A method for rapidly detecting the distribution of thermal sensitivity defects on the surface of an optical element

Abstract

The application discloses a method for rapidly detecting distribution of thermal sensitivity defects on a surface of an optical element, and particularly relates to the technical field of precise detection of optical elements, and comprises the following steps: laser continuous pumping system design, micro-interference microscopic imaging system design, and automatic control and data acquisition system design. The method can complete rapid, high-sensitivity and high-resolution detection of thermal sensitivity defects on the surface of the optical element with a diameter of 100 mm within several hours, and breaks through the technical bottleneck that detection precision and detection efficiency cannot be simultaneously achieved in traditional thermal sensitivity defect detection methods.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical element detection, in particular to a method for rapidly detecting the distribution of thermal sensitivity defects on the surface of an optical element. BACKGROUND

[0002] In high-energy strong laser devices, there are a large number of functional optical devices such as high-transmission, high-reflection, and polarization spectrometer. When there are thermal sensitivity defects on the surface of the optical substrate material or the thin film, the transmitted laser energy will be strongly absorbed and converted into heat through photoelectric or photoacoustic effect. The temperature rise or stress caused by this thermal field effect will break through the tolerance limit of the optical material, which can lead to laser-induced damage of the optical device. Therefore, detecting the thermal sensitivity defects of the optical element is of great significance for evaluating the manufacturing process level of the optical element and ensuring the stable operation of the high-energy laser system.

[0003] The thermal sensitivity defects mainly come from the absorption impurities, scratches, microcracks, and nodular defects introduced during the polishing or coating process of the element surface, which have micro-intermediate scale distribution characteristics from sub-micron to millimeter, and the absorption coefficient is usually in the weak absorption interval of 0.1ppm-100ppm. Although the traditional spectrophotometry and calorimetry methods are efficient, the detection precision cannot meet the requirements. The photothermal weak absorption detection technology based on the photothermal deflection principle can theoretically achieve a detection limit of ~1ppm, but it is limited to single-point testing area ≤50μm. For optical elements with a size ≥100mm, it takes months to achieve full-aperture coverage detection.

[0004] The prior art has the technical defect that the detection efficiency and detection precision cannot be balanced when detecting the full-aperture coverage of the 100mm-aperture optical element. SUMMARY

[0005] The purpose of the present application is to provide a method for rapidly and accurately detecting the distribution of thermal sensitivity defects on the surface of an optical element to solve the problems raised in the background art.

[0006] To achieve the above purpose, the present application provides the following technical solutions:

[0007] A method for rapidly detecting the distribution of thermal sensitivity defects on the surface of an optical element, comprising the following steps:

[0008] Design of detection system:

[0009] S1. Laser continuous pumping system: the wavelength of the laser output is consistent with or close to the wavelength of the actual application of the measured element, the laser beam is made to precisely coincide with the area of the surface of the optical element where the microscopic imaging interferes after the laser beam reaches the target surface by adjusting the reflecting mirror, and the beam shaper is used to shape the target surface spot into a flat-top uniform distribution and make the spot size consistent with the field size of the microscopic imaging interferes of the element surface, so that uniform strong laser pumping of the surface of the measured element is realized;

[0010] S2. Micro-area interference microscopic imaging system: used for microscopic interference imaging of the area of the measured element surface pumped by the laser, a typical Michelson interference imaging system is adopted, and different magnification switchable microscopic objectives are configured according to the needs of transverse resolution and detection speed;

[0011] S3. Automatic control and data acquisition system: including an electric translation stage for clamping the measured optical element and realizing two-dimensional programmed movement, a continuous laser output control unit and a microscopic image acquisition and processing unit, in the actual detection process, the two-dimensional translation stage moves step by step in the X and Y directions according to the program setting coordinates and path, the moving step is matched with the field of microscopic imaging, and each step is closed by the laser control unit, then the original microscopic interference image of the element surface under the condition of no pumping is collected, and then the laser output shutter is opened to collect the thermal pumping microscopic interference image of the element surface under the condition of laser output.

