Optical element vacuum picosecond laser damage on-line detection device and method based on scattered light imaging

Abstract

A kind of vacuum picosecond laser damage on-line detection device and method of optical element based on scattered light imaging is implemented, the device includes: picosecond laser, first mirror, vacuum cabin, two-dimensional displacement sample table, CCD imaging system, first absorption trap, second absorption trap, first continuous variable beam expander, second mirror, first probe laser, second continuous variable beam expander, third mirror, second probe laser and control computer.The method utilizes the CCD camera to capture the dark field scattered light image generated by the probe laser irradiation element damage test area, and according to the scattered light image contrast before and after the test laser irradiation, the precise diagnosis of optical element vacuum picosecond laser damage is realized.The method is flexible and convenient, the probe light can be incident from the front surface or rear surface according to the property of the optical element to be measured, the scattered light enhancement signal of damage point image is used, and the detection resolution limit of typical point-shaped damage morphology induced by picosecond laser is improved.

Description

Technical Field

[0001] The present invention relates to optical element picosecond laser damage detection, and in particular to a device and method for online detection of optical element vacuum picosecond laser damage based on scattered light imaging. Background Art

[0002] In picosecond high-energy petawatt laser systems, laser damage to the terminal optical components severely limits the system's peak output capability, making the study of picosecond laser damage to optical components crucial. Accurate online detection of picosecond laser damage to optical components is a prerequisite for conducting picosecond laser damage research.

[0003] Traditionally, plasma flash and brightfield CCD imaging methods are used for online detection of laser damage on component surfaces. Long-pulse nanosecond laser damage testing is conducted in an air environment. The minimum size and depth of nanosecond laser damage to optical components are often in the micron range, so both methods are effective for online detection of laser damage to components. However, during short-pulse picosecond laser damage testing, the laser action timescale is significantly shortened, and the material cannot undergo sufficient heat transfer after absorbing the laser energy. This significantly reduces the intensity of the thermal plasma flash signal generated by the damage, and also causes the initial damage point size to be smaller than that of nanosecond damage. Taking dielectric reflective films and dielectric film pulse compression gratings, typical terminal optical components of picosecond laser damage systems, as an example, the minimum size and depth of picosecond laser damage are often in the hundreds of nanometers. However, the damage diagnosis resolution of the two online detection systems mentioned above is often in the micron range, making it difficult to accurately diagnose picosecond laser damage to optical components.

[0004] Furthermore, during short-pulse picosecond laser damage testing, testing often requires vacuum to prevent the focused, ultra-high peak power beam from penetrating the air or causing self-focusing effects that could affect the accuracy of test results. The complex structure of the vacuum chamber not only limits the flexibility and freedom of the detection system but can also lead to increased testing costs and operational complexity. These factors constitute significant challenges in the current field of online picosecond laser damage detection.

[0005] Therefore, the development of an online detection technology suitable for picosecond laser damage, with high resolution and able to operate effectively in a vacuum environment has become a key issue that needs to be addressed urgently. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a device and method for online detection of vacuum picosecond laser damage to optical components based on scattered light imaging. This method can effectively improve the accuracy of online detection of vacuum picosecond laser damage to optical components.

[0007] The technical solutions of the present invention are as follows:

[0008] In one aspect, the present invention provides an online detection device for optical component vacuum picosecond laser damage based on scattered light imaging, which is characterized in that the device comprises:

[0009] The vacuum chamber is provided with an incident window, a diffraction window, a detection window, a reflection window, and a transmission window, which are respectively used to introduce damage test laser and to output diffracted light, reflected light, and transmitted light generated by the damage test laser through the optical element to be tested;

[0010] A two-dimensional displacement sample stage is placed in the vacuum chamber, used to fix the optical element to be measured and can be moved on a two-dimensional plane, and the moving plane is perpendicular to the normal of the detection window of the vacuum chamber;

[0011] A CCD imaging system is fixed to the detection window of the vacuum chamber and is perpendicular to the surface of the optical element to be tested, and is used to capture an image of the surface of the optical element to be tested to detect laser damage;

[0012] The first detection laser and the second detection laser are used to generate detection light for irradiating the front surface and the rear surface of the optical element to be tested, respectively, so as to cover the laser damage test area;

[0013] The computer is used to control the CCD imaging system to take pictures of the damaged test area before and after a single laser pulse irradiates the optical element to be tested, and automatically establish defect information tables of the test area before and after laser irradiation based on the two images taken. By comparing the defect position coordinates and sizes in the two tables, it is determined whether the test area is damaged.

[0014] Furthermore, it also includes a first continuous zoom beam expander and a second continuous zoom beam expander, which are respectively arranged on the output light paths of the first detection laser and the second detection laser, and are used to adjust the spot area of ​​the detection light to be larger than the spot area of ​​the damage test laser to ensure that all damage test areas are covered.

