Online detection device and method for vacuum picosecond laser damage of optical element based on scattered light imaging

Abstract

The invention discloses a device and a method for realizing vacuum picosecond laser damage on-line detection of an optical element based on scattered light imaging. The device comprises a picosecond laser, a first reflector, a vacuum chamber, a two-dimensional displacement sample table, a CCD imaging system, a first absorption well, a second absorption well, a first continuous zoom beam expander, a second reflector, a first detection laser, a second continuous zoom beam expander, a third reflector, a second detection laser and a control computer. According to the method, the CCD camera is used for capturing dark field scattered light images generated by a detection laser irradiation element damage test area, and accurate diagnosis of the vacuum picosecond laser damage of the optical element is realized according to comparison of the scattered light images before and after test laser irradiation. The method is flexible and convenient, the probe light can be incident from the front surface or the rear surface of the optical element to be detected according to the property of the optical element to be detected, the scattered light of the object image of the damage point is utilized to enhance the signal, and the detection resolution limit of an imaging system on typical point damage morphology generated by picosecond laser induction 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 realizing 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 terminal optical components severely limits the peak output capacity of the system, making the study of picosecond laser damage to optical components extremely important. Accurate online detection of picosecond laser damage to optical components is a necessary prerequisite for conducting picosecond laser damage research.

[0003] Traditionally, plasma flash method and bright field CCD imaging method are used to detect laser damage on the surface of components online. Long pulse nanosecond laser damage test is carried out in air environment. The minimum size and depth of nanosecond laser damage of optical components are often in the micron level. Therefore, both methods can perform good online detection of laser damage of components. However, in the process of short pulse picosecond laser damage test, due to the significant shortening of laser action time scale, the material cannot fully transfer heat after absorbing laser energy, which greatly 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 nanosecond damage. Taking the typical terminal optical elements of picosecond laser damage system - dielectric reflective film and dielectric film pulse compression grating as an example, the minimum size and depth of picosecond laser damage are often in the hundreds of nanometers, while the damage diagnosis resolution of the above two online detection systems is often in the micron level, which makes it difficult to accurately diagnose the picosecond laser damage of optical components.

[0004] In addition, in the process of short-pulse picosecond laser damage testing, in order to avoid the focused ultra-high peak power beam from breaking through the air or causing self-focusing effects that affect the accuracy of the test results, it is often necessary to conduct the test in a vacuum environment. The complex structure of the vacuum chamber not only limits the flexibility and freedom of the detection system, but may also lead to increased detection costs and increased operational complexity. These factors constitute major challenges in the current field of online picosecond laser damage detection.

[0005] Therefore, the development of 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 picosecond laser damage to optical components in a vacuum environment.

[0007] The technical solution of the present invention is as follows:

[0008] On the one hand, the present invention provides an online detection device for vacuum picosecond laser damage of optical components 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 export 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 move 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, and judge whether the test area is damaged by comparing the defect position coordinates and size in the two tables.

[0014] Furthermore, it also includes a first continuous variable power beam expander and a second continuous variable power 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 detection light diffraction angle θ 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 detection laser wavelength. 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: 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 of optical components based on scattered light imaging, which specifically comprises the following steps:

[0017] ① Evacuate the vacuum chamber, set the picosecond laser to output the picosecond laser required for the test 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 existing 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 have not changed, 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 environmental and imaging system shaking.

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

[0022] 1) The present invention utilizes the scattered light enhancement signal of the damaged point to perform imaging, which can improve the size accuracy of the smallest damaged point that can be resolved by the imaging system, 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 setting a separate detection window.

[0024] 3) The CCD imaging system is used to capture the scattered light image on 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 It is a schematic diagram of an 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 incident and rear surface incident detection modes, wherein (a) is front surface incident detection, and (b) is rear surface incident detection;

[0027] Figure 3The device of the present invention is used to collect the dark field scattered light image of the surface of the optical element before and after laser irradiation and the offline microscope magnified image of the damage position; Figure 4 This is a comparison chart of the online detection and offline microscope detection results of picosecond laser damage test on dielectric film grating samples. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with embodiments and drawings, but the protection scope of the invention shall not be limited thereto.

[0029] See also Figure 1 , Figure 1 Schematic diagram of an optical component vacuum picosecond laser damage online detection device. As shown in the figure, the optical component vacuum picosecond laser damage online detection device includes:

[0030] A picosecond laser 101, used to generate picosecond laser pulses required for damage testing;

[0031] A first reflector 102 guides the picosecond laser pulse into a vacuum chamber 103;

[0032] 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, wherein 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 and to export diffracted light, reflected light and transmitted light generated by the damage test laser through the optical element 105 to be tested;

[0033] The two-dimensional displacement sample stage 106 is placed in the vacuum chamber 103, used to fix the optical element to be tested, and can be moved on a two-dimensional plane, and the moving plane is perpendicular to the normal of the detection window 109, so as to realize the detection of different areas;

[0034] The CCD imaging system 110 is used to capture an image of the surface of the optical element 105 to detect laser damage, and 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 edge areas of the image due to defocusing;

[0035] 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;

[0036] The first detection laser 116 and the second detection laser 119 respectively generate detection light incident from the front surface and the back surface of the optical element 105 to be tested, so as to cover the laser damage test area.

