A high-resolution automatic detection device and method for optical element surface defects

Abstract

A high-resolution automatic detection device for optical element surface defects based on the principle of frequency domain expansion of structured light illumination includes a laser light source, a collimating lens, an acousto-optic modulator, a spatial filter, a polarization control module, a reflector, an aperture, a spatial light modulator, a lens, a shading plate, a holographic diffraction plate, a beam splitter, a microscope objective, a planar array detector, and a control system capable of realizing synchronization of the acousto-optic modulator, the polarization direction controller, the spatial light modulator, and the planar array detector. At the same time, the present invention also discloses a high-resolution automatic detection method for optical element surface defects, which solves the problems of strong speckle noise and anisotropy of imaging modulation degree in the existing laser interference structured light illumination, can automatically perform synchronous switching of illumination light direction and phase, and real-time matching of polarization state, thus improving system stability, and can realize super-diffraction limit scale rapid measurement of surface defects or microstructures of optical components without special sample treatment or addition of fluorescent agent.

Description

Technical Field

[0001] The present invention belongs to the field of optical element surface defect detection, and in particular relates to a high-resolution automatic detection device and method for optical element surface defects. Background Art

[0002] With the rapid development of advanced laser technology, lasers are advancing towards higher power and higher energy, placing higher demands on the damage resistance of optical components under these extreme conditions. Existing research indicates that surface micro- and nano-scale defects generated during the manufacturing process of optical components are the fundamental cause of their damage resistance. Effectively suppressing these micro- and nano-scale defects has become a critical scientific challenge in ultra-precision manufacturing and laser damage resistance. Detection, the eyes of ultra-precision manufacturing, is a core prerequisite for effectively suppressing these micro- and nano-scale defects. Currently, methods for detecting surface defects in optical components include microscopy, optical scattering, interferometry, and laser confocal imaging. Limited by the Abe-Rayleigh diffraction limit, these traditional methods can only detect defects within the diffraction limit. The current insufficient detection resolution directly restricts further advancements in ultra-precision manufacturing of optical components, hindering the upgrading of defect suppression and surface processing technologies. Since the late 20th century, physics has achieved significant breakthroughs in numerous attempts to break the diffraction limit and achieve high-resolution imaging. Researchers have developed a variety of high-resolution microscopy techniques, including AFM, SNOM (Scanning Near-field Optical Microscopy), SEM, stimulated emission depletion (STED) microscopy, structured illumination microscopy (SIM), stochastic optical reconstruction microscopy (STORM), and photoactivated localization microscopy (PALM). AFM and SNOM offer ultra-high spatial resolution (<1 nm for AFM and tens of nanometers for SNOM), but their detection efficiency is extremely low and their working distance is very short, making their application in optical component inspection extremely challenging. SEM, on the other hand, is generally only suitable for inspecting the surface topography of metallic components. Optical components require treatment such as gold sputtering, which can introduce invasive contamination and makes it unsuitable for surface defect detection in precision optical components. Other high-resolution imaging techniques (such as STED, STORM, and PALM) require fluorescent labeling and are therefore limited to biomedical imaging. It was not until Littleton et al. proposed high-resolution microscopy technology based on structured light illumination that they used structured light illumination to encode high-frequency information into the low-frequency region to expand the spectrum range of the optical microscopy system and obtain information on higher-frequency components. This made it possible to break through the diffraction limit without the aid of fluorescent molecular labels, while also having the advantages of wide-field imaging and three-dimensional tomography.

[0003] In the early days, high-resolution microscopy technology based on structured light illumination was achieved by adding a sinusoidal grating to the illumination light path. The ±1-order light generated by diffraction interfered on the sample surface to produce structured illumination light. However, in a system based on grating-based structured light illumination, the grating must be rotated and moved to obtain sample images with different illumination directions and phases. This mechanical motion device will reduce the stability of the system and affect the high-resolution imaging effect.

[0004] Invention Patent 201110448980.8 discloses a high-speed structured illumination optical microscopy system and method based on a digital micromirror device (DMD). This method utilizes a digital micromirror device (DMD) to load fringe images with different illumination directions and phases, which are then miniaturized onto the sample through a microscope system to achieve structured light illumination, avoiding mechanical motion and improving system stability. However, this method utilizes direct fringe projection, which produces significantly lower illumination fringe modulation than dual-beam interferometry. This method is unsuitable for optical components made of transparent materials such as glass, affecting phase estimation accuracy and high-resolution image reconstruction. Furthermore, the incoherent LED light source employed has a certain bandwidth, which can introduce artifacts in high-resolution image reconstruction.

