Aspheric interference measurement system based on spatial light modulator

Abstract

The invention discloses an aspheric interference measurement system based on a spatial light modulator. The device comprises a laser light source, a filtering, beam expanding and collimating module, a core interference light path, a 4f relay imaging module and an image sensor, the filtering, beam expanding and collimating module is used for generating high-quality collimated parallel light beams, and the core interference light path is used for dividing an incident light beam into a reference light path and a measurement light path and carrying out programmable wavefront phase modulation on the measurement light beam; and the 4f relay imaging module is used for transmitting the interference pattern to a detection plane without distortion. Through the flexible wavefront modulation capability of the spatial light modulator, rapid adaptive measurement of different aspheric surface shapes is realized, a traditional physical compensator does not need to be replaced, and the method has the advantages of high measurement precision, high universality and low cost, is suitable for surface shape detection of various spherical and aspheric optical elements, and has wide application prospects. The problems of poor system universality and low measurement efficiency caused by dependence on a special compensator in the prior art are solved.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, specifically to an aspherical interferometric measurement system based on a spatial light modulator. Background Technology

[0002] Aspherical optical elements, due to their unique advantages in correcting aberrations, simplifying system structure, reducing weight, and improving imaging performance, have become core components of modern high-end optical systems and are widely used in fields such as space remote sensing, photolithography equipment, astronomical observation, and high-end imaging. However, high-precision manufacturing heavily relies on high-precision inspection. The precise and efficient inspection of aspherical surface shapes remains a key technological bottleneck restricting their development.

[0003] Currently, mainstream aspherical surface inspection methods are mainly divided into two categories: contact measurement and non-contact measurement. Contact measurement, such as stylus profilometers, can directly obtain surface profile and height information, but its point-by-point scanning method is inefficient and carries the risk of scratching precision-polished surfaces, making it difficult to meet the needs of large-scale production. Non-contact measurement includes optical scanning methods and interferometry. Scanning methods are inefficient and require prior knowledge of the surface profile. Interferometry, due to its high sensitivity and ability to perform zero-point high-precision measurements, is highly anticipated, but it still faces many challenges in practical applications.

[0004] Aspherical interferometry presents the following challenges: Traditional compensated interferometry (including null compensation and partial compensation) relies on specially designed compensating optical elements (such as compensators and computational holograms) to convert aspherical wavefronts into easily measurable spherical or planar wavefronts. While this method offers high accuracy, the custom-matching nature of the compensators leads to poor versatility, long design and manufacturing cycles, and high costs. Furthermore, assembly and surface errors of the compensators directly affect the final measurement results, reducing measurement reliability.

[0005] Furthermore, interferometry faces a core dilemma when dealing with aspherical surfaces with large deviations (high asphericity): the trade-off between dynamic range and measurement accuracy. For high asphericity components, the resulting wavefront deviation is enormous, leading to excessively dense fringes in the interference field, even exceeding the spatial sampling limit of the detector. This results in the fringes being unable to be correctly resolved, a phenomenon known as "fringe aliasing." While fringe density can be reduced by decreasing sensitivity (e.g., using a longer wavelength), this directly sacrifices the system's measurement accuracy. Therefore, effectively expanding the interferometer's dynamic range without sacrificing accuracy has become a major challenge in aspherical surface detection.

[0006] In recent years, interferometry techniques based on spatial light modulators (SLMs) have offered promising solutions to the aforementioned problems. As a programmable optical element, the spatial light modulator can flexibly modulate the wavefront of light waves through electronic control, thus theoretically replacing traditional fixed compensators. How to construct an optical system that fully leverages the advantages of spatial light modulator technology while possessing high precision, high stability, and excellent anti-interference capabilities has become an important research direction in this field. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an aspherical interferometry measurement system based on a spatial light modulator, which has the advantages of rapid and high-precision detection of aspherical interference, strong versatility, and no need for traditional compensators, thus solving the aforementioned technical problems.

