Large-aperture optical element defect positioning device and method based on defocus diffraction field

Abstract

The application provides a device and method for large-aperture optical element defect positioning based on a defocus diffraction field, uses a large-aperture lens, collects a defocus diffraction field at a defocus position near a focus point of the lens, and realizes positioning of defects of the large-aperture optical element through the defocus diffraction field. Compared with a traditional defect positioning mode by means of line scanning or plane scanning, the method can complete positioning through a single diffraction pattern, has a simple optical path, and has small data volume. Moreover, since the method is for positioning defects by means of a diffraction ring, the application has high resolution. Therefore, the application has the advantages that simple optical path is used, a large-target-surface camera is not needed, and high-precision defect positioning of a large-aperture element can be realized through a single image.

Description

Technical Field

[0001] This invention relates to the field of optical component defect detection, specifically to a device and method for rapid localization of defects in large-aperture optical components. Background Technology

[0002] Large-aperture optical components have important and wide applications in astronomy, aerospace, high-power lasers, and high-energy-density physics. Due to factors such as manufacturing processes, coating processes, and operating environments, optical components can contain defects of different sizes and types, which severely affect the performance of the device. Therefore, precise detection of defects in large-aperture optical components is essential for improving the performance of optical systems.

[0003] The main challenge in inspecting large-aperture optical components lies in the difficulty of simultaneously achieving high inspection accuracy and high efficiency due to their large size. High-precision defect characterization in a single test often requires a high-quality inspection component with a diameter similar to that of the component under test, as well as a camera with a large target surface and high pixel count, significantly increasing inspection costs. An effective inspection solution is to first use a lower-precision, high-efficiency inspection method to initially locate defects in the component, followed by further characterization using more precise inspection methods. Scanning inspection methods, such as array scanning or line scanning, can locate or inspect components of the entire aperture, but the inspection efficiency is reduced to varying degrees, and the scanning operation generates a large amount of data, causing inconvenience for subsequent data processing. Therefore, a more efficient and precise method for rapid defect location is one of the key requirements for meeting the current defect inspection needs of large-aperture components, thereby improving the output performance of optical systems.

[0004] Traditional methods for locating large-aperture optical components often involve line scanning or area scanning to achieve initial defect localization. However, these methods have low detection efficiency, errors caused by mechanical displacement during scanning are difficult to eliminate, and the data volume is enormous, causing inconvenience for subsequent data processing and storage. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a device and method for locating defects in optical components based on defocused diffraction fields. This invention uses a large-aperture lens to collect a single defocused diffraction field at a defocused position near the lens focal point, thereby locating defects in large-aperture optical components. This method features a simple optical path, low data volume, and by collecting the diffraction signals of the defects, only a single image is needed to locate micron-level defects on large-aperture components measuring hundreds of millimeters.

[0006] The technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a defect location device for optical elements based on defocused diffraction field, characterized in that it includes a helium-neon laser, a beam expander, an optical element under test, a lens, a photodetector, and a computer;

[0008] The laser 1 outputs light through a beam expander in its forward direction. The beam beam after the beam expander illuminates the optical element under test. After passing through the optical element under test, the beam beam illuminates the lens. The photodetector is placed at a defocused position after the focal point of the lens. The photodetector is connected to a computer. The photodetector is used to collect the defocused diffraction field and transmit it to the computer for processing.

[0009] Preferably, the beam expander is used to expand the incident beam into a parallel beam output that is similar in size to the component under test.

[0010] On the other hand, the present invention also provides a method for locating defects in optical elements based on defocused diffraction fields, characterized in that the method includes the following steps:

[0011] Step ①: Set up the optical path: Place the beam expander, lens and photodetector in sequence in the direction of the laser output light. The photodetector is placed at the defocus position after the focal point of the lens and connected to an external computer.

[0012] In step ②, the photodetector acquires the defocused diffraction field E and transmits it to the computer;

[0013] Step ③ involves the computer performing maximum value normalization on the acquired defocused diffraction field E to obtain the normalized defocused diffraction field. The complex amplitude E' is calculated using the following formula:

[0014]

[0015] In the formula, k is the phase conversion coefficient, and A0 is the plane wave.