[0005] Invention patent 201510544833.9 discloses a dual-channel structured light illumination high-resolution imaging method and device. This method uses a laser light source and a spatial light modulator to generate structured light illumination. This method offers improved light source coherence and higher fringe modulation, making it suitable for transparent materials such as optical glass. However, this method requires the addition of fluorescent materials, which introduces contamination. Furthermore, it also suffers from issues such as laser speckle background noise caused by a strongly coherent light source and uneven resolution improvement due to the anisotropy of the modulation of illumination fringe stripes in different directions. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the present invention provides a high-resolution automatic detection device and method for surface defects of optical components, which solves the problems of uneven resolution improvement and reduced imaging quality caused by strong laser speckle noise and anisotropy of modulation of illumination stripes in different directions in the existing technology.

[0007] In order to achieve the above objectives, the technical solutions adopted by the present invention are as follows:

[0008] A high-resolution automatic detection device for optical component surface defects features a laser light source, with a collimating lens, an acousto-optic modulator (AOM), a spatial filter, a polarization control module, a reflector, an aperture, a spatial light modulator (SLM), a lens, a light shield, a lens, a holographic diffraction plate, a lens, a spectroscope, a microscope objective, a sample, a lens, an area array detector, and a synchronization control system positioned sequentially along the propagation direction of the laser light source beam. By synchronously controlling the AOM, polarization direction controller, SLM, and SLM, a high-resolution image of the optical component is acquired simultaneously, enabling rapid measurement of defects and microstructures beyond the diffraction limit without requiring special sample treatment or the addition of fluorescent agents.

[0009] The acousto-optic modulator is used for rapid modulation of the on-off state of the laser beam, thereby avoiding a decrease in the modulation degree of the structured light illumination stripes caused by an intermediate state when the pixel state of the spatial light modulator is switched.

[0010] The spatial filter is used to filter out the side lobe components of the Gaussian beam, improve the illumination uniformity, and eliminate the annular background noise of the collected image.

[0011] The polarization control module is used for synchronous matching of polarization states when the structured light illumination direction is switched, thereby achieving isotropic imaging modulation.

[0012] The incident angle of the light incident on the spatial light modulator is less than 10°.

[0013] The light shielding plate is used to block other orders of diffracted light diffracted by the spatial light modulator and only allow ±1st order diffracted light to pass through.

[0014] The rotating holographic diffraction plate is placed at the intersection of the +1st order diffraction light and the -1st order diffraction light, and is used to appropriately reduce the coherence of the laser beam and weaken the laser speckle background noise of the collected image.

[0015] A structured light illumination imaging method based on a high-resolution automatic detection device for optical component surface defects is characterized in that the method includes the following steps (taking three-directional illumination as an example):

[0016] Step 1: Set the spatial light modulator to load fringe images as multiple images, switch them in sequence at a certain time interval, and load fringe images in the following order: illumination direction is 0°, phase is 0, illumination direction is 0°, phase is 2π / 3, illumination direction is 0°, phase is 4π / 3, illumination direction is 60°, phase is 0, illumination direction is 60°, phase is 2π / 3, illumination direction is 60°, phase is 4π / 3, illumination direction is 120°, phase is 0, illumination direction is 120°, phase is 2π / 3, illumination direction is 120°, phase is 4π / 3.

[0017] Step 2: Control the synchronization of the acousto-optic modulator, polarization direction controller, array detector and spatial light modulator. The detector sequentially collects nine original images of sample defects / microstructures illuminated by structured light at different phases in three illumination directions.

[0018] Step 3: Perform image preprocessing on the nine collected original images of the structured light illumination samples, including flat field correction to eliminate the impact of non-uniform illumination, slight toe cutting to reduce edge artifacts during Fourier transform, etc.

[0019] Step 4: Perform a two-dimensional Fourier transform operation on the preprocessed image in the frequency domain to obtain the spectrum distribution information corresponding to the image.

