-beams. Each sub-aperture in the array maintains consistent diameter, focal length, and shape. Materials include quartz glass, K9 glass, and silicon single crystal.

[0028] The photodetector is equipped with a low-noise readout circuit and automatic gain control function, and is installed on the focal plane of the microlens array to collect the position of the light spot formed by the reflected wavefront with high precision, so as to achieve sub-micron centroid offset measurement;

[0029] The data processing module includes a centroid extraction unit, a wavefront reconstruction unit, a propagation correction calculation unit and a surface shape calculation unit; wherein:

[0030] The centroid extraction unit is used to calculate the centroid position of each light spot in the image and output the offset;

[0031] The wavefront reconstruction unit calculates the reflected wavefront phase distribution by a Zernike polynomial fitting algorithm according to the centroid offset;

[0032] The propagation correction calculation unit is used to calculate the distortion of the reflected wavefront in the propagation path and restore the true wavefront shape at the reflection point;

[0033] The surface shape calculation unit realizes accurate restoration of the mirror surface morphology by constructing a conjugate point mapping relationship.

[0034] Compared with the existing technology, the measuring device of the present invention has the following technical characteristics and functional advantages:

[0035] (1) By introducing the wavefront backtracking and equal-phase reflection conjugate model, the measurement deviation caused by wavefront propagation distortion and reflection mapping error in the traditional Shack-Hartmann wavefront sensor is effectively corrected, greatly improving the accuracy and stability of optical mirror surface reconstruction.

[0036] (2) The propagation correction model is used to trace the wavefront backwards. Even in the presence of millimeter-level propagation distance disturbances, high-precision measurement can be maintained. It has good error tolerance and environmental adaptability and is suitable for high-precision measurement tasks in complex optical path systems.

[0037] (3) Compared with complex structural solutions such as multi-sensor fusion and interferometer compensation, the present invention only relies on the existing Shack-Hartmann wavefront sensor structure and achieves accuracy improvement through algorithm optimization without adding additional optical elements or interference modules. It has high cost-effectiveness and engineering feasibility.

[0038] (4) Thanks to the model's ability to jointly correct non-ideal propagation paths and aspheric reflection characteristics, the present invention is particularly suitable for mirror measurement tasks with small curvature radius or large edge gradient. In actual tests, it has shown stability and resolution superior to traditional interferometers and ordinary Shack-Hartmann wavefront sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1This is a schematic structural diagram of a measuring device of a propagation-corrected Shack-Hartmann wavefront sensor according to an implementation method of the present invention.

[0040] Figure 2 This is a schematic flow chart of a propagation-corrected Shack-Hartmann wavefront sensor implementing the method of the present invention.

[0041] Figure 3 It is a schematic flow chart of the wavefront propagation correction model and the equal-phase reflection conjugate model for implementing the method of the present invention.

[0042] Figure 4 Schematic diagram of wavefront distortion.

[0043] Figure 5 Schematic diagram of the wavefront propagation correction model and the equal-phase reflection conjugate model for the implementation method of the present invention.

[0044] Figure 6 Schematic diagram of a propagation distance measurement method implemented in the present invention.

[0045] Figure 7 This is a measurement effect diagram of the propagation-corrected Shack-Hartmann wavefront sensor implementing the method of the present invention.

[0046] The numbers in the figure are: 1 is the collimated light source; 2 is the beam splitter prism; 3 is the microlens array; 4 is the photodetector; 5 is the fixture; 6 is the slide; 7 is the linear guide and stepper motor drive module; 8 is the optical mirror to be measured; 9 is the data processing module. DETAILED DESCRIPTION

[0047] The present invention will be further described below by way of examples in conjunction with the accompanying drawings. The examples are only used to illustrate the technical solutions of the present invention, but do not limit the scope of protection of the present invention.

