High-precision local mapping measurement system and method for surface damage density of optical elements

Abstract

This invention discloses a high-precision local mapping measurement system and method for surface damage density of optical components, aiming to solve the problem of non-uniform distribution of the laser spot affecting the measurement accuracy in existing technologies. This application establishes a coordinate mapping transformation relationship between the laser target irradiation spot and the optical component, realizing the spatial mapping between the optical component damage points and the local laser target spot. It then calculates and records the local flux, the number of damage points, and the number of pixels contained in each local region, and calculates the damage density for each flux interval. Innovatively, this application replaces the "average flux of the entire laser spot" in the prior art with the "local flux of the optical component damage points," achieving spatial mapping between the optical component damage points and the local laser target spot, effectively mitigating the impact of non-uniform laser spot distribution on the measurement accuracy of surface damage density.

Description

Technical Field

[0001] This invention belongs to the field of optical detection technology, and relates to the measurement of surface damage of optical elements, and particularly to the local mapping measurement of surface damage density of optical elements. Background Technology

[0002] Laser-induced damage has always been the biggest bottleneck restricting the development of high-power solid-state lasers to higher and stronger levels. Furthermore, laser-induced damage is characterized by its broad disciplinary scope, short duration, complex physical phenomena, numerous intertwined influencing factors, and difficulties in decoupling analysis, posing serious challenges to the measurement and evaluation of component damage performance. Currently, offline damage measurement is commonly used to characterize the damage performance of optical components. To continuously improve component fabrication processes, extend component lifespan, and enhance the reliability of laser devices, it is necessary to continuously improve the measurement accuracy of optical component damage performance characterization parameters.

[0003] Damage to optical components typically manifests as surface damage, and its performance can be characterized by four indicators: damage morphology, damage point geometry, damage threshold, and / or damage density. Damage morphology is generally used to study the physical mechanisms of laser-optical component interactions but is difficult to quantitatively characterize the damage performance of optical components. Damage point geometry is closely related to physical processes such as damage repair, damage growth, and laser operating strategies, and is generally not directly used to characterize damage performance. Compared to damage morphology and damage point geometry, the damage threshold is the most commonly used parameter to characterize the initial damage performance of optical components.

[0004] Patent application CN201610169105.9 discloses an optical element surface damage threshold testing system and its testing method. The system includes a laser, a liquid crystal light valve, a beam splitter, a first lens, a test sample, an absorption trap, a direct-viewing CCD, a reflector, an energy calorimeter, a second lens, and a monitoring CCD, arranged sequentially along the optical path. The laser beam emitted by the laser is shaped by the liquid crystal light valve and then reaches the beam splitter. The laser beam in the transmission direction of the beam splitter is focused onto the surface of the test sample by the first lens. The laser beam passing through the test sample passes through the absorption trap and reaches the direct-viewing CCD. The laser beam in the reflection direction of the beam splitter passes through the reflector, and the laser beam in the reflection direction of the reflector reaches the second lens and finally converges onto the monitoring CCD. The laser beam in the transmission direction of the reflector reaches the energy calorimeter. The testing method is as follows: Step 1): Establish the coordinate relationship between the liquid crystal light valve and the target surface, and calculate the transfer function; Step 2): Collect the coordinate information of the damage point in the direct-view CCD, and use the transfer function to calculate the position information on the liquid crystal light valve; Step 3): Obtain the position information on the monitoring CCD, select a certain pixel matrix, and use the near-field distribution of the light spot collected by the monitoring CCD to calculate the local flux. The testing principle is as follows: Near-field mapping technology is added to existing damage threshold testing methods (R-on-1 or 1-on-1 testing methods). Specifically, an online microscopic imaging system is used to observe whether damage has occurred on the surface of the component until a damage point is observed. The near-field distribution of the laser beam at the point of damage is acquired by a monitoring CCD. Absorption traps are removed, and the laser output is adjusted to low energy to acquire a near-field distribution image of the laser beam on a direct-viewing CCD. This near-field distribution image contains the position information of the damage point within the laser spot. Using this position information, the coordinates on the monitoring CCD are calculated based on the transfer function. Using these coordinates as the center, a suitable area is selected in the near-field image acquired by the monitoring CCD to calculate the flux. The calculation area is determined based on the size of the damage point. The flux calculation is based on the grayscale values ​​of the near-field image acquired by the monitoring CCD, thus completing the damage threshold calculation. The flux in the local area of ​​the damage point is the damage threshold of the defect.

[0005] Similar to existing damage threshold measurements, this testing method has advantages such as wide applicability and the existence of corresponding international standards (ISO 21254) to regulate the measurement. However, it also has significant drawbacks. Specifically, it is easily affected by damage measurement conditions such as spot size, measurement method, and sampling rate, and the damage threshold is difficult to directly correlate with the damage performance of the laser device. Unlike the damage threshold, damage density not only characterizes the initial damage performance of optical components but also the damage performance under high-throughput laser pulses. More importantly, damage density is more easily correlated with the damage performance of the laser device; therefore, high-precision measurement of damage density is currently a focus of attention.

