CN116256367A - High-precision local mapping measurement system and method for surface damage density of optical element - Google Patents
High-precision local mapping measurement system and method for surface damage density of optical element Download PDFInfo
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Abstract
The invention discloses a high-precision local mapping measurement system and method for the surface damage density of an optical element, relates to high-precision measurement of the surface damage density of the optical element, and aims to solve the problem that the test precision is affected by the non-uniformity distribution of light spots after splicing in the prior art. The method and the device realize the space mapping of the damage point of the optical element and the local light spot of the laser target surface by establishing the coordinate mapping transformation relation between the irradiation light spot of the laser target surface and the optical element; and calculating and recording the local flux, the number of damaged points and the number of pixel points contained in the local area of each local area, and calculating the damage density of each flux interval, wherein the 'average flux of the whole light spot' in the prior art is creatively replaced by the 'local flux of the damaged point of the optical element' in the application, so that the spatial mapping of the damaged point of the optical element and the local light spot of the laser target surface is realized, and the influence of the non-uniform distribution of the light spot on the measurement precision of the surface damage density is effectively relieved.
Description
Technical Field
The invention belongs to the technical field of optical detection, relates to surface damage measurement of an optical element, and particularly relates to local mapping measurement of surface damage density of the optical element.
Background
The laser-induced damage is always the biggest bottleneck factor for restricting the high-power solid laser to develop to higher and stronger, and has the characteristics of large subject span, short duration, complex physical phenomenon, interleaving of a plurality of influencing factors, difficult decoupling analysis and the like, thereby bringing serious challenges to the measurement and evaluation of the damage performance of the element. At present, offline damage measurement is generally adopted to represent the damage performance of the optical element, and in order to continuously improve the element processing technology, prolong the service life of the element and improve the reliability of a laser device, the measurement precision of the damage performance representation parameter of the optical element is required to be continuously improved.
Optical element damage is typically manifested as surface damage, and surface damage performance can be characterized by 4 indices of damage morphology, damage point geometry, damage threshold, and/or damage density. The damage morphology is generally used for physical mechanism research of interaction of laser and optical elements, and the damage performance of the optical elements is difficult to quantitatively characterize; the geometric dimension of the damage point is closely related to physical processes such as damage repair, damage growth, laser operation strategies and the like, and is not generally directly used for representing damage performance. Compared with the damage morphology and the damage point geometric size, the damage threshold is the most commonly used parameter for representing the initial damage performance of the optical element.
The invention patent application with the application number of CN201610169105.9 discloses an optical element surface damage threshold testing system and a testing method thereof, wherein the system comprises a laser, a liquid crystal light valve, a beam splitter, a first lens, a test sample, an absorption trap, a direct-view CCD, a reflecting mirror, an energy calorimeter, a second lens and a monitoring CCD which are sequentially arranged according to a light path, a laser beam emitted by the laser, after being shaped by the liquid crystal light valve, reaches the beam splitter, laser in the transmission direction of the beam splitter is converged on the surface of the test sample through the first lens, laser in the reflection direction of the beam splitter reaches the direct-view CCD through the absorption trap, laser in the reflection direction of the reflecting mirror reaches the second lens and finally converged on the monitoring CCD, and laser in the transmission direction of the reflecting mirror reaches the energy calorimeter. The testing method comprises the following steps: step 1): establishing a coordinate relation between a liquid crystal light valve and a target surface, and calculating a transfer function; step 2): collecting coordinate information of a damage point in a direct-view CCD, and calculating position information on a liquid crystal light valve by using a transfer function, wherein the step 3) is as follows: position information on the monitoring CCD is obtained, a certain pixel matrix is selected, and local flux is calculated by using the near-field distribution of light spots collected by the monitoring CCD. The testing principle is as follows: the method comprises the steps of adding a facula near-field mapping technology to an existing damage threshold testing method (R-on-1 testing method or 1-on-1 testing method), specifically, observing whether damage occurs on the surface of an element or not by using an online microscopic imaging system until the damage point is observed on the surface of the element, collecting laser beam near-field distribution generated by damage through a monitoring CCD, removing an absorption trap, adjusting a laser to output low-energy to collect laser beam near-field distribution images on a direct-view CCD, wherein the near-field distribution images comprise position information of the damage point in the facula, calculating coordinates on the monitoring CCD according to a transfer function by using the position information, selecting a proper area to calculate flux from the near-field images when the monitoring CCD collects the damage by taking the coordinates as a center, determining a calculated area according to the size of the damage point, calculating the flux according to the gray value of the near-field image collected by the monitoring CCD, and finishing calculation of the damage threshold, wherein the flux of the local area of the obtained damage point is the damage threshold of the defect.