[0015] When the optical element to be measured is a dielectric film grating, the diffraction angle θ of the detection light can be calculated according to the grating diffraction equation: D(sinθ0+sinθ)=λ, where D is the grating period, θ0 is the incident angle, and λ is the wavelength of the detection laser. In order to make the diffracted light deviate from the field of view of the CCD imaging system, the detection laser wavelength λ must satisfy the following equation: Where d and f are the aperture diameter and focal length of the CCD imaging system respectively.

[0016] On the other hand, the present invention also provides a method for online detection of vacuum picosecond laser damage to optical components based on scattered light imaging, which specifically includes the following steps:

[0017] ① Evacuate the vacuum chamber, set the picosecond laser to output the required picosecond laser at a fixed frequency of 10 Hz, and start the first or second detection laser;

[0018] ② The computer controls the CCD imaging system to take pictures of the damaged test area before and after a single laser pulse irradiates the optical element to be tested;

[0019] ③ The computer automatically creates a table of defect information in the test area before and after laser irradiation based on the two images taken by the CCD imaging system, including the defect location coordinates and size;

[0020] ④ The computer compares the two established defect information tables. If the position coordinates and sizes of the defects in the two tables do not change, it is determined that the test area is not damaged. If the table after laser irradiation shows new defect points (defects appear at different coordinate positions) or the defect points become larger (the defect size becomes larger) compared to the table before laser irradiation, it is determined that the test area is damaged. The comparison of the defect position coordinates and size can be set with a certain tolerance according to the situation to eliminate errors caused by shaking of the environment and imaging system.

[0021] The technical effects of the present invention are:

[0022] 1) The present invention utilizes the scattered light of the damage point to enhance the signal for imaging, which can improve the dimensional accuracy of the minimum damage point that the imaging system can distinguish, and effectively realizes the online and accurate detection of picosecond laser damage testing of optical components.

[0023] 2) The present invention does not have high requirements for the beam quality of the detection laser, and the detection light can be directly incident from the laser incident window or the transmission window, rationally and cleverly utilizing the test laser window of the vacuum chamber, without the need to set up a separate detection window.

[0024] 3) The CCD imaging system is used to capture the scattered light image of the surface of the optical component to be tested, and the damage condition is judged by comparing the image difference before and after laser irradiation. It not only has the advantages of non-contact and non-destructive, but also can achieve high-precision online detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the online detection device for vacuum picosecond laser damage of optical components based on scattered light imaging of the present invention;

[0026] Figure 2 Schematic diagram of front surface incidence and back surface incidence detection methods, where (a) is front surface incidence detection and (b) is back surface incidence detection;

[0027] Figure 3The device of the present invention is used to collect dark field scattered light images of the surface of the optical element before and after laser irradiation and offline microscope magnified images of the damage location;

[0028] Figure 4 This is a comparison chart of the online detection and offline microscope detection results of the picosecond laser damage test on the dielectric film grating sample. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the embodiments and drawings, but the scope of protection of the invention shall not be limited thereby.

[0030] See also Figure 1 , Figure 1 The figure is a schematic diagram of an online detection device for optical component vacuum picosecond laser damage. As shown in the figure, the online detection device for optical component vacuum picosecond laser damage includes:

[0031] Picosecond laser 101, used to generate picosecond laser pulses required for damage testing;

[0032] A first reflecting mirror 102 guides the picosecond laser pulse into a vacuum chamber 103;

[0033] The vacuum chamber 103 is provided with an incident window 104, a diffraction window 107, a detection window 109, a reflection window 111, and a transmission window 113. The incident window 104, the diffraction window 107, the reflection window 111, and the transmission window 113 are respectively used to introduce damage test laser light and to output diffracted light, reflected light, and transmitted light generated by the damage test laser light through the optical element to be tested 105.

[0034] The two-dimensional displacement sample stage 106 is placed in the vacuum chamber 103 and is used to fix the optical element to be measured. It can be moved on a two-dimensional plane, and the moving plane is perpendicular to the normal of the detection window 109 to achieve detection of different areas.

[0035] The CCD imaging system 110 is used to capture images of the surface of the optical element 105 to detect laser damage. The CCD imaging system 110 is fixed to the detection window 109 of the vacuum chamber 103 and is perpendicular to the surface of the optical element 105 to avoid blurring of the left and right edges of the image due to defocusing.

[0036] The first absorption well 108 and the second absorption well 112 respectively absorb the diffracted light and the reflected light generated by the test laser passing through the optical element to be tested 105;

[0037] The first detection laser 116 and the second detection laser 119 generate detection light incident from the front surface and the back surface of the optical element to be tested 105, respectively, to cover the laser damage test area.