[0006] Patent application CN202010353966.9 discloses a device and method for testing the damage density of large-aperture optical elements. The device includes: a laser source, an energy-adjusting waveplate, an energy-adjusting prism, a cylindrical lens, a first sampling mirror, an absorber, a beam quality analyzer, a second sampling mirror, an energy meter, a photoelectric probe, an oscilloscope, a long-focal-length microscope, a white light illumination source, a rotary motor controller, a data acquisition card, a three-dimensional sample motion motor controller for placing the large-aperture optical element sample, a microscope motion motor controller, and a computer. The laser beam output from the laser source is sequentially incident on the first sampling mirror through the energy-adjusting waveplate, the energy-adjusting prism, and the cylindrical lens. The reflected light from the front surface of the first sampling mirror is received by the beam quality analyzer to measure the light field distribution and transmit it to the data acquisition card. The reflected light from the rear surface of the first sampling mirror is incident on the second sampling mirror, and the reflected light from the second sampling mirror is received by the photoelectric probe. The photoelectric probe outputs... The first sampling mirror is connected to the input end of the oscilloscope, and the output end of the oscilloscope is connected to the data acquisition card. The transmitted light transmitted through the second sampling mirror is received by the energy meter to measure the beam energy and transmitted to the data acquisition card. The transmitted light transmitted through the first sampling mirror is incident on the test area of ​​the large-aperture optical element sample, and the remaining laser is absorbed by the absorber. After the long-focal microscope moves into the large-aperture optical element sample, it is illuminated by the white light source. The long-focal microscope is controlled by the microscope motion motor controller to detect damage at different locations of the optical element. The microscope motion motor controller is connected to the computer. The shutter of the laser source is connected to the computer to control the laser output. The energy adjustment waveplate is controlled by the rotary motor controller to adjust the test laser energy. The rotary motor controller is connected to the computer. The data acquisition card and the three-dimensional sample motion motor controller are respectively connected to the computer.

[0007] Similar to damage density measurements in existing technologies, this method employs a spliced ​​scanning irradiation method to measure damage density. The purpose of small-spot splicing scanning is to ensure that the surface of the component is irradiated over a large area in a nearly uniform manner. However, this method has high requirements for the stability of the spot flux and the stability of the spot spatial distribution, and it is difficult to mitigate the impact of the non-uniform distribution of the spot after splicing on the measurement. Summary of the Invention

[0008] The purpose of this invention is to address the problem in existing technologies where the non-uniform distribution of the laser spot after splicing affects the measurement results. This invention provides a high-precision local mapping measurement system and method for the surface damage density of optical components, enabling spatial mapping between damage points on the optical component and the local laser spot on the laser target surface, effectively mitigating the impact of non-uniform spot distribution on measurement accuracy.

[0009] The technical solution adopted in this invention is as follows:

[0010] A high-precision local mapping measurement system for surface damage density of optical components includes: a laser, a first mirror, a second mirror, an energy meter, a half-wave plate, a polarizer, a mapping transformation device, a lens, an optical wedge, a first three-dimensional translation stage, a second three-dimensional translation stage, an absorption trap, a microscope, a phototube-oscilloscope, a CCD camera, and a computer.

[0011] Before irradiation, the optical element under test (DUT) and microscope are moved using the first and second three-dimensional translation stages to select the measurement area of ​​the DUT. The microscope then acquires a pre-irradiation image of the measurement area of ​​the DUT. During irradiation, the laser beam is incident on the first reflecting mirror. The transmitted light after passing through the first reflecting mirror is incident on the energy meter. The reflected light after being reflected by the first reflecting mirror passes sequentially through the second reflecting mirror, a half-wave plate, a polarizer, a mapping transformation device, and a lens before being incident on the optical wedge. The reflected light reflected by the front surface of the optical wedge is received by a CCD camera to measure the near-field distribution of the target surface spot. The reflected light reflected by the rear surface of the optical wedge is received by a phototube-oscilloscope to measure the time waveform. The transmitted light transmitted through the optical wedge is incident on the DUT placed on the first three-dimensional translation stage. The transmitted light transmitted through the DUT is incident on an absorption trap on the second three-dimensional translation stage. After irradiation, the microscope is moved using the second three-dimensional translation stage, and the microscope acquires a post-irradiation image of the measurement area of ​​the DUT.

[0012] Furthermore, the laser output beam has pulse energy stability ≤ 5% RMS, laser polarization state is linearly polarized light with polarization degree ~100:1, pulse time waveform is smooth and stable, and pulse width instability ≤ 5% RMS.

[0013] Furthermore, the reflected light, after being reflected by the first mirror, passes sequentially through the second mirror, a half-wave plate, and a polarizer to obtain a measurement pulse with an extinction ratio on the order of ~100:1.

[0014] Furthermore, the relative distance between the target plane and the lens does not change during translation and rotation of the I-3D translation stage; the positioning accuracy of the I-3D translation stage during horizontal and vertical translation is ≤0.05mm; and the positioning accuracy of the I-3D translation stage during rotation is ≤0.1°.

[0015] Furthermore, the mapping transformation device is a mapping transformation device that can be moved into and out of the optical path without affecting the operation of the laser after being moved into the optical path, and that contains at least 3 non-collinear and distinguishable marker points that can generate intensity modulation of the downstream beam.

[0016] Furthermore, the microscope's field of view is larger than the size of the light spot on the optical element under test, the microscope's lateral resolution is ≤1μm, the microscope uses a broadband light source, and the maximum output power of the broadband light source is ≥100mW and the output power is continuously adjustable.

[0017] A high-precision local mapping measurement method for surface damage density of optical components, comprising the following steps:

[0018] Step S1, Optical path preparation

[0019] Select a clean measurement environment with humidity ≤50RH%, set up the measurement optical path, and adjust the half-wave plate and polarizer to obtain a measurement pulse with an extinction ratio on the order of ~100:1; the light spot sampled from the front surface of the optical wedge is used to measure the near-field distribution of the light spot on the target surface, and the light spot sampled from the rear surface of the optical wedge is used to measure the time waveform.