[0048] The measurement device structure of the propagation-corrected Shack-Hartmann wavefront sensor provided by the embodiment of the present invention is shown in FIG. Figure 1As shown, it includes: a collimated light source 1, which is used to adjust the divergence angle and uniformity of the output light beam to form a collimated plane wavefront with high uniformity and low wavefront error, which is used to illuminate the mirror surface 8 to be measured; a beam splitter prism 2, which guides the reflected wavefront to the direction of the microlens array 3 and minimizes the wavefront distortion caused by chromatic aberration and multiple reflections; the microlens array 3, which has high filling rate and low optical aberration characteristics to ensure high-precision spatial sampling of the reflected wavefront; a photodetector 4, which is equipped with a low-noise readout circuit and automatic gain control function, and is installed at the focal plane of the microlens array 3. The surface is used to collect the spot position formed by the reflected wavefront with high precision to achieve sub-micron center of mass offset measurement; the fixture 5 is used to achieve stable clamping of the mirror to be measured; the slide 6 connects the fixture with the guide rail and the stepper motor drive 7 to achieve distance setting to achieve high repeatability and stability of the displacement, thereby ensuring the controllable wavefront propagation path and improving measurement accuracy; the data processing module 9 reconstructs the reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor, and restores the true surface shape of the mirror to be measured 8 based on the wavefront propagation correction model and the equal phase reflection conjugate model.

[0049] The collimating light source 1 includes a semiconductor laser, a single-mode optical fiber, a fiber coupler, and a focus-adjustable collimating lens group.

[0050] The beam splitter prism 2 is a 45°-45° optical glass prism having an anti-reflection coating and a partial reflection coating structure to maintain the quality of the incident wavefront.

[0051] The microlens array 3 is a regularly arranged plano-convex lens array, the aperture of the microlens unit is no more than 300 μm, and the focal length is less than 20 mm.

[0052] The photodetector 4 is a high-resolution CMOS image sensor with a pixel size of less than 2 μm and an effective pixel array larger than 3000×3000.

[0053] The displacement control system includes a fixture 5, a slide 6 and a stepper motor drive module 7, which are used to accurately adjust the position of the mirror surface 7 to be measured along the normal direction and provide position information of the mirror surface 8 to be measured.

[0054] During the measurement process, a collimated light source 1 emits a plane wavefront to illuminate the mirror surface 8 to be measured. The reflected wavefront is guided by a beam splitter prism 2 into a microlens array 3, forming a light spot array on a photodetector 4. In the data processing module 9, the centroid extraction unit first calculates the spot offset to obtain the local wavefront slope. Subsequently, the wavefront reconstruction unit reconstructs the reflected wavefront shape using a Zernike polynomial fitting method. Next, the propagation correction calculation unit applies a wavefront backtracking method to calculate the true wavefront at the reflection point based on the wavefront gradient. Distortion introduced by the medium and distance in the optical path is eliminated through refractive index conversion and inverse propagation path calculation. Finally, the surface shape calculation unit accurately restores the mirror surface topography by constructing a phase conjugate point mapping relationship. This mapping, in other words, maps the reflected wavefront back to the actual mirror surface topography, achieving precise three-dimensional topography reconstruction. This invention addresses the problem of sample loss during wavefront retraction by using grid interpolation and boundary extrapolation techniques within the sampling area, ensuring wavefront continuity and stability.

[0055] The following describes a method for measuring a propagation-corrected Shack-Hartmann wavefront sensor using the above optical mirror measuring apparatus. Figure 2 A schematic flow chart of the method is shown in FIG. Figure 2 As shown, the method includes the following steps:

[0056] S210, the collimated light source 1 is used to adjust the divergence angle and uniformity of the output light beam to form a collimated plane wavefront with high uniformity and low wavefront error, which is used to illuminate the mirror surface 7 to be measured.

[0057] S220 , the beam splitter prism 2 guides the reflected wavefront toward the microlens array 3 and minimizes the wavefront distortion caused by chromatic aberration and multiple reflections.

[0058] S230, the microlens array 3 has high filling rate and low optical aberration characteristics to ensure high-precision spatial sampling of the reflected wavefront.

[0059] S240, the photodetector 4 is provided with a low-noise readout circuit and an automatic gain control function, and is installed on the focal plane of the microlens array 3 to collect the position of the light spot formed by the reflected wavefront with high precision to achieve sub-micron level centroid offset measurement.