[0020] Step 5: Separate the spectrum information of the 0° lighting direction based on the three-phase linear separation method, solve the linear equations, and separate the low-frequency information of the 0° lighting direction (D N ) and high-frequency information of zero-frequency position dislocation (D P and D S ), calculate the spatial frequency k0 and phase of the structured light illumination stripes.

[0021] Step 6: Separate the high-frequency information D of the zero-frequency position dislocation in the 0° lighting direction P Spectrum shift +k0, D S Spectrum shift -k0, the shifted spectrum information D P 、D S In the frequency domain and D N Perform spectrum superposition and synthesis to complete the spectrum expansion in the 0° lighting direction.

[0022] Step 7: Repeat steps 5 and 6 to complete spectrum expansion in the 60° and 120° lighting directions. Superimpose the spectrum information after expansion in the 0°, 60°, and 120° lighting directions in the frequency domain to obtain isotropic frequency domain broadening.

[0023] Step 8: Perform inverse Fourier transform on the spectrum information obtained after three-way superposition and broadening to obtain a high-resolution image of the sample surface defects / microstructures.

[0024] Compared with the prior art, the advantages of the present invention are:

[0025] A laser light source is combined with a spatial light modulator to generate highly modulated structured light illumination stripes, and a rotating holographic diffraction plate is used to appropriately reduce coherence and weaken the laser speckle background noise, thereby improving imaging quality. At the same time, the illumination light direction and phase can be switched synchronously, polarization states can be matched in real time, images can be acquired, and high-resolution images can be reconstructed, achieving a uniform improvement in resolution in each illumination direction, thereby improving system stability and imaging speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the optical path of the high-resolution automatic detection device for optical element surface defects of the present invention;

[0027] Figure 2 It is a schematic diagram of the sunshade design of the present invention;

[0028] Figure 3 This is the fringe image sequentially loaded by the spatial light modulator of the present invention (partially enlarged view);

[0029] Figure 4 It is a schematic diagram of the computer synchronous control system of the present invention;

[0030] Figure 5 It is a structured light image of the surface defects of the fused silica optical element collected by the detector of the present invention;

[0031] Figure 6 is a spectrum information diagram of surface defects of the fused silica optical element of the present invention;

[0032] Figure 7 These are a high-resolution image and a common microscopic image of the surface defects of the fused silica optical element of the present invention. DETAILED DESCRIPTION

[0033] The present invention provides a high-resolution automatic detection device and method for optical element surface defects based on structured light illumination. Figure 1 The optical system shown in the figure is used to construct a detection device. This example uses four-directional structured light illumination as an example to perform high-resolution automatic imaging detection of surface defects on fused silica optical components.

[0034] The laser beam generated by the laser 1 in the device passes through the collimating lens 2, the acousto-optic modulator 3, and the spatial filter 4 to produce a collimated beam with a uniform light field distribution. The polarization state of the beam is quickly adjusted by the polarization control module 5 (including the polarizer 501, the half-wave plate 502, the electrically controlled liquid crystal polarization direction rotator 503 and the quarter-wave plate 504). After passing through the reflector 6 and the aperture 7, it is incident on the spatial light modulator at an incident angle of about 5°. After diffraction by the spatial light modulator 8, multi-level diffraction light is generated. The shading plate 10 is made of shading material and contains eight small holes in four directions, which are symmetrically distributed at 45° intervals (such as Figure 2As shown, when the direction of the loaded stripes on the spatial light modulator is changed, the directions of the ±1st-order diffracted light beams change. The spacing between the holes in the same direction must match the position and focal length of the first lens 9, so that the ±1st-order diffracted light beams in all four directions generated by the spatial light modulator can pass through the small holes, while the light beams of other diffracted orders are blocked by the light shield. The ±1st-order diffracted light beams pass through the second lens 11, the holographic diffraction plate 12, the third lens 13, and the beam splitter 14 before entering the microscope objective 15, forming interference structured light stripes on the surface of the optical element. The relative position of the +1st-order diffracted light beam and the -1st-order diffracted light beam is observed through a light screen. The holographic diffraction plate 12 is placed at the intersection of the +1st-order diffracted light beam and fixed on a rotating shaft. It is driven by a motor to rotate at a constant speed to appropriately reduce the coherence of the passing laser beam and weaken the laser speckle background noise in the captured image. The light returning from the sample is collected by the microscope objective 15 and imaged on the area array detector 18, obtaining an image of the defect in the optical element illuminated by structured light.