[0020] Step S2, Measurement Preparation

[0021] After adjusting the microscope's observation parameters and light source illumination parameters, the microscope's field of view, magnification, resolution, and depth of field were calibrated. The energy meter and phototube-oscilloscope underwent traceability calibration, and the nonlinearity and uniformity of the CCD camera's response were calibrated.

[0022] Step S3, calibrate the sampling coefficients

[0023] The sampling coefficient γ is measured using a traceable calibrated energy meter. The sampling coefficient γ refers to the transmitted energy E of the transmitted light after passing through the first reflecting mirror. w And the transmission energy E of the transmitted light after passing through the optical wedge. t The ratio:

[0024]

[0025] Among them, the transmission energy E w This is the reading on the energy meter;

[0026] Step S4, calibrate the mapping transformation relationship

[0027] The mapping transformation device is moved into the optical path for laser irradiation. The horizontal and vertical coordinates of the characteristic regions on the CCD camera and the calibration sample induced by the mapping transformation device are obtained, and the coordinate mapping transformation relationship between the laser irradiation spot and the damage point of the optical element is calculated.

[0028] Step S5, calibrate the mapping error

[0029] The mapping transformation device is translated perpendicular to the optical path direction, and laser irradiation is performed to obtain the x-coordinate of the characteristic region on the calibration sample where the light intensity induced by the mapping transformation device is concentrated. CCD_measure y-axis Combining the calculation formula in step S4, the x-coordinate of the corresponding feature region on the calibration sample on the CCD camera is calculated. CCD_cal y-axis CCD_cal Then calculate the coordinate transformation error Δ between the laser irradiation spot and the damage point of the calibration sample. single The calculation formula is:

[0030]

[0031] Repeat the above steps to obtain multiple coordinate transformation errors. Perform Gaussian fitting on all coordinate transformation errors to obtain the standard deviation Δ of the mapping error. st-er Remove the mapping and transformation device from the measurement optical path;

[0032] Step S6: Obtain the image before irradiation.

[0033] In step S2, the observation parameters of the microscope and the illumination parameters of the light source remain unchanged. The mapping transformation device is moved out of the optical path, the optical element under test is replaced with the calibration sample, the microscope is moved by the third-dimensional translation stage II, and the area under test of the optical element under test is photographed by the microscope to obtain the image before irradiation.

[0034] Step S7: Obtain the post-irradiation image.

[0035] After setting up the absorption trap, the mapping transformation device is moved into the optical path, allowing the laser beam to interact with the optical element under test. The total pulse energy E of the target plane is then calculated based on the energy meter reading and the sampling coefficient γ calibrated in step S3. T Then, the target surface spot area of ​​the measurement pulse is obtained by combining the spatial distribution of the beam measured by the CCD camera; the absorption trap is removed, the microscope is moved by the second three-dimensional translation stage, and the area to be tested of the optical element under test is photographed by the microscope to obtain the irradiated image.

[0036] Step S8, Image Processing

[0037] By subtracting the image before irradiation obtained in step S6 from the image after irradiation obtained in step S7, an effect image after laser pulse action is obtained; then, based on the centroid of each scattering point in the effect image, the horizontal and vertical coordinates of the damage points on the surface of the optical element under test are obtained.

[0038] Step S9, Local area division of light spot

[0039] The near-field distribution of the laser spot on the target surface acquired by the CCD camera is binarized (1 / e²) to obtain the outline of the laser irradiation spot. If the spot outline is circular, the circumscribed rectangle of the spot outline is first obtained, and then several square local regions are divided inside the circumscribed rectangle. If the spot outline is square, several directional local regions are directly divided inside the spot outline. The side length of each local region is the standard deviation Δ. st-er 6 to 10 times;

[0040] Based on the horizontal and vertical coordinates of the damage points on the surface of the optical element under test obtained in step S8, coordinate transformation is performed using the calculation formula in step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage points on the CCD camera; then, combined with the local area division of the light spot, the number of damage points in each local area is counted.

[0041] Step S10: Calculate the flux in the local region.

[0042] Based on the number of damage points in a local area, calculate the local area flux of the corresponding local area, and record the local area flux, the number of damage points, and the number of pixels contained in each local area.

[0043] Step S11: Repeat the measurement and calculate the damage density.

[0044] Adjust the second three-dimensional translation stage, select different measurement areas of the optical element under test, and repeat steps S6-S10 for each measurement area until the measurement of the entire light-transmitting area of ​​the optical element under test is completed.

[0045] The local flux values ​​of all local regions are statistically analyzed and sorted in ascending order. The minimum local flux value is used as the flux starting point. Each subsequent flux step is used as a flux interval, and so on, to classify the flux of all local regions into multiple flux intervals. The number of damage points and the number of pixels corresponding to each flux interval are summed, and the average value of all fluxes in each flux interval is used as the flux value of that flux interval. The damage density of each flux interval is then calculated.

[0046] Furthermore, in step S4, when calculating the coordinate mapping transformation relationship between the laser irradiation spot and the damage point of the optical element, the specific calculation formula is as follows:

[0047]

[0048] in, To calibrate the x and y coordinates of the characteristic regions on the sample, and The coordinate mapping transformation matrix, These are the horizontal and vertical coordinates of the corresponding feature regions on the calibrated sample on the CCD camera.