[0008] To achieve the above objectives, the present invention provides the following technical solution: an aspherical interferometric measurement system based on a spatial light modulator, comprising a laser source (1), a filtering beam expansion and collimation module, a core interferometric optical path, a 4f relay imaging module and an image sensor (12); The laser source (1) is used to emit laser light; The filtering, beam expanding, and collimating module includes a filter (2), a microscope objective (3), a spatial filter pinhole (4), and a collimating objective (5) arranged sequentially along the optical path. The light emitted by the laser source (1) passes through the filter (2) and the microscope objective (3) in sequence. The microscope objective (3) focuses the light beam onto the spatial filter pinhole (4) at its focal plane. The light beam passing through the spatial filter pinhole (4) is collimated into parallel light by the collimating objective (5) and enters the core interference optical path. The core interference optical path includes a first beam splitter (6), a second beam splitter (7), a third beam splitter (8), a fourth beam splitter (9), a plane mirror (13), a spatial light modulator (14), and a measured aspherical mirror (15) arranged along the optical path. Parallel light emitted from the collimating objective (5) passes through the first beam splitter (6) and is split into two beams by the second beam splitter (7). One beam is a reference beam, which is transmitted through the second beam splitter (7) to the plane mirror (13), reflected back by the plane mirror (13), and passes through the second beam splitter (7) again to reach the first beam splitter (6), and then... The beam is reflected from the second beam splitter (6) to the fourth beam splitter (9) and finally transmitted through the fourth beam splitter (9); another beam is the measurement beam, which is reflected from the second beam splitter (7) to the spatial light modulator (14), modulated by the spatial light modulator (14), reflected back to the second beam splitter (7) and transmitted through, then enters the third beam splitter (8) and is transmitted to the aspherical mirror under test (15); after being reflected by the aspherical mirror under test (15), the beam returns to the third beam splitter (8) along the original path, and is reflected at this cube to the fourth beam splitter (9), and finally reflected at the fourth beam splitter (9); The 4f relay imaging module includes a first relay lens (10) and a second relay lens (11) arranged sequentially along the optical path; the reference light emitted from the fourth beam splitter (9) and the measurement light are combined into one path, and pass through the first relay lens (10) and the second relay lens (11) in sequence, and finally converge on the target surface of the image sensor (12).

[0009] As a preferred technical solution of the present invention, the spatial filter pinhole (4) in the filter beam expanding and collimating module is located on the image-side focal plane of the microscope objective (3) and simultaneously on the object-side focal plane of the collimating objective (5).

[0010] As a preferred technical solution of the present invention, the spatial light modulator (14) is a reflective phase-type liquid crystal spatial light modulator, whose pixel array is programmably loaded with aspherical compensation phase distribution.

[0011] As a preferred technical solution of the present invention, the 4f relay imaging module is composed of a first relay lens (10) and a second relay lens (11) with different focal lengths. The distance between the two lenses is the sum of their focal lengths, forming a beam-shrinking optical path to proportionally reduce the size of the interference beam to match the target surface of the image sensor (12).

[0012] As a preferred technical solution of the present invention, the aspherical reflector (15) under test is mounted on a multi-dimensional adjustment frame to adjust its position and orientation in order to achieve precise alignment and focusing of the beam.

[0013] As a preferred technical solution of the present invention, the plane mirror (13) is mounted on a precision displacement platform, and the optical path of the reference optical path is changed by adjusting the precision displacement platform.

[0014] As a preferred technical solution of the present invention, the front focal plane of the first relay lens (10) of the 4f relay imaging module coincides with the beam combining plane of the fourth beam splitting cube (9), and the rear focal plane of the second relay lens (11) coincides with the target surface of the image sensor (12).

[0015] Compared with the prior art, the present invention provides an aspherical interferometric measurement system based on a spatial light modulator, which has the following advantages: 1. This invention uses a spatial light modulator as a fixed component of the system, combined with an optimized optical path structure, to achieve rapid adaptation and measurement of different aspherical mirrors. This avoids the problem of needing to replace different compensators in traditional interferometers, and improves the versatility and measurement efficiency of the equipment.

[0016] 2. Through its unique structural layout, the present invention maintains a compact system structure and improves the stability and reliability of the system by arranging multiple beam-splitting cubes in a specific position and using a fixed configuration of a 4f relay imaging system.