[0012] Preferably, the imaging CCD collects and outputs the microscopic interference images of the same detection area before and after thermal pumping in real time, the interference fringe changes caused by the heat package generated by the heat-sensitive defect absorbing laser energy are extracted through image processing, and the interference fringe distortion caused by the non-heat-sensitive defects on the surface of the optical element is avoided to interfere with the detection result.

[0013] Preferably, according to the extracted fringe distortion information, the local heat package deformation degree in the field of microscopic imaging of the element surface is calculated by combining the fringe distortion factor and the theoretical calculation formula of the surface profile fluctuation, and the relative light absorption intensity of the heat-sensitive defect is finally reflected.

[0014] Preferably, through the combination of step scanning and image splicing, the two-dimensional distribution and relative intensity information of the heat-sensitive defects in the full aperture range of the optical element can be realized, which is used for evaluating the defect control level of the surface of the optical element and predicting the laser damage resistance performance of the element under strong laser loading.

[0015] Compared with the prior art, the beneficial effects of the present application are:

[0016] The patent proposes a kind of optical element surface thermal sensitivity defect rapid detection method based on "continuous heat pumping + micro interference high-resolution microscopic imaging", can complete the rapid high-sensitivity, high-resolution detection of the surface thermal sensitivity defect of 100mm aperture optical element in several hours, break through the technical bottleneck of "detection precision and detection efficiency cannot be compatible" of traditional thermal sensitivity defect detection means. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is the schematic diagram of the detection of the present application;

[0018] Figure 2 It is the schematic diagram of the detection of the present application; DETAILED DESCRIPTION

[0019] EMBODIMENT

[0020] The main implementation principle of the present application is "continuous heat pumping + micro interference high-resolution imaging". The light source of heat pumping is a high-power solid continuous laser consistent with the wavelength of the actual application of optical element. The continuous laser (or quasi-continuous) emitted by the laser reaches the surface of the measured optical element after beam shaping, forming a uniform distribution of flat-top pumping spot of about 1mm x 1mm. When there is a thermal sensitivity defect in the irradiation area of the spot, the temperature gradient is generated between the defect site after absorbing laser energy and the non-defect site, and then thermal expansion and stress field are generated, which will cause a slight thermal package deformation of the optical film or substrate material of the defect site. The transverse and longitudinal dimensions of the deformation are related to the light absorption intensity of the defect, the defect size, the thermal diffusion length of the substrate material and the thermal expansion coefficient. Usually, the transverse scale is in the range of 1 μm ~ 100 μm, and the longitudinal height is several nm to hundreds of nm. Microscopic imaging of the 1mm x 1mm pumping area is carried out, and the height and transverse scale of the thermal package deformation of the region can be detected through the distortion degree and range of interference fringes, and then the size and light absorption intensity of the thermal sensitivity defect in the laser pumping region are reflected. In actual detection, the power density of pumping laser can be controlled to be close to the laser damage threshold of the measured element, so as to maximize the thermal package effect of the defect to improve the detection sensitivity. The specific technical principle is shown in the figure Figure 1 As shown in the figure, the laser pumping in the first field of view range does not cause the change of air stripe, and there is no thermal sensitivity defect. The interference fringes around the second defect produce slight distortion, and there is low-absorption thermal sensitivity defect. The interference fringes around the third defect produce serious distortion, and there is high-absorption thermal sensitivity defect.