[0049] Furthermore, in step S10, when calculating the local flux of the corresponding local region, the specific calculation formula is as follows:

[0050]

[0051] Among them, E T g represents the total energy of the target plane pulse obtained in step S7. i，j Let ∑g be the grayscale value of the pixel at the i-th row and j-th column within the light spot outline. i，j ∑ represents the total pixel grayscale within the light spot outline. local g i，j N represents the total pixel grayscale value within a local area of ​​the light spot. t S represents the total number of pixels within a local area of ​​the light spot. pixel CCD is the area of ​​a single pixel unit.

[0052] Furthermore, in step S11, the specific calculation formula for calculating the damage density in each flux interval is as follows:

[0053]

[0054] in, For the damage density within a specific flux range, N m n represents the number of damage points within a specific flux range. f S represents the number of pixels within a specific flux range. pixel This represents the area of ​​a CCD pixel.

[0055] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0056] In this invention, a coordinate mapping transformation relationship between the laser irradiation spot and the damage points of the optical element is established to realize the spatial mapping between the optical element and the local spot of the laser target surface. Then, the local flux, the number of damage points, and the number of pixels contained in each local area are calculated and recorded. The flux intervals are divided according to a certain rule, and the damage density of each flux interval is calculated using the number of damage points and the number of pixels corresponding to each flux interval. Innovatively, the "local flux of optical element damage points" in this application replaces the "average flux of the entire spot" in the prior art, effectively mitigating the influence of non-uniform spot distribution on the accuracy of surface damage density measurement. Attached Figure Description

[0057] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is a schematic diagram of the optical path structure of the present invention;

[0059] Figure 2 This is a schematic diagram of the local area division of the light spot in this invention, wherein the outer rectangular frame represents a local area of ​​the light spot that has been divided, the circular frame represents the outline of the light spot within the local area, and the small dots represent damage points;

[0060] The attached figures are labeled as follows:

[0061] 1-Laser, 2-Second reflecting mirror, 3-First reflecting mirror, 4-Half-wave plate, 5-Energy meter, 6-Polarizer, 7-Mapping transformation device, 8-Lens, 9-Optical wedge, 10-I-Three-dimensional translation stage, 11-II-Three-dimensional translation stage, 12-Microscope, 13-Absorption trap, 14-Phototube-oscilloscope, 15-CCD camera, 16-Computer, 17-Optical element under test. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are merely some embodiments of the invention, not all embodiments. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of this invention.

[0063] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0064] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0065] The preferred embodiment of the present invention provides a high-precision local mapping measurement system for surface damage density of optical components, comprising a laser 1, a first reflecting mirror 3, a second reflecting mirror 2, an energy meter 5, a half-wave plate 4, a polarizer 6, a mapping transformation device 7, a lens 8, an optical wedge 9, a first three-dimensional translation stage 10, a second three-dimensional translation stage 11, an absorption trap 13, a microscope 12, a phototube-oscilloscope 14, a CCD camera 15, and a computer 16.

[0066] The beam output from laser 1 is adjusted for propagation azimuth by two mirrors, first mirror 3 and second mirror 2. The irradiation energy of the target surface is monitored by measuring the transmitted light through the first mirror 3. Then, the reflected light from the second mirror 2 passes through a half-wave plate 4 and a polarizer 6 to obtain a measurement pulse with a high extinction ratio (on the order of ~100:1). The pulse is then sampled by an optical wedge 9 to measure the near-field and time waveforms of the laser pulse on the target surface (the sampled spot from the first reflective surface of the optical wedge is used to measure the near-field distribution of the target surface spot, and the sampled spot from the second reflective surface of the optical wedge is used to measure the time waveform). The near-field distribution of the target surface spot meets the requirements of conjugate measurement. Laser 1 can repeatedly output laser pulses with a near-flat-top single-longitudinal-mode or a predetermined bandwidth, and its output wavelength matches the working wavelength of the test sample. The pulse energy stability of the beam output from laser 1 is ≤5% RMS, the laser polarization state is linearly polarized with a polarization degree of ~100:1, the pulse time waveform is smooth and stable, and the pulse width instability is ≤5% RMS.

[0067] The first three-dimensional translation stage 10 must meet the following requirements: it should be able to fix the sample, perform three-dimensional translation and one-dimensional rotation, and the implementation of the scanning translation and rotation functions should not change the relative distance between the target plane and the focusing lens; when translating in the horizontal and vertical directions, the horizontal and vertical translation stroke should meet the requirements of the test sample, and the positioning accuracy should be ≤0.05mm; when rotating, the rotation angle range should be sufficient to ensure that the laser incident angle meets the test requirements, and the positioning accuracy should be ≤0.1°; the lateral movement distance must be greater than the lateral geometric dimension of the component under test.

[0068] The second three-dimensional translation stage 11 must meet the following requirements: it should be able to achieve microscope focusing and vertical plane scanning functions, and the axial movement accuracy should be less than the microscope depth of field, and the lateral movement distance should be greater than the geometric size of the target surface spot.

[0069] The mapping transformation device 7 must meet the following requirements: it must contain at least 3 non-collinear and distinguishable markers that can produce an intensity suppression effect on the downstream beam, and be easy to move into and out of the optical path, and not affect the safe operation of the laser after being moved into the optical path.