[0060] S250, one-dimensional motion sample fixture 5 supports closed-loop control to achieve high repeatability and stability of displacement, thereby ensuring the controllable wavefront propagation path and improving measurement accuracy.

[0061] S260, the data processing module 6 reconstructs the reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor, and restores the true surface shape of the mirror 7 according to the wavefront propagation correction model and the equal phase reflection conjugate model.

[0062] In one embodiment, the data processing module process in S260 is as follows: Figure 3 As shown, the specific steps are:

[0063] S310, a centroid extraction unit, is used to calculate the centroid position of each light spot in the image and output an offset.

[0064] S320, a wavefront reconstruction unit, calculates the reflected wavefront phase distribution according to the centroid offset by using a Zernike polynomial fitting algorithm, that is, reconstructs the reflected wavefront phase incident on the surface of the Shack-Hartmann wavefront sensor;

[0065] S330, a propagation correction calculation unit, is a wavefront propagation correction model used to calculate the distortion of the reflected wavefront in the propagation path and restore the true wavefront shape at the reflection point.

[0066] S340, the surface shape calculation unit is an equal phase reflection conjugate model, which realizes accurate restoration of the mirror surface morphology by constructing a conjugate point mapping relationship.

[0067] The specific process of reconstructing the phase of the reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor in step S320 is as follows:

[0068] Wavefront reconstruction is performed using the modal method. The x- and y-axis offsets (Δx and Δy) of each spot are calculated using the spot centroid calculation unit. Based on these offsets and the microlens focal length, the average wavefront slope k at each subaperture is calculated.

[0069] The wavefront W is then expanded using Zernike polynomials:

[0070]

[0071] Where C i is the coefficient of the i-th Zernike polynomial, which can be obtained by the following formula:

[0072] K=Z·C (2) , (2)

[0073] Where K is the wavefront slope matrix, and Z is the partial derivative matrix of each Zernike polynomial.

[0074] Figure 4A schematic diagram of wavefront distortion is shown. The traditional reflection mapping model assumes that the reflected wavefront W1 and the mirror surface topography W0 satisfy the simplified relationship: 2W0 ≈ W1. This approximation is highly accurate for mirrors with large curvature radii, but introduces significant errors for small curvature radii. The reflected wavefront W1 propagates through three paths before reaching the microlens array: free space L1, a beamsplitter L2, and additional free space L3. Because the reflection process does not introduce an optical path difference, this path can be equated to the wavefront propagating distance L2 in a glass medium with a refractive index of n, where the internal wavefront is denoted as W2. When tracing forward from W2 to W3, the wavefront is distorted in the x and y directions due to the change in the normal vector, and these distortions are proportional to the propagation distance. Furthermore, due to the size limitations of the microlens array and image sensor, the ultimately measured wavefront W′ is only a fraction of W3, with an effective aperture of D. Wavefront propagation distortion and errors in the reflection mapping model together reduce the accuracy of the Shack-Hartmann wavefront sensor for optical mirror surface measurement.

[0075] This method eliminates the reflected wavefront distortion caused by the relative position change between the optical mirror and the component during the measurement process.

[0076] Figure 5 Schematic diagrams of the wavefront propagation correction model and the equal-phase reflection conjugate model are shown. These models correct for reflected wavefront distortion caused by changes in the relative positions of optical components during measurement. During backtracking, the reflected wavefront W1 is solved based on the normal vector of the measured wavefront W′. A 256×256 sampling grid is used to reduce fitting errors. The normal vectors Nx, Ny, and Nz are calculated from the wavefront gradient within the same medium:

[0077]

[0078] When a wavefront propagates through a medium interface, its normal vector needs to be scaled. For example, when it enters a glass medium from free space, the normal vectors in the x and y directions are scaled to Nx / n and Ny / n, respectively, corresponding to W3→W2; conversely, when it goes from W2 to W1, the normal vectors are scaled to Nx·n and Ny·n, respectively. After the wavefront propagates in the opposite direction along the z axis, the forward wavefront W front (x,y,z) is converted to the backward wavefront W back (x′,y′,z′), the expression is as follows:

[0079]

[0080] where +P is the piston term. However, due to the change in the wavefront boundary, the aperture becomes D′. Specifically, for the converging wavefront, D′ increases after backtracing and can be restored to D by interpolation at the original sampling interval. For the diverging wavefront, D′ decreases, and radial polynomial extrapolation is required to fill the missing area to restore the aperture D. Considering the potential error introduced by boundary extrapolation, 90% of the original aperture D is ultimately retained. Therefore, the backtracing model established according to equations (3) and (4) is used to determine the reflected wavefront W1(x1, y1, z1) at the mirror position.

[0081] Plane wavefront O front It is incident and reflected on the mirror W0, forming a reflected wavefront W1. In the phase conjugate reflection mapping model, another plane wavefront O(x0,y0,0) is introduced at the lowest point of W1. According to the principle of equal phase plane, O front Each point on W0 propagates a distance T toward W0, and W0 is divided into two parts, t and t'. Taking a point A on W0 as an example, its projection point on plane wave O is A", and point A', which is in phase with A, satisfies: |AA'| = |AA"| = t, where AA' is the normal vector of W0 and W1. Therefore, points A, A', and A" form conjugate points, achieving phase conjugate reflection. This principle ensures that the normal distance from the conjugate points on W1 and O to W0 is equal to t. Based on this, the parameters x0', y0', z0' and t can be calculated, where x0', y0', z0' are the reconstructed mirror surface topography W0:

[0082]

[0083] Figure 6 The figure shows a schematic diagram of the propagation distance measurement method. For static distances, including the distance between the microlens array and the beam splitter prism or the size of the beam splitter prism, multiple contact measurements are used to minimize errors. For variable distances, including the distance between the beam splitter prism and the lowest point of the measured surface, the distance L from the beam splitter prism surface to the fixture is measured. ref As a reference, see the figure. Then use the previous arrow height L sag (design value or contact measurement value) and the displacement L of the fixture on the one-axis platform move Calculate the wavefront propagation distance L.

[0084] L=L ref +L move -L sag , (6)

[0085] Figure 7The measurement effect diagram of the propagation-corrected Shack-Hartmann wavefront sensor is shown, and an experimental measurement is carried out on a concave spherical mirror. The curvature radius R of the concave mirror is measured using a Taylor-Hobson profilometer, and the result is 200.17 mm. In order to reduce the influence of the wavefront propagation distance error, the mirror is measured at multiple positions. When the mirror to be measured is located at positions 1, 2, and 3, the total propagation distance (L1+L2+L3) is between 165 mm and 305 mm, which meets the reconstruction conditions. The influence of the wavefront propagation distance on the reconstruction accuracy can be derived from the differential form of formula (4), and the result is:

[0086] ΔW' max ≈1 / 5000·ΔL, (7)

[0087] Where ΔW′ max represents the maximum wavefront error, and ΔL represents the propagation distance perturbation. This result demonstrates the model's excellent stability under propagation distance perturbations. In the experiment, perturbations of ±0.2mm, ±0.4mm, ±0.6mm, ±0.8mm, and ±1mm were applied to L1, and the measurement was repeated three times to minimize the effects of vibration. The standard deviation of the final results was less than 0.3%.

[0088] Using a traditional Shack-Hartmann wavefront sensor alone would result in an inverted wavefront distribution, leading to a curvature radius error (ΔR) exceeding 125%. In contrast, this proposed model significantly improves measurement accuracy over a propagation distance of up to 140 mm, achieving an average curvature radius of 201.01 mm and a ΔR error within 2.5%.