As with the damage threshold measurement in the prior art, the test method has the advantages of wide application range, standard measurement by corresponding international standard (ISO 21254) and the like. However, this test method also has significant drawbacks in that it is susceptible to damage measurement conditions such as spot size, measurement method, sampling rate, etc., and the damage threshold is difficult to directly correlate with the damage performance of the laser device. Unlike the damage threshold, the damage density not only characterizes the initial damage performance of the optical element, but also characterizes the damage performance of the optical element under the action of high-flux laser pulses, and more importantly, the damage density is easier to be related to the damage performance of the laser device, so high-precision measurement of the damage density is the focus of current attention.
The invention patent application with the application number of CN202010353966.9 discloses a device and a method for testing damage density of a large-caliber optical element, wherein the device comprises the following steps: the system comprises a laser light source, an energy adjusting wave plate, an energy adjusting prism, a cylindrical lens, a first sampling mirror, an absorber, a light beam quality analyzer, a second sampling mirror, an energy meter, a photoelectric probe, an oscilloscope, a long-focus microscope, a white light illumination light source, a rotating motor controller, a data acquisition card, a three-dimensional sample motion motor controller for placing a sample of a large-caliber optical element to be measured, a microscope motion motor controller and a computer, wherein the three-dimensional sample motion motor controller is used for placing the sample of the large-caliber optical element to be measured; the laser source outputs a light beam which is sequentially incident to the first sampling mirror through the energy adjusting wave plate, the energy adjusting prism and the cylindrical lens, and reflected light reflected by the front surface of the first sampling mirror is received by the light beam quality analyzer and is used for measuring light field distribution and transmitting the light field distribution to the data acquisition card; the reflected light reflected by the rear surface of the first sampling mirror is incident to the second sampling mirror, the reflected light reflected by the second sampling mirror is received by the photoelectric probe, the output end of the photoelectric probe is connected with the incident end of the oscilloscope, and the output end of the oscilloscope is connected with the data acquisition card; the transmitted light transmitted by the second sampling mirror is received by the energy meter, is used for measuring the energy of the light beam, and is transmitted to the data acquisition card; the transmitted light transmitted by the first sampling mirror is incident to a region to be tested of the large-caliber optical element sample to be tested, and the residual laser is absorbed by the absorber; after the long-focus microscope moves into the large-caliber optical element sample to be detected, the long-focus microscope is illuminated by the white light illumination light source, the long-focus microscope is controlled by the microscope motion motor controller to realize the detection of damage conditions at different positions of the optical element, and the microscope motion motor controller is connected with the computer; the shutter of the laser light source is connected with the computer to control laser output; the energy adjusting wave plate is controlled by the rotating motor controller to realize the adjustment of the energy of the test laser, the rotating motor controller is connected with the computer, and the data acquisition card and the three-dimensional sample motion motor controller are respectively connected with the computer.
As with the damage density measurement in the prior art, the damage density measurement is carried out by adopting the irradiation method of splice scanning, and the purpose of splice scanning of small light spots is to make the surface of an element subjected to approximately uniform large-area irradiation.
Disclosure of Invention
The invention aims at: in order to solve the problem that the measurement effect is affected by the non-uniform distribution of the spliced light spots in the prior art, the system and the method for high-precision local mapping measurement of the surface damage density of the optical element are provided, the spatial mapping of the damage points of the optical element and the local light spots of the laser target surface is realized, and the influence of the non-uniform distribution of the light spots on the measurement precision is effectively relieved.