[0070] The field of view (which can be spliced) of the microscope 12 must be larger than the size of the light spot on the optical element 17 to be tested. The lateral resolution of the microscope 12 is ≤1μm. The microscope 12 uses a broadband light source (such as white light) for bright field illumination. The purpose of selecting a broadband light source is to avoid interference between the scattered light induced by the damage point and the probe light. The maximum output power of the broadband light source used is ≥100mW and the output power is continuously adjustable.

[0071] Using the above-mentioned components, construct as follows Figure 1 The optical path shown.

[0072] Before irradiation, the optical element under test 17 and microscope 12 are moved using the first three-dimensional translation stage 10 and the second three-dimensional translation stage 11, and the measurement area of ​​the optical element under test 17 is selected. The microscope 12 acquires an image of the measurement area of ​​the optical element under test 17 before irradiation. During irradiation, the beam output from the laser 1 is incident on the first reflecting mirror 3. The transmitted light after passing through the first reflecting mirror 3 is incident on the energy meter 5. The reflected light after being reflected by the first reflecting mirror 3 passes sequentially through the second reflecting mirror 2, the half-wave plate 4, the polarizer 6, the mapping transformation device 7, and the lens 8 before being incident on the optical wedge 9. The reflected light from the front surface is received by the CCD camera 15 and used to measure the near-field distribution of the target surface spot; the reflected light from the rear surface of the optical wedge 9 is received by the phototube-oscilloscope 14 and used to measure the time waveform; the transmitted light transmitted through the optical wedge 9 is incident on the optical element under test 17 placed on the first three-dimensional translation stage 10, and the transmitted light transmitted through the optical element under test 17 is incident on the absorption trap 13 on the second three-dimensional translation stage 11; after irradiation, the microscope 12 is moved through the second three-dimensional translation stage 11, and the microscope 12 acquires the irradiated image of the measurement area of ​​the optical element under test 17.

[0073] When using the above-mentioned measurement system, the specific measurement method is as follows:

[0074] Step S1, Optical path preparation

[0075] Select a clean measurement environment with humidity ≤50RH%, set up the measurement optical path, and adjust the two reflectors together so that the measurement pulse output by the laser is incident on the optical component sample under test at a suitable angle; adjust the half-wave plate 4 and polarizer 6 to obtain a measurement pulse with an extinction ratio on the order of ~100:1; the light spot sampled from the front surface of the optical wedge 9 is used to measure the near-field distribution of the target surface light spot, and the light spot sampled from the rear surface of the optical wedge 9 is used to measure the time waveform; the near-field distribution of the target surface light spot meets the requirements of conjugate measurement.

[0076] Step S2, Measurement Preparation

[0077] The field of view (which can be stitched together) of microscope 12 must be larger than the geometric size of the target surface spot, and the lateral resolution of microscope 12 must be ≤1μm. A broadband light source (such as white light) is selected for bright-field illumination to avoid interference between the scattered light induced by the damage point and the probe light. The maximum output power of the broadband light source must be ≥100mW and the output power must be continuously adjustable. The axial movement accuracy of the second three-dimensional translation stage 11 must be less than the depth of field of microscope 12, and the lateral movement distance must be greater than the geometric size of the target surface spot. The optical element to be tested 17 is flexibly placed and fixed on the first three-dimensional translation stage 10. The nonlinearity and uniformity of the response of CCD camera 15 must be calibrated. The energy meter 5 and phototube-oscilloscope 14 must be traced and calibrated. Adjust microscope 12 so that microscope 12 is perpendicular to the side of the optical element to be tested 17. Repeatedly adjust the parameters of the illumination source and microscope 12 to obtain the best observation effect, and keep the aforementioned observation parameters unchanged in subsequent measurements. Then calibrate the following parameters of microscope 12 (or use the data in the microscope's factory inspection report): field of view, resolution, magnification, and depth of field.

[0078] It should be noted that there is no requirement for the order of calibration of the sampling coefficients in step S3 and calibration of the mapping transformation relationship and mapping error in steps S4-S5. You can calibrate the sampling coefficients in step S3 first and then calibrate the mapping transformation relationship and mapping error in steps S4-S5, or you can calibrate the mapping transformation relationship and mapping error in steps S4-S5 first and then calibrate the sampling coefficients in step S3.

[0079] Step S3, calibrate the sampling coefficients

[0080] The sampling coefficient γ is measured using a traceable calibrated energy meter 5. The sampling coefficient γ refers to the transmitted energy E of the transmitted light after passing through the first reflecting mirror 3. w And the transmission energy E of the transmitted light after passing through the optical wedge 9 tThe ratio:

[0081]

[0082] Among them, the transmission energy E w This is the reading on the energy meter;

[0083] At the same time, the relationship curve between the delay and output energy between each amplification stage of laser 1 can also be measured.

[0084] Step S4, calibrate the mapping transformation relationship

[0085] The calibration sample is placed at the position where the optical element 17 to be tested should be placed. The mapping transformation device 7 is moved into the optical path for laser irradiation. The horizontal and vertical coordinates of the characteristic regions on the CCD camera 15 and the calibration sample induced by the mapping transformation device 7 are obtained. The coordinate mapping transformation relationship between the laser irradiation spot and the damage point of the optical element is calculated. The specific calculation formula is as follows:

[0086]

[0087] in, To calibrate the x and y coordinates of the characteristic regions on the sample, and The coordinate mapping transformation matrix, The x and y coordinates are the corresponding feature regions on the calibrated sample on the CCD camera (15).