[0017] 3. By adopting a structural combination of conventional optical elements, this invention significantly reduces manufacturing costs while ensuring measurement accuracy. The beam-splitting cube adopts a conventional size design, avoiding the use of high-cost micro beam-splitting elements in traditional Fizeau interferometers, thus achieving good economic benefits. Attached Figure Description

[0018] Figure 1 Optical path diagram of an aspherical interferometry system based on a spatial light modulator; Figure 2 This is a diagram of the surface of the aspherical reflecting mirror being measured in the measurement system. Figure 3 This is an optical path data diagram for beam collimation in the measurement system; Figure 4 It is the phase diagram that needs to be loaded into the spatial light modulator in the measurement system; Figure 5 This is a diagram of the modulated coherent optical path data in the measurement system; Figure 6 It is a cross-sectional view of the light phase reflected from the aspherical mirror under test in the measurement system; Among them, 1-laser source; 2-filter; 3-microscope objective; 4-spatial filter pinhole; 5-collimating objective; 6-first beam splitter cube; 7-second beam splitter cube; 8-third beam splitter cube; 9-fourth beam splitter cube; 10-first relay lens; 11-second doublet lens; 12-image sensor; 13-plane mirror; 14-spatial light modulator (SLM); 15-aspheric mirror under test. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Please see Figures 1-6 An aspherical interferometric measurement system based on a spatial light modulator includes: a laser source (1), a filtering, beam expanding, and collimating module, a core interferometric optical path, a 4f relay imaging module, and an image sensor (12), such as Figure 1 As shown; The laser source (1) is used to emit laser light; The filtering, beam expanding, and collimating module includes a filter (2), a microscope objective (3), a spatial filter pinhole (4), and a collimating objective (5) arranged sequentially along the optical path. The light emitted by the laser source (1) passes through the filter (2) and the microscope objective (3) in sequence. The filter is used to purify the beam spectrum. The microscope objective (3) focuses the beam onto the spatial filter pinhole (4) at its focal plane. The beam passing through the spatial filter pinhole (4) is collimated into parallel light by the collimating objective (5) and enters the core interference optical path. The spatial filter pinhole (4) in the filter beam expanding and collimating module is located on the image-side focal plane of the microscope objective (3) and simultaneously on the object-side focal plane of the collimating objective (5). The core interference optical path includes a first beam splitter (6), a second beam splitter (7), a third beam splitter (8), a fourth beam splitter (9), a plane mirror (13), a spatial light modulator (14), and a measured aspherical mirror (15) arranged along the optical path. Parallel light emitted from the collimating objective (5) passes through the first beam splitter (6) and is split into two beams by the second beam splitter (7). One beam is a reference beam, which is transmitted through the second beam splitter (7) to the plane mirror (13), reflected back by the plane mirror (13), passes through the second beam splitter (7) again, and reaches the first beam splitter (6). The beam is reflected from the first beam splitter (6) to the fourth beam splitter (9), and finally transmitted through the fourth beam splitter (9); the other beam is the measurement beam, which is reflected by the second beam splitter (7) to the spatial light modulator (14), modulated by the spatial light modulator (14) and reflected back to the second beam splitter (7) and transmitted through, then enters the third beam splitter (8) and is transmitted to the aspherical mirror under test (15); after being reflected by the aspherical mirror under test (15), the beam returns to the third beam splitter (8) along the original path, and is reflected at this cube to the fourth beam splitter (9), and finally reflected at the fourth beam splitter (9); The spatial light modulator (14) is a reflective phase-type liquid crystal spatial light modulator, whose pixel array is programmably loaded with aspherical compensation phase distribution; The aspherical mirror under test (15) is mounted on a multi-dimensional adjustment frame to adjust its position and orientation to achieve precise alignment and focusing of the beam; The plane mirror (13) is mounted on a precision displacement platform, and the optical path of the reference optical path is changed by adjusting the precision displacement platform; The 4f relay imaging module includes a first relay lens (10) and a second relay lens (11) arranged sequentially along the optical path. The reference light emitted from the fourth beam splitter (9) and the measurement light are combined into one path and pass through the first relay lens (10) and the second relay lens (11) in sequence, finally converging on the target surface of the image sensor (12). The focal lengths of the first relay lens and the second relay lens are matched to form a standard 4f imaging optical path, which relays the interference pattern at the beam splitter surface of the fourth beam splitter to the target surface of the image sensor without aberration or magnification. When the optical path difference between the reference light and the measurement light is matched, clear interference fringes are formed at the image sensor. The accurate measurement of the surface shape of the aspherical mirror under test is achieved by analyzing the interference fringes. The 4f relay imaging module consists of a first relay lens (10) and a second relay lens (11) with different focal lengths. The distance between the two lenses is the sum of their focal lengths, forming a beam-shrinking optical path to proportionally reduce the size of the interference beam to match the target surface of the image sensor (12). The front focal plane of the first relay lens (10) of the 4f relay imaging module coincides with the beam combining plane of the fourth beam splitting cube (9), and the rear focal plane of the second relay lens (11) coincides with the target surface of the image sensor (12). The spatial light modulator (SLM), as a programmable virtual compensator, achieves flexible modulation of the wavefront of the measurement beam by loading different digital phase diagrams; the 4f relay imaging system ensures high-quality transmission of the interferogram; and the core interferometric optical path achieves a high degree of integration between the reference optical path and the measurement optical path through the compact layout of multiple beam-splitting cubes.

