A low-noise Fizeau interferometry test device and method based on all-solid-state random phase ring light source

Abstract

The present invention discloses a low-noise Fizeau interferometry test device and method based on an all-solid-state random phase ring light source. The device and method primarily include an all-solid-state random phase ring light source module, comprising a laser, a microscope objective lens, a pinhole filter, a collimating lens assembly, an axicon lens assembly, a focusing lens assembly, a beam splitter, a spatial light modulator, an alignment camera, and the like, as well as a Fizeau interferometry test module. The device and method of the present invention, through the all-solid-state random phase ring light source module, can reduce the impact of coherent noise generated by surface defects and dust particles on optical components during Fizeau interferometry testing, thereby improving the intermediate frequency transmission capability of the interferometer and achieving low-noise, high-precision measurement of the surface shape of planar optical components. The device also features a point light source operating mode, adapting to spherical, aspheric, free-form surface measurements, and transient measurements. The device also avoids the use of mechanical motion components, improving reliability and stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical precision detection, and in particular relates to a low-noise Fizeau interferometry testing device and method based on an all-solid-state random phase annular light source. Background Art

[0002] Interference detection technology, as the most accurate and widely used optical surface detection technology today, is the cornerstone of the modern optical industry. Its detection accuracy limits the maximum precision of optical processing. To improve the brightness and contrast of the interference image, modern interferometers typically use lasers as light sources. However, the coherence of lasers causes them to diffract and scatter at surface defects and dust on optical components, resulting in coherent superposition of the reference and detection beams with various stray light, generating a large amount of circular and streak-like coherent noise, reducing image quality and the system's transfer function. In large-aperture interferometers designed for extreme detection tasks such as ultra-high-power laser systems and extreme ultraviolet lithography systems, coherent noise is even more mixed with the required intermediate frequency information, severely restricting detection accuracy.

[0003] Existing low-coherence and extended-light sources can achieve good coherent noise suppression in some systems. However, due to reduced temporal or spatial coherence, the interference fringe contrast in long-cavity Fizeau interferometers, commonly used for interferometric detection, is reduced to almost zero, rendering them unusable. Placing a rotating frosted glass in the interferometer's imaging section has limited coherent noise suppression capabilities and can blur the interference fringes, degrading the system's transfer function.

[0004] Virtual ring light sources, such as rotating lenses, gratings, and wedge-shaped plates, have been shown to effectively suppress coherent noise in interferometers, but they are difficult to assemble and adjust, and suffer from poor stability. Real ring light sources, considered a simple and effective method for suppressing coherent noise and employed by companies such as ZYGO, still rely on mechanical moving elements such as rotating frosted glass. Furthermore, the introduction of these moving elements inevitably introduces additional errors, reducing the reliability and stability of the system. However, there are few reports, both domestically and internationally, on coherent noise suppression methods for Fizeau interferometers without mechanical moving elements, and on the corresponding all-solid-state, low-coherent-noise Fizeau interferometer test devices. Summary of the Invention

[0005] The present invention provides a low-noise Fizeau interferometry testing device and method based on an all-solid-state random phase ring light source, which is used for low-noise measurement of the surface shape of planar optical elements. It also provides a point light source working mode, which is suitable for spherical, aspheric, free-form surface measurement and transient measurement.

[0006] A low-noise Fizeau interferometer test device based on an all-solid-state random phase ring light source, comprising an all-solid-state random phase ring light source module and a Fizeau interferometer test module;

[0007] The all-solid-state random phase ring light source module includes: a laser, a microscope objective lens, a pinhole filter, a first collimating lens group, an axicon, a guide rail, a focusing lens group, a first beam splitter, a spatial light modulator, a neutral density filter, and an alignment camera; laser light is emitted from the laser and focused onto the pinhole filter through the microscope objective lens to complete spatial filtering; the point light source formed after filtering is collimated into parallel light by the first collimating lens group, and a Bessel beam is generated by the axicon; after the light beam passes through the focusing lens group and the first beam splitter, the transmitted light beam is focused on the spatial light modulator to form a ring light source, and the reflected light beam passes through the neutral density filter for light intensity attenuation and is focused onto the alignment camera to assist in alignment; the axicon is installed on the guide rail and is moved out of or into the light path by the guide rail to achieve switching between point light source mode and ring light source mode;

[0008] The Fizeau interferometer test module includes: a second beam splitter, a second collimator lens group, a standard transmission flat crystal, a measured plane, an imaging lens group and an imaging camera; the random phase ring light source provided by the all-solid-state random phase ring light source module is emitted from the spatial light modulator, reflected by the first beam splitter and enters the Fizeau interferometer test module, transmitted through the second beam splitter, and collimated by the second collimator lens group into parallel light, which is irradiated onto the standard transmission flat crystal and the measured plane and reflected respectively; the two beams of reflected light are coherently superimposed to form interference fringes, which are reflected by the second beam splitter and deflected to the imaging light path, and the imaging lens group images the interference fringes onto the imaging camera.