[0016] Step ④ Calculate the transmission distance d3 of the complex amplitude E', using the following formula:

[0017]

[0018] d″=(d2-d′)

[0019]

[0020] f2 = d2 - f1

[0021] In the formula, d1 is the distance between the element under test and the lens, f1 is the focal length of the lens, and d2 is the distance between the lens and the photodetector;

[0022] Step 5: The light field is obtained by transmitting a beam with complex amplitude E' a distance d3 in free space, and the defect location of the component under test is obtained according to the light field distribution diagram.

[0023] Furthermore, in step ⑤, the defect location of the component under test is obtained based on the light field distribution map, specifically as follows:

[0024] Step 5.1 Flip the light field distribution map vertically first, then flip it horizontally;

[0025] Step 5.2 Convert the flipped light field distribution map to... The scale is adjusted to obtain a scaled light field distribution map, which is used to locate the defects in the component under test.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1) This invention utilizes a defocused diffraction field to locate defects in large-aperture components. The location is achieved through a single diffraction pattern, resulting in a simple optical path and a small amount of data.

[0028] 2) This invention locates defects by collecting their diffraction information. Compared to the defects themselves, the diffraction information of the defects has a larger spatial dimension. Therefore, this invention has high resolution and can locate defects in large-diameter components with high precision using a single image without relying on a large-area camera.

[0029] 3) Although the present invention uses a large-diameter detection element, the quality of the detection element will not have a significant impact on the positioning results. Attached Figure Description

[0030] Figure 1 This is an optical path diagram of the large-aperture optical element defect localization device based on defocused diffraction field according to the present invention.

[0031] Figure 2 Defect distribution in the component under test

[0032] Figure 3 Defocused diffraction field

[0033] Figure 4 Defect distribution calculated from the defocused diffraction field

[0034] Figure 5 .Distribution map of flipping defects Detailed Implementation

[0035] The present invention will be further described below with reference to embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0036] Please refer to the following first. Figure 1 , Figure 1The figure shows the optical path diagram of the large-aperture optical element defect localization device based on off-axis diffraction field according to the present invention. The device includes: a laser 1, a beam expander 2, an optical element under test 3, a lens 4, a photodetector 5, and a computer 6. The optical path of this device is explained below:

[0037] The laser output light travels through a beam expander. The beam, after passing through the beam expander, illuminates the optical element under test (DUT). After passing through the DUT, the beam travels a distance d1 before illuminating a lens with a focal length of f1. A photodetector is placed at a distance d2 behind the lens. The photodetector is connected to a computer.

[0038] The beam expander expands the incident beam into an outgoing parallel beam with a size similar to that of the device under test.

[0039] The photodetector is placed at a defocused position after the focal point of the lens, i.e., d2≠f1.

[0040] A method for locating defects in optical components based on off-axis diffraction fields, comprising the following steps:

[0041] The photodetector collects the defocused diffraction field E and transmits it to the computer, where it is normalized to its maximum value to obtain... The phase is converted by a certain coefficient k and combined with the plane wave A0 to form a new complex amplitude. This process can be represented by the following equation:

[0042]

[0043] The recombined complex amplitude E' propagates in free space. This propagation process can be described by the following partial differential equation.

[0044]

[0045] Where A is the complex amplitude of the light field, and k1 is the wavenumber in free space. By transmitting a distance d3, the defect of the component under test can be located from the obtained light field.

[0046] The method for determining d3 is as follows:

[0047] Suppose an ideal lens with a focal length of f2 = (d2 - f1) is placed at the location of the photodetector. This hypothetical lens and the lenses in the optical path will form a beam-constriction imaging system. The optical element under test (DUT) will be imaged at a position d4 behind the hypothetical lens after passing through this hypothetical beam-constriction imaging system. d4 can be calculated using object-image correspondence. Since the ideal lens is a pure phase-modulating element, this assumption has no effect on the image captured by the camera. First, calculate the image position of the DUT after passing through the first lens:

[0048]

[0049] in Let be the distance between the image plane and the first lens, where the image is formed after passing through the first lens. The distance from this image plane to the second lens is d″ = (d² - d′). Then, the image position after passing through the second lens is:

[0050]

[0051] have to Therefore, the image acquired by the photodetector is the diffraction field after traveling a distance d4 from the image plane. Once d4 is determined, d3 = d4, and the distance required to travel after defocusing the diffraction field can be confirmed.