[0038] The first continuously zooming beam expander 114 , the second reflecting mirror 115 , the second continuously zooming beam expander 117 and the third reflecting mirror 118 are used to guide and adjust the path and spot size of the detection light.

[0039] The picosecond laser pulses output by the picosecond laser 101 pass through the first reflector 102 and the incident window 104 of the vacuum chamber 103 and irradiate the surface of the optical element to be measured 105; the detection light output by the first detection laser 116 passes through the second reflector 115 and the first continuously variable power beam expander 114 in sequence, and is incident through the transmission window 113 of the vacuum chamber 103, irradiating the rear surface of the optical element to be measured 105, covering the laser damage test area; the detection light output by the second detection laser 119 passes through the third reflector 118, the second continuously variable power beam expander 117 and the first reflector 102 in sequence, and is incident through the incident window 104 of the vacuum chamber 103, irradiating the front surface of the optical element to be measured 105, covering the laser damage test area.

[0040] In dark field imaging, it is necessary to prevent the reflected light or diffracted light generated by the detection light on the surface of the optical element to be measured 105 from entering the CCD imaging system 110 and reducing the imaging contrast.

[0041] When the optical element 105 to be measured is a dielectric film grating, the diffraction angle θ of the detection light can be calculated according to the grating diffraction equation: D(sinθ0+sinθ)=λ, where D is the grating period, θ0 is the incident angle, and λ is the wavelength of the detection laser. In order to make the diffracted light deviate from the field of view of the CCD imaging system 110, the detection laser wavelength λ must satisfy: Where d and f are the aperture diameter and focal length of the CCD imaging system 110 respectively.

[0042] The continuous laser wavelength output by the first detection laser 116 and the second detection laser 119 is 655 nm, which meets the above detection laser wavelength condition range. The output power thereof is continuously adjustable, and the peak output power is 1 W.

[0043] The expansion ratio adjustment range of the first continuously variable beam expander 114 and the second continuously variable beam expander 117 is 2 to 6 times, which can flexibly adjust the detection light spot area after beam expansion to be larger than the test laser spot area output by the picosecond laser, thereby ensuring that all damage test areas can be irradiated by the detection light.

[0044] A method for online detection of vacuum picosecond laser damage to optical components based on scattered light imaging includes the following steps:

[0045] ① Evacuate the vacuum chamber 108, set the picosecond laser 101 to output the picosecond laser required for the test at a fixed frequency of 10 Hz, and start the first detection laser 116 or the second detection laser 119;

[0046] ② The computer 120 controls the CCD imaging system 110 to take pictures of the damaged test area before and after a single laser pulse irradiates the optical element to be tested 105;

[0047] ③ The computer 120 automatically creates a table of defect information existing in the test area before and after laser irradiation based on the two images taken by the CCD imaging system 110, including the defect position coordinates and size;

[0048] ④ The computer 120 compares the two established defect information tables. If the position coordinates and sizes of the defects in the two tables do not change, it is determined that the test area is not damaged. If the table after laser irradiation shows new defect points (defects appear at different coordinate positions) or the defect points become larger (the defect size becomes larger) compared to the table before laser irradiation, it is determined that the test area is damaged. The comparison of the defect position coordinates and size can be set with a certain tolerance according to the situation to eliminate errors caused by shaking of the environment and the imaging system.

[0049] See also Figure 2 . Figure 2 The following diagrams illustrate the detection of a 70° dielectric reflective film component using the front-surface incident detection method. To prevent the reflected light from entering the CCD camera and affecting the dark field imaging contrast, the following conditions must be met: In the present invention, the aperture diameter d and the focal length f of the CCD imaging system are 44 mm and 122 mm respectively, so the reflection angle θ needs to be greater than 10.2°. For the dielectric reflective film, the reflection angle of the detection light is 70°, which meets the above conditions, so the reflected light will not enter the CCD imaging field of view; (b) is the rear surface incident detection method to detect the 70° dielectric film grating element. At this time, the -1 order transmission diffraction light of the detection light should be prevented from entering the CCD, so the -1 order transmission diffraction angle also needs to be greater than 10.2°. The -1 order transmission diffraction angle can be calculated according to the grating diffraction equation: D(sinθ0+sinθ)=λ, where the incident angle θ0 is 70°. The dielectric film grating line density to be measured in the present invention is 1740 lines / mm, and its corresponding grating period D is 574.7 nm. The detection laser wavelength λ is 655 nm. It can be calculated that the -1 order transmission diffraction angle is 11.5°, which meets the condition requirement that the diffracted light deviates from the CCD imaging field of view.