[0021] The general design idea of the detection system mainly contains three parts. 1. Laser continuous pumping system: the laser output wavelength is consistent or close to the laser loading wavelength of the actual application of the measured element. The continuous laser beam is made to precisely coincide with the micro-interference imaging area on the surface of the optical element by adjusting the reflecting mirror. The beam shaper is used to shape the target surface spot into a flat-top uniform distribution and make the spot size consistent with the field size of the micro-interference imaging, so as to realize the uniform strong laser pumping of the measured element surface. 2. Micro-interference microscopic imaging system: used for micro-interference microscopic imaging of the laser pumped area on the surface of the measured element. A typical Michelson interference imaging system can be used. According to the needs of transverse resolution and detection speed, different magnification switchable microscopes can be configured, such as 2mm×2mm, 1mm×1mm and 0.5mm×0.5mm. For a typical 1mm×1mm detection field, the time for full coverage detection of a 100mm aperture optical element can be controlled within about 3 hours. 3. Automatic control and data acquisition system: including an electric translation stage for clamping the measured optical element and realizing two-dimensional programmed movement, a continuous laser output control unit and a microscopic image acquisition and processing unit. In the actual detection process, the two-dimensional translation stage moves step by step in X and Y directions according to the program setting coordinates and path, and the moving step is matched with the microscopic imaging field. Each step is closed by the laser control unit, and the original microscopic interference image of the element surface under the condition of no pumping is collected, and then the laser output shutter is opened to collect the thermal pumping microscopic interference image of the element surface under the condition of laser output. The imaging CCD collects and outputs the microscopic interference images of the same detection area before and after thermal pumping in real time. Through image processing, the interference fringe changes caused by the heat package generated by the heat-sensitive defect absorbing laser energy are extracted, so as to avoid the interference of the interference fringe distortion caused by the non-heat-sensitive defects on the surface of the optical element. According to the extracted fringe distortion information, combined with the theoretical calculation formula of the fringe distortion factor and the surface profile, the local deformation degree of the heat package in the microscopic imaging field of the element surface can be calculated, and the relative light absorption intensity of the heat-sensitive defect is finally reflected. Through step scanning and image splicing, the two-dimensional distribution and relative intensity information of the heat-sensitive defects in the full aperture range of the optical element can be realized, which is used to evaluate the defect control level of the surface of the optical element and predict the anti-laser damage performance of the element under strong laser loading.

Claims

1. A method for rapidly detecting the distribution of thermally sensitive defects on the surface of an optical element, characterized in that, The method comprises the following steps: Design of the detection system: S1. Laser continuous pumping system: the laser output wavelength is consistent or close to the optical element, and the laser beam is made to precisely coincide with the micro-interference imaging area of the surface of the optical element by adjusting the reflecting mirror after reaching the target surface, and a beam shaper is used to shape the target surface spot into a flat-top uniform distribution and make the spot size consistent with the field size of the micro-interference imaging of the surface of the optical element, so as to realize uniform high-power laser pumping on the surface of the optical element; S2. Micro-interference microscopic imaging system: used for micro-interference microscopic imaging of the area of the surface of the optical element pumped by the laser, a typical Michelson interference imaging system is adopted, and different magnification switchable micro-objectives are configured according to the needs of transverse resolution and detection speed; S3. Automatic control and data acquisition system: including an electric translation stage for clamping the optical element and realizing two-dimensional programmed movement, a continuous laser output control unit and a microscopic image acquisition and processing unit, in the actual detection process, the electric translation stage moves step by step in X and Y directions according to the program setting coordinates and path, the moving step is matched with the field of the microscopic imaging, and each step is controlled by the continuous laser output control unit to close the laser output, the original microscopic interference image of the surface of the optical element under the condition of no pumping is collected, then the laser output shutter is opened to collect the thermal pumping microscopic interference image of the surface of the optical element under the condition of laser output; The imaging CCD collects and outputs the microscopic interference images of the same detection area before and after thermal pumping in real time, the interference fringe changes caused by the heat package generated by the heat-sensitive defects absorbing laser energy are extracted through image processing, and the interference fringe distortion caused by the non-heat-sensitive defects on the surface of the optical element is avoided to interfere with the detection result; According to the extracted fringe distortion information, the local heat package deformation degree in the microscopic imaging field of the surface of the optical element is calculated by combining the fringe distortion factor and the theoretical calculation formula of the surface profile, and finally the relative light absorption intensity of the heat-sensitive defects is reflected; Through the combination of step scanning and image splicing, the two-dimensional distribution and relative light absorption intensity information of the heat-sensitive defects in the full aperture range of the optical element can be realized, which is used for evaluating the defect control level of the surface of the optical element and predicting the anti-damage performance of the optical element under the action of high-power laser.

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