[0052] The calculated defect distribution map is not a direct representation of the defect distribution of the component under test. Due to lens imaging, the calculated defect distribution map and the actual distribution have a vertical and horizontal flipping relationship. Furthermore, the calculated defect distribution map and the actual distribution are scaled, with the specific scaling ratio as follows: After these conversions, we can obtain the defect distribution map of the component under test from the calculated defect distribution map.

[0053] In this embodiment: the laser is a helium-neon laser with a beam length of 632.8 nm. The beam expander is a large-aperture beam expander that expands the laser beam size to a size comparable to the device under test. The photodetector is a CCD with a resolution of 1600*1200 and a pixel size of 5.5 μm. The sample under test is a planar optical element containing defects. The lens is a lens with a focal length of 400 mm.

[0054] The output light from laser 1 travels through beam expander 2, and the beam after beam expander 2 illuminates the optical element under test 3. After passing through optical element 3, the beam travels a distance of 200mm before illuminating a lens with a focal length of 400mm. A photodetector is placed 600mm behind the lens, i.e., 200mm off-focus. The photodetector is connected to a computer. The computer reads the off-focus diffraction field E measured by the photodetector.

[0055] E is normalized, and then applied to the plane wave with a phase conversion coefficient k = 1 to obtain the recombined complex amplitude E'.

[0056] The complex amplitude E' is transmitted in free space over a distance of -200mm using the angular spectrum transmission algorithm, and the defect distribution of the optical element under test can be obtained from the obtained light field.

[0057] Figure 2The diagram shows the defect distribution in the optical element under test. The spatial coordinates of the three defects are (1,1)mm, (1.5,-1.5)mm, and (-1,-1)mm, respectively. Their diameters are 600mm, 400mm, and 200mm, respectively. Figure 3 This refers to the defocused diffraction field obtained under the parameters of the above embodiment. Figure 4 This is a defect distribution map obtained by the method described in this patent under the parameters of the above embodiments. With the defocus setting, the measured defect distribution and the actual defect distribution have a flipped and scaled correspondence. In this embodiment, it is equivalent to flipping vertically and horizontally while shrinking by a factor of 2. Figure 5 This is the flipped defect distribution map. Here, the spatial coordinates of the three defects are (0.5, 0.5) mm, (0.75, -0.75) mm, and (-0.5, -0.5) mm. Doubling these coordinates gives the required spatial coordinates of the three defects.

Claims

1. A method for locating defects in optical elements based on defocused diffraction fields, characterized in that, The method includes the following steps: Step ①: Set up the optical path: Place the beam expander, lens and photodetector in sequence in the direction of the laser output light. The photodetector is placed at the defocus position after the focal point of the lens and connected to an external computer. Step ②: The photodetector collects the defocused diffraction field E and transmits it to the computer; Step ③ The computer performs maximum value normalization on the acquired defocused diffraction field E to obtain the normalized defocused diffraction field. The complex amplitude E' is calculated using the following formula: In the formula, The phase conversion coefficient, It is a plane wave. Step ④ Calculate the transmission distance d3 of the complex amplitude E', using the following formula: In the formula, The distance between the element under test and the lens. The focal length of the lens is... This is the distance between the lens and the photodetector; Step 5: The light field is obtained by transmitting the beam with complex amplitude E' a distance d3 in free space, and the defect location of the component under test is obtained according to the light field distribution diagram.

2. The optical element defect localization method based on defocused diffraction field according to claim 1, characterized in that, In step ⑤, the defect location of the component under test is obtained based on the light field distribution map, specifically as follows: Step 5.1 Flip the light field distribution map vertically first, then flip it horizontally; Step 5.2 Convert the flipped light field distribution map to... The scale is adjusted to obtain a scaled light field distribution map, which is used to locate the defects in the component under test.

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