[0088] Step S5, calibrate the mapping error

[0089] After the coordinate transformation relationship is calculated, the mapping transformation device 7 is translated along the direction perpendicular to the optical path, and laser irradiation is performed to obtain the x-coordinate of the characteristic region on the calibration sample where the light intensity induced by the mapping transformation device 7 is concentrated. CCD_measure y-axis Combining the calculation formula in step S4, the abscissa x of the corresponding feature region on the calibration sample on the CCD camera 15 is calculated. CCD_cal y-axis CCD_cal Then, based on the horizontal and vertical coordinates of the characteristic regions on the calibration sample and the corresponding horizontal and vertical coordinates of the characteristic regions on the calibration sample on the CCD camera 15, the coordinate transformation error Δ between the laser irradiation spot and the damage point of the calibration sample is calculated. single The calculation formula is:

[0090]

[0091] Repeat the above steps to obtain multiple coordinate transformation errors. Perform Gaussian fitting on all coordinate transformation errors to obtain the standard deviation Δ of the mapping error. st-erAfter completing the aforementioned work, the mapping and transformation device 7 will be removed from the measurement optical path.

[0092] Step S6: Obtain the image before irradiation.

[0093] In step S2, the observation parameters of microscope 12 and the illumination parameters of the light source remain unchanged. The mapping transformation device (7) is moved out of the optical path, and the optical element to be tested 17 is replaced with the calibration sample. Before damage measurement, a certain area of ​​the optical element to be tested 17 needs to be photographed. If there are many scattering points in the area, the first three-dimensional translation stage 10 needs to be moved and a measurement area with better quality needs to be selected. If there are no obvious scattering points in the area to be tested, the area to be tested is photographed. During the photographing, microscope 12 is moved by the second three-dimensional translation stage 11, and the area of ​​the optical element to be tested 17 is photographed by microscope 12 to obtain the image before irradiation. After the photographing is completed, microscope 12 is moved away by the second three-dimensional translation stage 11 and the lens of microscope 12 is protected.

[0094] Step S7: Obtain the post-irradiation image.

[0095] Select the delay parameters between each amplification stage of the laser to obtain the target output energy; set up the absorption trap 13, move the mapping transformation device 7 into the optical path, so that the beam output by the laser 1 interacts with the optical element under test 17, and calculate the total pulse energy E of the target plane based on the reading of the energy meter and the sampling coefficient γ calibrated in step S3. T The target area of ​​the measurement pulse is obtained by combining the spatial distribution of the beam measured by the CCD camera 15. The absorption trap 13 is removed, and the microscope 12 is moved by the second three-dimensional translation stage 11 so that the microscope 12 can observe the optical element 17 under test from the front. In step S2, the observation parameters and light source illumination parameters of the microscope 12 remain unchanged. The microscope 12 is used to take pictures of the area under test of the optical element 17 to obtain the irradiated image. After the picture is taken, the microscope 12 is moved away by the second three-dimensional translation stage 11 and the lens of the microscope 12 is protected.

[0096] Step S8, Image Processing

[0097] Using common image processing software, the image after irradiation obtained in step S7 is subtracted from the image before irradiation obtained in step S6 to obtain the effect image after laser pulse action. For the newly obtained image, the influence of dust on the sample surface on the result is first eliminated. Then, according to the relevant algorithm (the algorithm in the existing technology is sufficient), the background light is eliminated, binarized, and the centroid of each scattering point is obtained by using image moments to obtain the horizontal and vertical coordinates of the damage points on the surface of the optical element 17 under test.

[0098] Step S9, Local area division of light spot

[0099] The near-field distribution of the target surface spot acquired by the CCD camera (15) is binarized (1 / e 2 The outline of the laser irradiation spot is obtained, and then the local area is divided according to the shape of the outline of the spot.

[0100] If the light spot's outline is circular, first obtain the circumscribed rectangle of the outline, then divide the area within the circumscribed rectangle into several square local regions. If the light spot's outline is square, directly divide the area within the outline into several directional local regions. During division, the side length of each local region is the standard deviation Δ. st-er 6 to 10 times.

[0101] After the local area is divided, the coordinates of the horizontal and vertical coordinates of the surface damage points of the optical element 17 to be tested are obtained in step S8. The coordinates are transformed using the calculation formula in step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage points on the CCD camera 15. Then, combined with the local area division of the light spot, the number of damage points in each local area is counted.

[0102] Step S10: Calculate the flux in the local region.

[0103] Based on the number of damage points in a local area, calculate the local area flux of the corresponding local area, and record the local area flux, the number of damage points, and the number of pixels contained in each local area.

[0104] The specific formula for calculating the local flux in a corresponding local region is as follows:

[0105]

[0106] Among them, E T g represents the total energy of the target plane pulse obtained in step S7. i，j Let ∑g be the grayscale value of the pixel at the i-th row and j-th column within the light spot outline. i，j ∑ represents the total pixel grayscale within the light spot outline. local g i，j N represents the total pixel grayscale value within a local area of ​​the light spot. t S represents the total number of pixels within a local area of ​​the light spot. pixel CCD is the area of ​​a single pixel unit.

[0107] Step S11: Repeat the measurement and calculate the damage density.

[0108] Adjust the third-dimensional translation stage 11, select different measurement areas of the optical element 17 to be tested, and repeat steps S6-S10 for each measurement area until the measurement of the entire light-transmitting area of ​​the optical element 17 to be tested is completed.