[0035] A high-resolution automatic detection method for surface defects of optical components based on structured light illumination comprises the following steps:

[0036] Step 1: Load the fringe image on the spatial light modulator. Set it to load multiple images and switch them in sequence at a certain time interval. The order of loading fringe images is as follows (e.g. Figure 3 The phase in the 0° direction is φ1, the phase in the 0° direction is φ2, and the phase in the 0° direction is φ3; the phase in the 45° direction is φ1, the phase in the 45° direction is φ2, and the phase in the 45° direction is φ3; the phase in the 90° direction is φ1, the phase in the 90° direction is φ2, and the phase in the 90° direction is φ3; the phase in the 135° direction is φ1, the phase in the 135° direction is φ2, and the phase in the 135° direction is φ3.

[0037] Step 2: Synchronize the AOM, polarization direction controller, light shield, array detector and spatial light modulator through computer control, such as Figure 4 As shown. When the spatial light modulator is loaded with a stripe image, the acousto-optic modulator is controlled to allow the laser beam to pass through and be incident on the spatial light modulator, and the polarization direction controller and the light shield are controlled to automatically rotate to match the polarization state of the light beam and the illumination direction, thereby obtaining a high-modulation structured light field corresponding to the illumination direction. When the spatial light modulator switches the stripe image, the acousto-optic modulator is controlled to prevent the laser beam from passing through, thereby avoiding a decrease in the modulation index of the structured light illumination stripes caused by the intermediate state when the spatial light modulator pixel state is switched. At the same time, the array detector sequentially collects 12 original images of the sample defects / microstructures illuminated by structured light at different phases in four illumination directions, as shown. Figure 5 shown.

[0038] Step 3: Perform image preprocessing on the 12 collected original images of the structured light illumination samples, including flat field correction to eliminate the impact of non-uniform illumination, slight toe cutting to reduce edge artifacts during Fourier transform, etc.

[0039] Step 4: Perform a two-dimensional Fourier transform operation on the preprocessed image in the frequency domain to obtain the spectrum distribution information corresponding to the image.

[0040] Step 5: Separate the spectrum information of the 0° lighting direction based on the three-phase linear separation method, solve the linear equations, and separate the low-frequency information of the 0° lighting direction (D N ) and high-frequency information of zero-frequency position dislocation (D P and D S ), calculate the spatial frequency k0 and phase of the structured light illumination stripes.

[0041] Step 6: Separate the high-frequency information D of the zero-frequency position dislocation in the 0° lighting direction P Spectrum shift +k0, D S Spectrum shift -k0, the shifted spectrum information D P 、D S In the frequency domain and D N Perform spectrum superposition and synthesis to complete the spectrum expansion in the 0° lighting direction.

[0042] Step 7: Repeat steps 5 and 6 to complete the spectrum expansion of the 60° and 120° lighting directions, and perform frequency domain superposition on the spectrum information after expansion of the four lighting directions of 0°, 45°, 90°, and 135° to obtain the frequency domain widening of each isotropic component (such as Figure 6 The blue dashed circles represent the spectrum information after expansion at 0°, 45°, 90°, and 135° illumination directions, respectively. The red dashed circle represents the spectrum information after frequency domain superposition of the four illumination directions, which is significantly expanded compared to the spectrum of ordinary microscopic imaging (green dashed circle).

[0043] Step 8: Perform inverse Fourier transform on the spectrum information obtained after four-way superposition and broadening to obtain a reconstructed high-resolution image of the optical element surface defect (such as Figure 7 (a)). Compared with the results of conventional microscopic imaging ( Figure 7 Compared with (b), the reconstructed high-resolution image has higher resolution and can provide richer detailed information on scratches and pit defects on the surface of optical components.

[0044] The above-described embodiments merely represent specific implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of protection of the present application. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the technical concept of the present application, and all such variations and improvements fall within the scope of protection of the present application.