[0089] It should be pointed out that, for those skilled in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A measuring device for a propagation-corrected Shack-Hartmann wavefront sensor, characterized in that: include: Collimated light source, beam splitter, microlens array, photodetector, moving sample fixture, slide, stepper motor drive module, data processing module; including: The collimated light source is used to generate a plane detection light, that is, a plane wavefront beam, which is directly incident on the beam splitter prism in the horizontal direction; The beam splitter prism is precisely arranged between the collimated light source and the optical mirror to be measured, with its incident surface forming a 45° angle with the horizontal plane; after the plane wavefront beam penetrates the inclined surface of the beam splitter prism, it is irradiated on the optical mirror to be measured at a vertical angle; the optical mirror to be measured reflects the wavefront beam back to the inclined surface of the beam splitter prism, and after reflection from the inclined surface, the beam is turned to the direction of the microlens array perpendicular to the original incident beam path; The microlens array is parallel to the reflected light path of the beam splitter prism and is installed on the outgoing light path of the beam splitter prism, and is used to decompose the reflected wavefront into multiple sub-wavefronts and focus them; The photodetector is firmly fixed on the focal plane of the microlens array and is used to collect the focused light spot array image; The fixture is used to support and fix the optical mirror to be measured, and it is mounted on the slide. The stepper motor drive module provides linear motion guidance for the slide, which can move precisely along the normal direction of the mirror to be measured, thereby achieving precise adjustment of the mirror position. The data processing module is used to reconstruct the reflected wavefront incident on the surface of the Shack-Hartmann wavefront sensor and restore the true surface shape of the mirror according to the wavefront propagation correction model and the equal phase reflection conjugate model; the data processing module is deployed in a computer in the form of software; The computer is connected to the photoelectric detector and stepper motor drive module through cables. On the one hand, it receives the light spot position data collected by the photoelectric detector, and on the other hand, it controls the displacement operation of the optical mirror, while performing key tasks such as wavefront reconstruction and surface shape calculation.

2. The measuring device according to claim 1, characterized in that: The collimating light source includes a semiconductor laser, a single-mode optical fiber, a fiber coupler, and a focus-adjustable collimating lens group; The semiconductor laser is directly coupled to the input end of the single-mode optical fiber to introduce the laser energy into the optical fiber. The output end is connected to the input end of the optical fiber coupler, and the light beam is transmitted to the focus-adjustable collimating lens group to calibrate the divergent light beam into a plane wavefront beam. The beam splitter is a 45°-45° optical glass prism with an anti-reflection coating and a partial reflection coating structure to maintain the quality of the incident wavefront; The microlens array is a regularly arranged plano-convex lens array, the microlens unit aperture is no more than 300 μm, and the focal length is less than 20 mm; The photodetector is a high-resolution CMOS image sensor with a pixel size of less than 2 μm and an effective pixel array greater than 3000×3000; The fixture has the function of translation and tilt adjustment along the x-axis and y-axis, with a maximum tilt range of ±5°; The slide is a mechanical transmission actuator with a high-precision linear guide at the bottom, which is precisely matched with a ball slider or roller slider to provide stable support. The linear guide is longer than 200mm and the motion resolution is better than 1μ. The stepper motor drive module drives the slide to move along the linear guide rail.

3. The measuring device according to claim 1, wherein: The collimated light source is used to adjust the divergence angle and uniformity of the output light beam to form a collimated plane wavefront with high uniformity and low wavefront error, which is used to illuminate the mirror to be measured; The beam splitter prism guides the reflected wavefront toward the microlens array and minimizes wavefront distortion caused by chromatic aberration and multiple reflections; The microlens array has high filling rate and low optical aberration characteristics to ensure high-precision spatial sampling of the reflected wavefront; Specifically, the microlens array uses circular, hexagonal, or square apertures, arranged in an array format, to decompose incident light into a corresponding number of sub-beams; the diameter, focal length, and shape of each sub-aperture in the array remain consistent; The photodetector is equipped with a low-noise readout circuit and an automatic gain control function, and is installed on the focal plane of the microlens array to collect the position of the light spot formed by the reflected wavefront with high precision, so as to achieve sub-micron-level center of mass offset measurement.