[0109] The local flux values ​​of all local regions are statistically analyzed and sorted in ascending order. The minimum local flux value is used as the flux starting point. Each subsequent flux step is used as a flux interval, and so on, to classify the flux of all local regions into multiple flux intervals. The number of damage points and the number of pixels corresponding to each flux interval are summed, and the average value of all fluxes in each flux interval is used as the flux value of that flux interval. The damage density of each flux interval is then calculated.

[0110] The specific formula for calculating the damage density for each flux range is as follows:

[0111]

[0112] in, For the damage density within a specific flux range, N m n represents the number of damage points within a specific flux range. f S represents the number of pixels within a specific flux range. pixel This represents the area of ​​a CCD pixel.

[0113] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for high-precision local mapping measurement of surface damage density of an optical element, characterized in that, The steps are as follows: Step S1, optical path preparation; Select a clean measurement environment with humidity ≤50RH%, set up the measurement optical path, adjust the half-wave plate (4) and polarizer (6) to obtain a measurement pulse with an extinction ratio of ~100:1; the light spot sampled from the front surface of the optical wedge (9) is used to measure the near-field distribution of the light spot on the target surface, and the light spot sampled from the rear surface of the optical wedge (9) is used to measure the time waveform. Step S2, Measurement Preparation; Adjust the observation parameters and light source illumination parameters of the microscope (12), and calibrate the field of view, magnification, resolution and depth of field of the microscope; the energy meter (5) and phototube-oscilloscope (14) are calibrated by traceability, and the nonlinearity and uniformity of the response of the CCD camera (15) are calibrated. Step S3: Calibrate the sampling coefficients; The sampling factor is measured with a traceable energy meter (5), the sampling factor is the ratio of the transmitted energy of the transmitted light after the first mirror (3) and the transmitted energy of the transmitted light after the transmission through the optical wedge (9) ​ ； Among them, transmission energy The reading of the energy meter (5); Step S4: Define the mapping transformation relationship; The mapping transformation device (7) is moved into the optical path for laser irradiation. The light intensity induced by the mapping transformation device (7) is obtained and the horizontal and vertical coordinates of the characteristic area on the CCD camera (15) and the calibrated sample are obtained. The coordinate mapping transformation relationship between the laser irradiation spot and the damage point of the optical element is calculated. Step S5, calibrate the mapping error; The mapping transformation device (7) is translated perpendicular to the optical path direction and laser irradiation is performed to obtain the abscissa of the characteristic region on the calibration sample where the light intensity induced by the mapping transformation device (7) is located. y-axis Combined with the calculation formula in step S4, the abscissa of the corresponding feature region on the calibration sample on the CCD camera (15) is calculated. y-axis Then calculate the coordinate transformation error between the laser irradiation spot and the damage point of the calibration sample. The calculation formula is: ； Repeat the above steps to obtain multiple coordinate transformation errors. Perform Gaussian fitting on all coordinate transformation errors to obtain the standard deviation of the mapping error. Remove the mapping transformation device (7) from the measurement optical path; Step S6: Obtain the image before irradiation; In step S2, the observation parameters and light source illumination parameters of the microscope (12) remain unchanged. The mapping transformation device (7) is moved out of the optical path, the optical element to be tested (17) is replaced with the calibration sample, the microscope (12) is moved by the third-dimensional translation stage (11), and the microscope (12) is used to take pictures of the area to be tested of the optical element to be tested (17) to obtain the image before irradiation. Step S7: Obtain the image after irradiation; After setting up the absorption trap, the mapping transformation device (7) is moved into the optical path, allowing the beam output from the laser (1) to interact with the optical element (17) under test. The results are then determined based on the reading of the energy meter and the sampling coefficient calibrated in step S3. Calculate the total energy of the pulse on the target plane. ;Then, the target surface spot area of ​​the measurement pulse is obtained by combining the spatial distribution of the beam measured by the CCD camera (15);The absorption trap is removed, the microscope (12) is moved by the second three-dimensional translation stage (11), and the microscope (12) is used to take pictures of the area to be tested of the optical element (17) to be tested, and the irradiated image is obtained. Step S8, image processing; The image after irradiation is obtained by subtracting the image before irradiation obtained in step S6 from the image after irradiation obtained in step S7. The effect image after laser pulse action is obtained. Then, based on the centroid of each scattering point in the effect image, the horizontal and vertical coordinates of the damage points on the surface of the optical element (17) under test are obtained. Step S9, local area division of the light spot; The near-field distribution of the target surface light spot acquired by the CCD camera (15) is binarized to obtain the outline of the laser irradiation light spot; if the outline of the light spot is circular, the circumscribed rectangle of the light spot outline is first obtained, and then several square local regions are divided inside the circumscribed rectangle; if the outline of the light spot is square, several directional local regions are directly divided inside the light spot outline; the side length of each local region is the standard deviation. 6 to 10 times; Based on the horizontal and vertical coordinates of the surface damage points of the optical element (17) to be tested obtained in step S8, coordinate transformation is performed using the calculation formula in step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage points on the CCD camera (15); then, combined with the local area division of the light spot, the number of damage points in each local area is counted. Step S10: Calculate the flux in the local region; Based on the number of damage points in a local area, calculate the local area flux of the corresponding local area, and record the local area flux, the number of damage points, and the number of pixels contained in each local area. Step S11: Repeat the measurement and calculate the damage density; Adjust the third-dimensional translation stage (11), select different measurement areas of the optical element to be tested (17), and repeat steps S6-S10 for each measurement area until the measurement of the entire light-transmitting area of ​​the optical element to be tested (17) is completed; The local flux values ​​of all local regions are statistically analyzed and sorted in ascending order. The minimum local flux value is used as the flux starting point. Each subsequent flux step is used as a flux interval, and so on, to classify the flux of all local regions into multiple flux intervals. The number of damage points and the number of pixels corresponding to each flux interval are summed, and the average value of all fluxes in each flux interval is used as the flux value of that flux interval. The damage density of each flux interval is then calculated.