The technical scheme adopted by the invention is as follows:
an optical element surface damage density high-precision local mapping measurement system, comprising: the device comprises a laser, a first reflecting mirror, a second reflecting mirror, an energy meter, a half-wave plate, a polaroid, mapping transformation equipment, a lens, an optical wedge, an I three-dimensional translation stage, an II three-dimensional translation stage, an absorption trap, a microscope, a photoelectric tube-oscilloscope, a CCD camera and a computer;
before irradiation, moving the optical element to be measured and a microscope through the three-dimensional translation table I and the three-dimensional translation table II, selecting a measurement area of the optical element to be measured, and acquiring an image before irradiation of the measurement area of the optical element to be measured by the microscope; when the light beam is irradiated, the light beam output by the laser is incident to the first reflecting mirror, the transmitted light transmitted by the first reflecting mirror is incident to the energy meter, the reflected light reflected by the first reflecting mirror is sequentially transmitted to the optical wedge after passing through the second reflecting mirror, the half-wave plate, the polarizing plate, the mapping conversion equipment and the lens, and the reflected light reflected by the front surface of the optical wedge is received by the CCD camera and is used for measuring the near-field distribution of the target light spots; the reflected light reflected by the rear surface of the optical wedge is received by a photoelectric tube-oscilloscope for measuring time waveform; the transmitted light transmitted by the optical wedge is incident to an optical element to be measured which is placed on the third-dimensional translation stage I, and the transmitted light transmitted by the optical element to be measured is incident to an absorption trap on the third-dimensional translation stage II; after irradiation, the microscope is moved through the II three-dimensional translation stage, and the microscope acquires an irradiated image of the measurement area of the optical element to be measured.
Further, the pulse energy stability of the light beam output by the laser is less than or equal to 5% RMS, the polarization state of the laser is linear polarization with the polarization degree of 100:1, the pulse time waveform is smooth and stable, and the pulse width instability is less than or equal to 5% RMS.
Further, after the reflected light reflected by the first reflecting mirror sequentially passes through the second reflecting mirror, the half-wave plate and the polaroid, the extinction ratio reaches 100: measurement pulses of the order of 1.
Further, the relative distance between the target plane and the lens is not changed when the I three-dimensional translation stage translates and rotates; the positioning precision of the I three-dimensional translation stage is less than or equal to 0.05mm when the I three-dimensional translation stage translates in the horizontal and vertical directions; the positioning precision of the third-dimensional translation stage is less than or equal to 0.1 degree when the third-dimensional translation stage rotates.
Further, the mapping device is a mapping device which can be moved out of the optical path and which does not affect the operation of the laser after being moved into the optical path, and which comprises at least 3 non-collinear and distinguishable mark points which can produce a light intensity modulation on the downstream light beam.
Further, the view field of the microscope is larger than the size of a light spot on the optical element to be detected, the transverse resolution of the microscope is less than or equal to 1 mu m, the microscope adopts a broadband light source, the maximum output power of the broadband light source is more than or equal to 100mW, and the output power is continuously adjustable.
The high-precision local mapping measurement method for the surface damage density of the optical element comprises the following steps:
step S1, light path preparation
A clean measuring environment with humidity less than or equal to 50RH percent is selected, a measuring light path is built, and a half wave plate and a polaroid are regulated to obtain an extinction ratio reaching 100: a measurement pulse of magnitude 1; the light spot sampled by the front surface of the optical wedge is used for measuring the near-field distribution of the target surface light spot, and the light spot sampled by the rear surface of the optical wedge is used for measuring the time waveform;
step S2, measurement preparation
Adjusting observation parameters and light source illumination parameters of the microscope, and calibrating the field of view, magnification, resolution and depth of field of the microscope; the energy meter and the photoelectric tube-oscilloscope are calibrated by tracing, and the nonlinearity and uniformity of the response of the CCD camera are calibrated by:
s3, calibrating sampling coefficients
Measuring a sampling coefficient by using a traceably calibrated energy meter, wherein the sampling coefficient gamma refers to the transmission energy E of the transmitted light transmitted by the first reflector w And transmission energy E of transmitted light transmitted through the wedge t Ratio of:
wherein the energy E is transmitted w An indication of the energy meter;
s4, calibrating the mapping transformation relation
The mapping transformation equipment is moved into a light path to perform laser irradiation, the transverse coordinate and the longitudinal coordinate of the light intensity modulation generated by the mapping transformation equipment on the CCD camera and the characteristic area on the calibration sample are obtained, and the coordinate mapping transformation relation between the laser irradiation light spot and the optical element damage point is calculated;