[0050] See also Figure 3 , Figure 3Dark-field scattered light images of the surface of a dielectric film grating sample before and after laser damage testing are presented, along with offline microscope magnified images of the same damage location. (a) shows the initial dark-field image of the surface before the test laser irradiates the sample. At this time, no damage has occurred, and the entire field of view is completely dark. (b) shows the dark-field image of the surface after the test laser irradiates the sample. A large number of bright spots appear in the center of the image, providing high contrast and accurately indicating damage has occurred. (c) shows an offline microscope magnified image of the same damage location. The focused black spots appearing at the damage location correspond to the bright spots in (b), and the size of each black spot is smaller than the CCD camera's image resolution (5μm).

[0051] See also Figure 4 . Figure 4 This is a comparison chart of the online detection and offline microscope detection results of the picosecond laser damage test of the dielectric film grating sample. The grating zero probability damage threshold F is measured by online detection using scattered light. th The results were consistent with those of offline microscope detection, both of which were 2.3 J / cm 2 , and under the same test laser flux, the damage probability detected by the two methods is also the same, indicating that this detection method has excellent accuracy.

[0052] Experimental results show that the present invention can quickly and accurately perform online detection of damage points on the surface of optical components, improve the detection accuracy of the imaging system, and meet the actual needs of picosecond laser vacuum damage testing.

Claims

1. A method for detecting vacuum picosecond laser damage lines in optical components based on scattered light imaging, characterized in that: An optical element vacuum picosecond laser damage online detection device is used, the device comprising: a vacuum chamber (103) provided with an incident window (104), a diffraction window (107), a detection window (109), a reflection window (111) and a transmission window (113), respectively used for introducing damage test laser and deriving diffracted light, reflected light and transmitted light generated by the damage test laser through the optical element to be tested (105); A two-dimensional displacement sample stage (106) is placed in the vacuum chamber (103), used for fixing the optical element (105) to be measured and movable on a two-dimensional plane, wherein the movable plane is perpendicular to the normal of the detection window (109) of the vacuum chamber (103); A CCD imaging system (110) is fixed to a detection window (109) of the vacuum chamber (103) and is perpendicular to the surface of the optical element (105) to be tested, and is used to capture an image of the surface of the optical element (105) to detect laser damage; A first detection laser (116) and a second detection laser (119) are respectively used to generate detection light for irradiating the front surface and the rear surface of the optical element (105) to be tested, so as to cover the laser damage test area; A computer (120) is used to control a CCD imaging system (110) to take pictures of a damaged test area before and after a single laser pulse irradiates the optical element to be tested (105), and automatically create defect information tables of the test area before and after laser irradiation based on the two images taken, and determine whether the test area is damaged by comparing the defect position coordinates and sizes in the two tables; The invention also includes a first continuously variable beam expander (114) and a second continuously variable beam expander (117) respectively arranged on the output light paths of the first detection laser (116) and the second detection laser (119), and used to adjust the spot area of ​​the detection light to be larger than the spot area of ​​the damage test laser to ensure that all damage test areas are covered; The optical element (105) to be measured is a dielectric film grating. To ensure that the diffracted light deviates from the field of view of the CCD imaging system, the following formula must be satisfied: D(sinθ0+sinθ)=λ, Where θ0 is the incident angle, λ is the wavelength of the detection light, θ is the diffraction angle, D is the grating period, d and f are the aperture diameter and focal length of the CCD imaging system, respectively; The method comprises the following steps: ① Evacuate the vacuum chamber (108), set the picosecond laser (101) to output the picosecond laser required for the test at a fixed frequency, and start the first detection laser (116) or the second detection laser (119); ② The computer (120) controls the CCD imaging system (110) to take pictures of the damaged test area before and after the single laser pulse irradiates the optical element (105) to be tested; ③ The computer (120) automatically creates a table of defect information existing in the test area before and after laser irradiation based on the two images captured by the CCD imaging system (110), including the defect position coordinates and size; ④ The computer (120) compares the two established defect information tables. If the position coordinates and sizes of the defects in the two tables do not change, it is determined that the test area is not damaged. If new defect points appear in the table after laser irradiation or the defect points become larger than those in the table before laser irradiation, it is determined that the test area is damaged.

2. The method for detecting optical component vacuum picosecond laser damage lines based on scattered light imaging according to claim 1, characterized in that: The method further comprises the steps of: adjusting the incidence of the detection light from the front surface or the rear surface of the optical element (105) to be measured, as well as the incident angle and / or the spot size, according to the type of the optical element (105) to be measured, so as to meet dark field imaging conditions.

3. The method for online detection of vacuum picosecond laser damage to optical components based on scattered light imaging according to claim 1, characterized in that: The comparison of defect position coordinates and size can be set with a certain tolerance according to the situation to eliminate errors caused by environmental and imaging system shaking.

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