2. The method for high-precision local mapping measurement of surface damage density of optical elements as described in claim 1, characterized in that, The high-precision local mapping measurement system for surface damage density of optical components used in the measurement includes: a laser (1), a first mirror (3), a second mirror (2), an energy meter (5), a half-wave plate (4), a polarizer (6), a mapping transformation device (7), a lens (8), an optical wedge (9), a first three-dimensional translation stage (10), a second three-dimensional translation stage (11), an absorption trap (13), a microscope (12), a phototube-oscilloscope (14), a CCD camera (15), and a computer (16). Before irradiation, the optical element under test (17) and microscope (12) are moved by the first three-dimensional translation stage (10) and the second three-dimensional translation stage (11) to select the measurement area of ​​the optical element under test (17). The microscope (12) acquires the pre-irradiation image of the measurement area of ​​the optical element under test (17). During irradiation, the beam output by the laser (1) is incident on the first reflecting mirror (3). The transmitted light after passing through the first reflecting mirror (3) is incident on the energy meter (5). The reflected light after being reflected by the first reflecting mirror (3) passes through the second reflecting mirror (2), half-wave plate (4), polarizer (6), and lens (8) in sequence before being incident on the light wedge (9). The reflected light from the front surface is received by a CCD camera (15) and used to measure the near-field distribution of the target surface spot; the reflected light from the rear surface of the optical wedge (9) is received by a phototube-oscilloscope (14) and used to measure the time waveform; the transmitted light transmitted through the optical wedge (9) is incident on the optical element under test (17) placed on the first three-dimensional translation stage (10), and the transmitted light transmitted through the optical element under test (17) is incident on the absorption trap (13) on the second three-dimensional translation stage (11); after irradiation, the microscope (12) is moved through the second three-dimensional translation stage (11), and the microscope (12) acquires the irradiated image of the measurement area of ​​the optical element under test (17).

3. The method for high-precision local mapping measurement of surface damage density of optical components as described in claim 2, characterized in that: The laser (1) outputs a beam with pulse energy stability ≤ 5% RMS, a laser polarization state of linearly polarized light with a polarization degree of ~100:1, a smooth and stable pulse time waveform, and a pulse width instability ≤ 5% RMS.

4. The method for high-precision local mapping measurement of surface damage density of optical elements as described in claim 2, characterized in that: The reflected light after being reflected by the first mirror (3) passes through the second mirror (2), the half-wave plate (4), and the polarizer (6) in sequence to obtain a measurement pulse with an extinction ratio of ~100:

1.

5. The method for high-precision local mapping measurement of surface damage density of optical components as described in claim 2, characterized in that: The first three-dimensional translation stage (10) does not change the relative distance between the target plane and the lens (8) when translating or rotating; the positioning accuracy of the first three-dimensional translation stage (10) is ≤0.05mm when translating in the horizontal and vertical directions; the positioning accuracy of the first three-dimensional translation stage (10) is ≤0.1° when rotating.

6. The method for high-precision local mapping measurement of surface damage density of optical components as described in claim 2, characterized in that: The mapping transformation device (7) is a mapping transformation device (7) that can be moved into and out of the optical path and does not affect the operation of the laser (1) after being moved into the optical path, and contains at least 3 non-collinear and distinguishable markers that can generate intensity modulation of the downstream beam.

7. The method for high-precision local mapping measurement of surface damage density of optical components as described in claim 2, characterized in that: The field of view of the microscope (12) is larger than the size of the light spot on the optical element (17) to be tested. The lateral resolution of the microscope (12) is ≤1μm. The microscope (12) uses a broadband light source, and the maximum output power of the broadband light source is ≥100mW and the output power is continuously adjustable.

8. The method for high-precision local mapping measurement of surface damage density of optical elements as described in claim 1, characterized in that, In step S4, the specific calculation formula for calculating the coordinate mapping transformation relationship between the laser irradiation spot and the damage point of the optical element is as follows: ； in, To calibrate the x and y coordinates of the characteristic regions on the sample, and The coordinate mapping transformation matrix, The x and y coordinates are the corresponding feature regions on the calibrated sample on the CCD camera (15).

9. The method for high-precision local mapping measurement of surface damage density of optical elements as described in claim 1, characterized in that, In step S10, the specific calculation formula for calculating the local flux of the corresponding local region is as follows: ； in, The total energy of the target plane pulse obtained in step S7 is... Let the pixel at the i-th row and j-th column within the light spot outline be the gray level. This represents the total pixel grayscale within the light spot outline. The total pixel grayscale value within a local area of ​​the light spot. This represents the total number of pixels within the local area of ​​the light spot. This represents the area of ​​a single pixel unit.

10. The method for high-precision local mapping measurement of surface damage density of optical elements as described in claim 1, characterized in that, In step S11, the specific formula for calculating the damage density in each flux interval is as follows: ； in, Damage density for a specific flux range The number of damage points within a specific flux range. The number of pixels within a specific flux range. This represents the area of ​​a CCD pixel.

Images