Claims

1. A high-resolution automatic detection device for surface defects of optical components, characterized in that: The invention comprises a laser light source (1), a collimating lens (2), an acousto-optic modulator (3), a spatial filter (4), a polarization control module (5), a reflector (6), an aperture (7), a spatial light modulator (8), a first lens (9), a light shielding plate (10), a second lens (11), a holographic diffraction plate (12), a third lens (13), a beam splitter (14), a microscope objective lens (15) and a sample (16) distributed along the transmission direction of the beam splitter, a fourth lens (17) distributed along the reflection direction of the beam splitter, a planar array detector (18) and a computer are sequentially arranged along the propagation direction of the light beam of the laser light source (1). The computer synchronously controls the acousto-optic modulator (3), the polarization control module (5), the spatial light modulator (8), the area array detector (18) and the light shielding plate (10); The holographic diffraction plate (12) is placed at the intersection of the +1st order diffraction light and the -1st order diffraction light. The holographic diffraction plate is fixed on a rotating shaft and rotates at a certain speed to appropriately reduce the coherence of the passing laser beam and weaken the laser speckle background noise of the collected image. The acousto-optic modulator (3) performs rapid modulation through a voltage signal to control the rapid switching of the laser beam switch state, thereby avoiding a decrease in the modulation degree of the structured light illumination stripes caused by an intermediate state when the pixel state of the spatial light modulator (8) is switched; The spatial filter (4) is used to filter out the side lobe components of the Gaussian beam, improve the illumination uniformity, and eliminate the annular background noise of the collected image; The polarization control module (5) is used for synchronous matching of polarization states when the structured light illumination direction is switched, thereby achieving isotropic imaging modulation.

2. The high-resolution automatic detection device for surface defects of optical components according to claim 1, characterized in that: The incident angle of light incident on the spatial light modulator (8) is less than 10°.

3. The high-resolution automatic detection device for surface defects of optical components according to claim 1, characterized in that: The light shielding plate (10) is used to block other orders of diffracted light diffracted by the spatial light modulator (8) and only allow ±1st order diffracted light to pass through.

4. A structured light illumination imaging method using the optical element surface defect high-resolution automatic detection device according to claim 1, characterized in that: The method comprises the following steps: Step 1: The fringe image loading on the spatial light modulator is set to load multiple images, which are switched in sequence at a certain time interval. The order of loading fringe images is: the illumination direction is 0°, the phase is 0, the illumination direction is 0°, the phase is 2π / 3, the illumination direction is 0°, the phase is 4π / 3, the illumination direction is 60°, the phase is 0, the illumination direction is 60°, the phase is 2π / 3, the illumination direction is 60°, the phase is 4π / 3, the illumination direction is 120°, the phase is 0, the illumination direction is 120°, the phase is 2π / 3, and the illumination direction is 120°, the phase is 4π / 3; Step 2: Control the AOM, polarization direction controller, array detector and spatial light modulator to synchronize, and the detector sequentially collects 9 original images of sample defects / microstructures illuminated by structured light at different phases in three illumination directions; Step 3: Perform image preprocessing on the 9 collected original images of the structured light illumination samples, including flat field correction to eliminate the influence of non-uniform illumination and slight apodization to reduce edge artifacts during Fourier transform; Step 4: Perform a two-dimensional Fourier transform operation on the preprocessed image in the frequency domain to obtain the spectrum distribution information corresponding to the image; Step 5: Separate the spectrum information of the 0° lighting direction based on the three-phase linear separation method, solve the linear equations, and separate the low-frequency information D of the 0° lighting direction. N And the high frequency information D of zero frequency position dislocation P and D S , calculate the spatial frequency k0 and phase of the structured light illumination stripes; Step 6: Separate the high-frequency information D of the zero-frequency position dislocation in the 0° illumination direction P Spectrum shift +k0, D S Spectrum shift -k0, the shifted spectrum information D P , D S In the frequency domain and D N Perform spectrum superposition synthesis to complete spectrum expansion in the 0° lighting direction; Step 7: Repeat steps 5 and 6 to complete the spectrum expansion of the 60° and 120° lighting directions, and perform frequency domain superposition on the spectrum information after expansion of the three lighting directions of 0°, 60°, and 120° to obtain the frequency domain widening of each isotropic spectrum; Step 8: Perform inverse Fourier transform on the spectrum information obtained after three-way superposition and broadening to obtain a high-resolution image of the sample surface defects / microstructures.

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