[0009] Furthermore, the laser is a tunable semiconductor laser.

[0010] Furthermore, the microscope objective lens and the first collimating lens group are confocal, and the pinhole filter is located on a common focal plane of the microscope objective lens and the first collimating lens group.

[0011] Furthermore, the spatial light modulator is located on the focal plane of the focusing lens group, is reflective, pure phase type or amplitude and phase type, and has a pixel size of no more than 20 microns.

[0012] Furthermore, the alignment camera is located on another focal plane of the focusing lens assembly, symmetrical with the spatial light modulator about the beam splitting surface of the first beam splitter. The imaging camera is conjugate with the measured plane.

[0013] In the present invention, all components except the guide rail are coaxial at the same height, and the axis is defined as the optical axis of the device, and the guide rail is perpendicular to the optical axis of the device.

[0014] The device of the present invention has two working modes: annular light source and point light source. The axicon is mounted on a guide rail and is moved out of or into the optical path by the guide rail to switch between the two modes: point light source mode when moved out of the optical path and annular light source mode when moved into the optical path.

[0015] The present invention also provides a low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source, using the above-mentioned low-noise Fizeau interferometry test device, comprising the following steps:

[0016] (1) Install and adjust the low-noise Fizeau interferometer test device so that the all-solid-state random phase ring light source module and the Fizeau interferometer test module are aligned and properly matched;

[0017] (2) Repeatedly loading several sets of phase distributions on the spatial light modulator and setting the exposure time of the imaging camera so that the exposure time of the imaging camera matches the repetition period of the phase;

[0018] (3) After the device is adjusted, the interference pattern is collected: the wavelength of the laser is adjusted to shift the phase, and the imaging camera is used to collect the phase-shifted interference patterns with phase differences of 0, π / 2, π, and 3π / 2 in sequence;

[0019] (4) The interference pattern is phase demodulated, unwrapped, and the tilt factor of the ring light source is corrected to finally obtain a low-noise surface shape measurement result of the measured plane.

[0020] In step (1), when adjusting the all-solid-state random phase annular light source module, all the light spots on the alignment camera should overlap at the center of the field of view and have the minimum light spot width.

[0021] In step (2), the phase loaded by the spatial light modulator varies depending on the operating mode, as follows:

[0022] In the ring light source mode, the spatial light modulator repeatedly loads N sets of random phases at a frame rate of 1 / T, N>100, and the imaging camera exposure time is set to NT to achieve sufficient coherent noise suppression''

[0023] In point light source mode, the spatial light modulator is loaded with a uniform phase, and there is no constraint on the exposure time of the imaging camera.

[0024] In step (3), the wavelength tuning step of the laser is related to the equivalent interference cavity length d of the Fizeau interferometer test module. When the phase shift step is π / 2, the tuning step Δλ of the laser is:

[0025]

[0026] Where λ is the center wavelength of the laser.

[0027] In step (4), the original surface shape W is corrected for the tilt factor according to the radius r of the annular light source and the focal length f of the interferometer collimating lens. The formula is as follows:

[0028]

[0029] Where (x, y) is the spatial coordinate.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The device and method of the present invention, using an all-solid-state random phase ring light source module, can reduce the impact of coherent noise generated by surface defects and dust particles on optical components during Fizeau interferometry testing, improving the interferometer's intermediate frequency transmission capability. This allows for low-noise, high-precision measurement of the surface shape of planar optical components and is also adaptable to spherical surface measurements. Furthermore, the device features a point light source operating mode, adapting to spherical, aspheric, free-form surface measurements, and transient measurements. The device also avoids the use of mechanical motion components, improving reliability and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of a low-noise Fizeau interferometry test device based on an all-solid-state random phase ring light source according to the present invention. DETAILED DESCRIPTION

[0033] The present invention will be described in further detail below with reference to the accompanying drawings and examples. It should be noted that the following examples are intended to facilitate understanding of the present invention and do not have any limiting effect on the present invention.

[0034] like Figure 1 As shown, a low-noise Fizeau interferometer test device based on an all-solid-state random phase ring light source includes an all-solid-state random phase ring light source module and a Fizeau interferometer test module.