[0021] The data used in this embodiment is as follows: Laser specifications: Diameter 0.65mm; Wavelength 632.8mm; The specific parameters of the aspherical surface measured are: diameter 25.4 mm; aspherical coefficient -0.85; quartic coefficient of surface shape 2.5 × 10⁻⁷; hexagonal coefficient -8.3 × 10⁻¹¹.

[0022] The light emitted from laser source 1 passes sequentially through filter 2 and microscope objective 3. Microscope objective 3 focuses the beam onto a spatial filter pinhole 4 at its focal plane. The beam passing through spatial filter pinhole 4 is then collimated into high-quality parallel light by collimating objective 5 before entering the main optical path.

[0023] The formula for collimating the beam emitted from the spatial filter pinhole 4 in the filtering, beam expanding, and collimating module into parallel light is as follows: Total magnification: In the formula, The half-angle of the beam divergence at the pinhole. The focal length of microscope objective 3. This is the focal length of collimating objective lens 5.

[0024] Reference Figure 3 The microscope objective 3 has a magnification of 100x, the collimating objective 5 has a focal length of 70mm, and outputs parallel light.

[0025] The distance between the end of the microscope objective 3 and the spatial filter pinhole 4 is 3.8 mm; the distance between the spatial filter pinhole 4 and the collimating objective 5 is 73.54 mm, and the diameter of the output parallel beam is 23 mm.

[0026] The parallel light beam, after passing through the first beam splitter cube 6, is split into two beams by the second beam splitter cube 7. The reference light, after being transmitted through the second beam splitter cube 7 to the plane mirror 13, is reflected back along the same path, passes through the second beam splitter cube 7 again, reaches the first beam splitter cube 6, and is reflected at the first beam splitter cube 6 to the fourth beam splitter cube 9, and finally passes through the fourth beam splitter cube 9.

[0027] The other beam is the measurement beam, which is reflected by the second beam splitter 7 to the spatial light modulator SLM 14. After modulation, it is reflected back to the second beam splitter 7 and transmitted through, then enters the third beam splitter 8 and is transmitted through, reaching the aspherical mirror 15 under test. After being reflected by the aspherical mirror 15 under test, the beam returns to the third beam splitter 8 along the original path.

[0028] The compensation phase applied to the spatial light modulator SLM 14 can be calculated using the following formula: In the formula, The wavefront difference between the aspherical mirror 15 and the plane mirror 13 under test. λ is the wavelength of light in a vacuum; Reference Figure 5-6 The phase of the light reflected by the aspherical mirror 15 under test is basically the same as the phase of the reference light path. The maximum peak-to-valley difference of the wavefront is 0.5028 wavelengths, and the root mean square value is 0.0353 wavelengths. The output measurement light maintains essentially conjugate coherence, allowing for interferometric measurements. The measurement light is reflected at the third beam splitter cube to the fourth beam splitter cube, and finally reflected again at the fourth beam splitter cube.