4. The measuring device according to claim 1, characterized in that The data processing module includes a centroid extraction unit, a wavefront reconstruction unit, a propagation correction calculation unit and a surface shape calculation unit; wherein: The centroid extraction unit is used to calculate the centroid position of each light spot in the image and output the offset; The wavefront reconstruction unit calculates the reflected wavefront phase distribution by a Zernike polynomial fitting algorithm according to the centroid offset; The propagation correction calculation unit is used to calculate the distortion of the reflected wavefront in the propagation path and restore the true wavefront shape at the reflection point; The surface shape calculation unit realizes accurate restoration of the mirror surface morphology by constructing a conjugate point mapping relationship.

5. The measuring device according to claim 4, characterized in that The wavefront reconstruction unit calculates the reflected wavefront phase distribution according to the centroid offset using a Zernike polynomial fitting algorithm. The specific process is as follows: Assume that the offsets Δx and Δy of each spot in the x and y directions are obtained by the spot centroid calculation unit; based on these offsets and the focal length of the microlens, the average wavefront slope k on each subaperture is calculated; then, the wavefront W is expressed using the Zernike polynomial expansion: Where C i is the coefficient of the i-th Zernike polynomial, which is obtained by the following formula: K=Z·C (2) , (2) Where K is the wavefront slope matrix, and Z is the partial derivative matrix of each Zernike polynomial.

6. The measuring device according to claim 4, characterized in that The propagation correction calculation unit is used to calculate the distortion of the reflected wavefront in the propagation path and restore the true wavefront shape at the reflection point. The specific process is as follows: Starting from the normal vector of the measured wavefront W′, the reflected wavefront W1 is solved; using a 256×256 sampling grid, the normal vectors Nx, Ny, and Nz are calculated from the wavefront gradient in the same medium: When the wavefront propagates through the medium interface, its normal vector is scaled; for the wavefront from free space to glass medium, the normal vectors in the x and y directions are scaled to Nx / n and Ny / n respectively, corresponding to W3→W2; conversely, when W2→W1, the normal vectors are scaled to Nx·n and Ny·n respectively; after the wavefront propagates in the opposite direction along the z axis for a distance of -L, the forward wavefront W front (x,y,z) is converted to the backward wavefront W back (x′, y′, z′), the expression is as follows: Where +P is the piston term; due to the change in the wavefront boundary, the aperture becomes D′; specifically, for the converging wavefront, D′ increases after back tracing and can be restored to D by interpolation of the original sampling interval; while for the diverging wavefront, D′ decreases, and radial polynomial extrapolation is required to fill the missing area to restore the aperture D; considering that the boundary extrapolation may introduce errors, 90% of the original aperture D is retained in the end. Therefore, the back tracing model established according to formulas (3) and (4) is used to determine the reflected wavefront W1(x1, y1, z1) at the mirror position; Plane wavefront O front It is incident and reflected on the mirror W0, forming a reflected wavefront W1. In the phase conjugate reflection mapping model, another plane wavefront O(x0, y0, 0) is introduced at the lowest point of W1. According to the principle of equal phase surface, O front Each point on W0 propagates a distance T in the direction of W0, and is divided into two parts t and t' by W0; suppose a point A on W0, its projection point on the plane wave O is A", and the point A' with the same phase as A satisfies: |AA'|=|AA"|=t, where AA' is the normal vector of W0 and W1; therefore, points A, A' and A" constitute conjugate points, realizing phase conjugate reflection; this principle ensures that the normal distance from the conjugate points on W1 and O to W0 is equal to t; based on this, the parameters x0', y0', z0' and t are obtained, where x0', y0', z0' are the reconstructed mirror morphology W0: For static distance, including the distance between the microlens array and the beam splitter prism or the size of the beam splitter prism, multiple contact measurements are used to minimize errors. For variable distance, including the distance between the beam splitter prism and the lowest point of the measured surface, the distance L from the beam splitter prism surface to the fixture is measured. ref As a reference, the previous arrow height L is then used sag That is, the design value or contact measurement value and the displacement L of the fixture on the one-axis platform move Calculate the wavefront propagation distance L: L=L ref +L move -L sag , (6)。

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