[0035] In the all-solid-state random phase ring light source module, the laser light emitted by laser 1 passes through deflecting mirrors 2 and 3 before being focused by microscope objective 4 onto pinhole filter 5 for spatial filtering. The resulting point light source is collimated into parallel light by first collimating lens group 6, and a Bessel beam is generated by axicon 7. After passing through focusing lens group 8 and first beam splitter 9, the transmitted light beam is focused on spatial light modulator 10 to form a ring light source, which is then loaded with a dynamic random phase. The reflected light beam passes through neutral density filter 11 for light intensity attenuation and is focused onto alignment camera 12 to assist in alignment. Axicon 7 is mounted on guide rail 13 and can be moved into or out of the optical path via guide rail 13 to switch between point light source mode and ring light source mode.

[0036] A random phase ring light source is emitted from the spatial light modulator 10, reflected by the first beam splitter 9 and enters the Fizeau interferometer test module. It is transmitted through the second beam splitter 101 and collimated into parallel light by the second collimating lens group 102. It is irradiated onto the standard transmission flat crystal 103 and the measured plane 104 and reflected respectively. The two beams of reflected light are coherently superimposed to form interference fringes, which are reflected and deflected by the second beam splitter 101 to the imaging optical path. The interference fringes are imaged by the imaging lens group 105 and captured on the imaging camera 106.

[0037] In this embodiment of the present invention, the laser 1 is a tunable semiconductor laser. The microscope objective 4 and the first collimator lens group 6 are confocal, and the pinhole filter 5 is located on the common focal plane of the microscope objective 4 and the first collimator lens group 6. The spatial light modulator 10 is located on the focal plane of the focusing lens group 8 and is reflective, pure phase type, or amplitude and phase type, with a pixel size no larger than 20 microns. The alignment camera 12 is located on the other focal plane of the focusing lens group 8, symmetrically with the spatial light modulator 10 about the beam splitting surface of the first beam splitter 9. The imaging camera 106 is conjugate to the measured plane 104.

[0038] In the present invention, all components except the guide rail 13 are coaxial at the same height, and this axis is defined as the optical axis of the device, and the guide rail is perpendicular to the optical axis of the device.

[0039] The device of the present invention has two working modes: annular light source and point light source. The axicon 7 is mounted on a guide rail 13 and is moved out of or into the optical path by the guide rail 13 to switch between the two modes: point light source mode when moved out of the optical path and annular light source mode when moved into the optical path.

[0040] The method for performing low-noise Fizeau interferometry testing based on a solid-state random phase ring light source using the above device is as follows, taking plane testing as an example:

[0041] Step 1: Install and adjust the low-noise Fizeau interferometer test device based on an all-solid-state random phase ring light source, ensuring that the all-solid-state random phase ring light source module and the Fizeau interferometer test module are aligned and properly matched. During the device adjustment process, when the measured plane 104 is not in place, the image captured by the alignment camera 12 should show the reference ring light source reflected by the first beam splitter 9 and the self-collimated ring light source image formed by the reflection of the standard transmission flat crystal 103. Adjust the position of the components until the two rings overlap and have a minimum ring width. This indicates that the all-solid-state random phase ring light source module and the Fizeau interferometer test module are properly installed and properly matched. The optical path adjustment is complete, and the neutral density filter 11 and alignment camera 12 can be retained, removed, or used for other purposes.

[0042] Step 2: When testing the surface 104, the spatial light modulator 10 should remain on and continuously load a dynamic random phase. For surface testing, a ring light source mode is recommended. In this case, the exposure time of the imaging camera 106 should match the repetition period of the spatial light modulator 10: the spatial light modulator repeatedly loads N sets of random phases (N > 100) at a frame rate of 1 / T, and the imaging camera exposure time is set to NT to achieve sufficient coherent noise suppression.

[0043] Step 3: After the device is adjusted, collect the interferogram: adjust the wavelength of laser 1 to perform phase shifting, and use imaging camera 106 to sequentially collect phase shift interferograms with phase differences of 0, π / 2, π, and 3π / 2. The wavelength tuning step of laser 1 is related to the equivalent interferometer cavity length of the Fizeau interferometer test module: when the laser center wavelength λ and the equivalent interferometer cavity length d are equal to the phase shift step of π / 2, the laser tuning step Δλ is:

[0044]

[0045] Step 4: Phase demodulation, unwrapping, and tilt factor correction are performed on the interference pattern to finally obtain a low-noise surface shape measurement result of the measured plane. The tilt factor correction is performed on the surface shape W according to the radius r of the ring light source and the focal length f of the interferometer collimating lens:

[0046]