[0037] The first continuously variable beam expander 114, the second reflector 115, the second continuously variable beam expander 117 and the third reflector 118 are used to guide and adjust the path and spot size of the detection light.

[0038] The picosecond laser pulse output by the picosecond laser 101 passes through the first reflector 102 and the incident window 104 of the vacuum chamber 103, and then irradiates the surface of the optical element 105 to be tested; 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 then is incident through the transmission window 113 of the vacuum chamber 103, irradiating the rear surface of the optical element 105 to be tested, 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 then is incident through the incident window 104 of the vacuum chamber 103, irradiating the front surface of the optical element 105 to be tested, covering the laser damage test area.

[0039] 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 105 to be measured from entering the CCD imaging system 110 and reducing the imaging contrast.

[0040] 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 wavelength λ of the detection laser must satisfy: Wherein d and f are the aperture diameter and focal length of the CCD imaging system 110 respectively.

[0041] 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.

[0042] 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, and the detection light spot area after beam expansion can be flexibly adjusted 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.

[0043] The method for online detection of vacuum picosecond laser damage of optical components based on scattered light imaging comprises the following steps:

[0044] 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;

[0045] ② The computer 120 controls the CCD imaging system 110 to take pictures of the damage test area before and after the single laser pulse irradiates the optical element 105 to be tested;

[0046] ③ The computer 120 automatically creates a defect information table of 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;

[0047] ④ The computer 120 compares the two established defect information tables. If the position coordinates and sizes of the defects in the two tables have not changed, 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.

[0048] See also Figure 2 . Figure 2 This is a schematic diagram of the detection of the detection light from the front surface and the back surface of the test sample. (a) is the detection of the 70° dielectric reflective film element by the front surface incident detection method. In order to avoid the reflected light of the detection light 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 avoided 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 574.7nm, and the detection laser wavelength λ is 655nm. 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.

[0049] See also Figure 3 , Figure 3The dark field scattered light images of the surface of the dielectric film grating sample before and after the laser damage test and the offline microscope magnified images of the same damage position are given. (a) is the initial dark field image of the surface before the test laser irradiates the sample. At this time, no damage occurs and the entire field of view is completely dark; (b) is 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, which has a high contrast and can accurately determine that damage has occurred; (c) is the offline microscope magnified image of the same damage position. The focused black spots appearing at the damage position in this image correspond to the bright spots in (b). The size of a single black spot is smaller than the image resolution of the CCD camera (5μm).

[0050] 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 zero probability damage threshold F of the grating measured by the scattered light online detection th The results are consistent with those of offline microscope detection, both of which are 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.

[0051] The experimental results show that the present invention can quickly and accurately perform online detection of damage points on the surface of optical components and improve the detection accuracy of the imaging system, thus meeting the actual needs of picosecond laser vacuum damage testing.

Claims

1. An online detection device for vacuum picosecond laser damage of optical components based on scattered light imaging, characterized in that: The device includes: 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), which are respectively used to introduce damage test laser and to export diffracted light, reflected light and transmitted light generated by the damage test laser through the optical element (105) to be tested; A two-dimensional displacement sample stage (106) is placed in the vacuum chamber (103), used to fix the optical element (105) to be measured and can move on a two-dimensional plane, and the moving 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 be tested to detect laser damage; A first detection laser (116) and a second detection laser (119) are used to generate detection light for irradiating the front surface and the rear surface of the optical element (105) to be tested, respectively, so as to cover the laser damage test area; The computer (120) is used to control 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, and automatically establish defect information tables of the test area before and after the laser irradiation based on the two images taken, and judge whether the test area is damaged by comparing the defect position coordinates and sizes in the two tables.

2. The optical element picosecond laser damage online detection device based on scattered light imaging according to claim 1 is characterized in that: 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, so as to ensure that all damage test areas are covered.

3. The optical element picosecond laser damage online detection device based on scattered light imaging according to claim 1 is characterized in that: When the optical element (105) to be measured is a dielectric film grating, in order 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, Among them, 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.

4. The optical element picosecond laser damage online detection device based on scattered light imaging according to any one of claims 1 to 3, characterized in that: Also includes: Picosecond laser, used to generate the picosecond laser pulses required for damage testing; The first reflecting mirror guides the picosecond laser pulse into the vacuum chamber.

5. A method for detecting vacuum picosecond laser damage of optical components based on scattered light imaging, using the vacuum picosecond laser damage online detection device for optical components according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: ① evacuating the vacuum chamber (108), setting the picosecond laser (101) to output the picosecond laser required for the test at a fixed frequency, and starting 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 taken 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 the table after laser irradiation shows new defect points (defects appear at different coordinate positions) or the defect points become larger (defect size becomes larger) compared to the table before laser irradiation, it is determined that the test area is damaged.

6. The optical component vacuum picosecond laser damage line detection method based on scattered light imaging according to claim 5 is 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 incidence 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.

7. The method for online detection of vacuum picosecond laser damage of optical components based on scattered light imaging according to claim 5, 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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