[0029] The reference light and measurement light emitted from the fourth beam splitter 9 are combined into one path and pass through the first relay lens 10 and the second relay lens 11 in sequence, finally converging on the target surface of the image sensor 12.

[0030] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An aspherical interferometric measurement system based on a spatial light modulator, characterized in that: It includes a laser source (1), a filtering, beam expanding and collimating module, a core interference optical path, a 4f relay imaging module and an image sensor (12); The laser source (1) is used to emit laser light; The filtering, beam expanding, and collimating module includes a filter (2), a microscope objective (3), a spatial filter pinhole (4), and a collimating objective (5) arranged sequentially along the optical path. The light emitted by the laser source (1) passes through the filter (2) and the microscope objective (3) in sequence. The microscope objective (3) focuses the light beam onto the spatial filter pinhole (4) at its focal plane. The light beam passing through the spatial filter pinhole (4) is collimated into parallel light by the collimating objective (5) and enters the core interference optical path. The core interference optical path includes a first beam splitter (6), a second beam splitter (7), a third beam splitter (8), a fourth beam splitter (9), a plane mirror (13), a spatial light modulator (14), and a measured aspherical mirror (15) arranged along the optical path. Parallel light emitted from the collimating objective (5) passes through the first beam splitter (6) and is split into two beams by the second beam splitter (7). One beam is a reference beam, which is transmitted through the second beam splitter (7) to the plane mirror (13), reflected back by the plane mirror (13), and passes through the second beam splitter (7) again to reach the first beam splitter (6), and then... The beam is reflected from the second beam splitter (6) to the fourth beam splitter (9) and finally transmitted through the fourth beam splitter (9); another beam is the measurement beam, which is reflected from the second beam splitter (7) to the spatial light modulator (14), modulated by the spatial light modulator (14), reflected back to the second beam splitter (7) and transmitted through, then enters the third beam splitter (8) and is transmitted to the aspherical mirror under test (15); after being reflected by the aspherical mirror under test (15), the beam returns to the third beam splitter (8) along the original path, and is reflected at this cube to the fourth beam splitter (9), and finally reflected at the fourth beam splitter (9); The 4f relay imaging module includes a first relay lens (10) and a second relay lens (11) arranged sequentially along the optical path; the reference light emitted from the fourth beam splitter (9) and the measurement light are combined into one path, and pass through the first relay lens (10) and the second relay lens (11) in sequence, and finally converge on the target surface of the image sensor (12).

2. The aspherical interferometric measurement system based on a spatial light modulator according to claim 1, characterized in that: The spatial filter pinhole (4) in the filtering beam expanding and collimating module is located on the image-side focal plane of the microscope objective (3) and simultaneously on the object-side focal plane of the collimating objective (5).

3. The aspherical interferometric measurement system based on a spatial light modulator according to claim 1, characterized in that: The spatial light modulator (14) is a reflective phase-type liquid crystal spatial light modulator, whose pixel array is programmably loaded with aspherical compensation phase distribution.

4. The aspherical interferometric measurement system based on a spatial light modulator according to claim 1, characterized in that: The 4f relay imaging module consists of a first relay lens (10) and a second relay lens (11) with different focal lengths. The distance between the two lenses is the sum of their focal lengths, forming a beam-shrinking optical path to proportionally reduce the size of the interference beam to match the target surface of the image sensor (12).

5. The aspherical interferometric measurement system based on a spatial light modulator according to claim 1, characterized in that: The aspherical mirror under test (15) is mounted on a multi-dimensional adjustment frame to adjust its position and orientation to achieve precise alignment and focusing of the beam.

6. The aspherical interferometric measurement system based on a spatial light modulator according to claim 1, characterized in that: The plane mirror (13) is mounted on a precision displacement platform, and the optical path of the reference optical path is changed by adjusting the precision displacement platform.

7. The aspherical interferometric measurement system based on a spatial light modulator according to claim 4, characterized in that: The front focal plane of the first relay lens (10) of the 4f relay imaging module coincides with the beam combining plane of the fourth beam splitting cube (9), and the rear focal plane of the second relay lens (11) coincides with the target surface of the image sensor (12).

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