[0047] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low-noise Fizeau interferometer test device based on an all-solid-state random phase ring light source, characterized in that: Including all-solid-state random phase ring light source module and Fizeau interferometer test module; The all-solid-state random phase ring light source module includes: a laser, a microscope objective lens, a pinhole filter, a first collimating lens group, an axicon, a guide rail, a focusing lens group, a first beam splitter, a spatial light modulator, a neutral density filter, and an alignment camera; laser light is emitted from the laser and focused onto the pinhole filter through the microscope objective lens to complete spatial filtering; the point light source formed after filtering is collimated into parallel light by the first collimating lens group, and a Bessel beam is generated by the axicon; after the light beam passes through the focusing lens group and the first beam splitter, the transmitted light beam is focused on the spatial light modulator to form a ring light source, and the reflected light beam passes through the neutral density filter for light intensity attenuation and is focused onto the alignment camera to assist in alignment; the axicon is installed on the guide rail and is moved out of or into the light path by the guide rail to achieve switching between point light source mode and ring light source mode; The Fizeau interferometer test module includes: a second beam splitter, a second collimator lens group, a standard transmission flat crystal, a measured plane, an imaging lens group and an imaging camera; the random phase ring light source provided by the all-solid-state random phase ring light source module is emitted from the spatial light modulator, reflected by the first beam splitter and enters the Fizeau interferometer test module, transmitted through the second beam splitter, and collimated by the second collimator lens group into parallel light, which is irradiated onto the standard transmission flat crystal and the measured plane and reflected respectively; the two beams of reflected light are coherently superimposed to form interference fringes, which are reflected by the second beam splitter and deflected to the imaging light path, and the imaging lens group images the interference fringes onto the imaging camera.

2. The low-noise Fizeau interferometry test device based on an all-solid-state random phase ring light source according to claim 1, characterized in that: The microscope objective lens and the first collimating lens group are confocal, and the pinhole filter is located on a common focal plane of the microscope objective lens and the first collimating lens group.

3. The low-noise Fizeau interferometry test device based on a fully solid-state random phase ring light source according to claim 1, characterized in that: The spatial light modulator is located on the focal plane of the focusing lens group and is of reflective, pure phase type or amplitude and phase type, with a pixel size not greater than 20 microns.

4. The low-noise Fizeau interferometry test device based on an all-solid-state random phase ring light source according to claim 1, characterized in that: The alignment camera is located on another focal plane of the focusing lens assembly and is symmetrical with the spatial light modulator with respect to the beam splitting surface of the first beam splitter.

5. The low-noise Fizeau interferometry test device based on a fully solid-state random phase ring light source according to claim 1, characterized in that: The imaging camera is conjugate to the measured plane.

6. A low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source, characterized in that: Using the low-noise Fizeau interferometry test device according to any one of claims 1 to 5 comprises the following steps: (1) Install and adjust the low-noise Fizeau interferometer test device so that the all-solid-state random phase ring light source module and the Fizeau interferometer test module are aligned and properly matched; (2) Repeatedly loading several sets of phase distributions on the spatial light modulator and setting the exposure time of the imaging camera so that the exposure time of the imaging camera matches the repetition period of the phase; (3) After the device is adjusted, the interference pattern is collected: the wavelength of the laser is adjusted to shift the phase, and the imaging camera is used to collect the phase-shifted interference patterns with phase differences of 0, π / 2, π, and 3π / 2 in sequence; (4) The interference pattern is phase demodulated, unwrapped, and the tilt factor of the ring light source is corrected to finally obtain a low-noise surface shape measurement result of the measured plane.

7. The low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source according to claim 6, characterized in that: In step (1), when adjusting the all-solid-state random phase annular light source module, all the light spots on the alignment camera should overlap at the center of the field of view and have the minimum light spot width.

8. The low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source according to claim 6, characterized in that: In step (2), the phase loaded by the spatial light modulator varies depending on the operating mode, as follows: In ring light source mode, the spatial light modulator repeatedly loads N sets of random phases at a frame rate of 1 / T, where N>100, and sets the exposure time of the imaging camera to NT to achieve sufficient coherent noise suppression. In point light source mode, the spatial light modulator loads a uniform phase, and there is no constraint on the exposure time of the imaging camera.

9. The low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source according to claim 6, characterized in that: In step (3), the wavelength tuning step of the laser is related to the equivalent interference cavity length d of the Fizeau interferometer test module. When the phase shift step is π / 2, the tuning step Δλ of the laser is: Where λ is the center wavelength of the laser.

10. The low-noise Fizeau interferometry test method based on an all-solid-state random phase ring light source according to claim 6, characterized in that: In step (4), the original surface shape W is corrected for the tilt factor according to the radius r of the annular light source and the focal length f of the interferometer collimating lens. The formula is as follows: Where (x, y) is the spatial coordinate.

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