step S5, calibrating the mapping error
Translating the mapping transformation device perpendicular to the direction of the light path, and carrying out laser irradiation to obtain the abscissa x of the characteristic region of the light intensity modulation on the calibration sample, wherein the abscissa x is generated by the mapping transformation device and is induced by the light intensity modulation CCD_measure Ordinate and ordinate of
And combining the calculation formula in the step S4 to calculate the abscissa x of the characteristic region on the corresponding calibration sample on the corresponding CCD camera CCD_cal Y, ordinate CCD_cal The method comprises the steps of carrying out a first treatment on the surface of the Then calculating the coordinate transformation error delta between the laser irradiation light spot and the damage point of the calibration sample single The calculation formula is as follows: />
Repeating the above steps to obtain multiple coordinate transformation errors, performing Gaussian fitting on all the coordinate transformation errors to obtain standard deviation delta of the mapping error st-er The method comprises the steps of carrying out a first treatment on the surface of the Shifting the mapping transformation device out of the measuring light path;
step S6, acquiring an image before irradiation
Fixing the observation parameters of the microscope and the illumination parameters of the light source in the step S2, moving the mapping transformation equipment out of the light path, replacing a calibration sample with the optical element to be detected, moving the microscope through a II three-dimensional translation stage, shooting the area to be detected of the optical element to be detected by using the microscope, and obtaining an image before irradiation;
step S7, obtaining an irradiated image
Setting up an absorption trap, moving the mapping conversion device into a light path to enable the light beam output by the laser to act with the optical element to be tested, and calculating the total energy E of the target plane pulse according to the indication of the energy and the sampling coefficient gamma calibrated in the step S3 T The method comprises the steps of carrying out a first treatment on the surface of the And then the spatial distribution of the light beam measured by the CCD camera is combinedMeasuring the area of the target surface light spot of the pulse; removing the absorption trap, moving a microscope through a II three-dimensional translation stage, and shooting a region to be detected of the optical element to be detected by using the microscope to obtain an irradiated image;
step S8, picture processing
Subtracting the pre-irradiation image obtained in the step S6 from the post-irradiation image obtained in the step S7 to obtain an effect image after the laser pulse acts; obtaining the horizontal coordinate and the vertical coordinate of the surface damage point of the optical element to be detected according to the mass center of each scattering point in the effect image;
step S9, dividing local areas of the light spots
Binarizing (1/e 2) the near-field distribution of the target surface light spots acquired by the CCD camera to obtain the outline of the laser irradiation light spots; if the outline of the light spot is circular, firstly obtaining an circumscribed rectangle of the outline of the light spot, and then dividing a plurality of square local areas in the circumscribed rectangle; if the outline of the light spot is square, directly dividing a plurality of direction local areas inside the outline of the light spot; the side length of each local area is the standard deviation delta st-er 6 to 10 times of the total number of the components;
according to the horizontal and vertical coordinates of the surface damage point of the optical element to be detected obtained in the step S8, performing coordinate conversion by using the calculation formula in the step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage point on the CCD camera; combining with the division of the local areas of the light spots, and counting the number of damage points in each local area;
step S10, calculating local area flux
Calculating local area flux of the corresponding local area according to the number of the damaged points in the local area, and recording the local area flux, the number of the damaged points and the number of the pixel points contained in the local area of each local area;
step S11, repeatedly measuring and calculating the damage density
Adjusting a II three-dimensional translation stage, selecting different measurement areas of the optical element to be measured, and repeating the steps S6-S10 for each measurement area until the measurement of the light transmission area of the whole optical element to be measured is completed;
counting the local area flux values of all local areas, carrying out ascending arrangement, positioning the flux starting point by the minimum local area flux value, taking 1 flux step length as 1 flux interval every time after the flux starting point is increased, and so on, classifying the flux completion intervals of all local areas and forming a plurality of flux intervals; and summing the number of the damage points and the number of the pixel points corresponding to each flux interval, and calculating the damage density of each flux interval by taking the average value of all fluxes in each flux interval as the flux value of the flux interval.
Further, in step S4, when calculating the coordinate mapping transformation relationship between the laser irradiation spot and the optical element damage point, a specific calculation formula is as follows:
wherein ,
for calibrating the abscissa of the characteristic area on the sample,/->
and />
For the coordinate mapping transformation matrix, < >>
The horizontal and vertical coordinates of the characteristic area on the corresponding calibration sample on the CCD camera.
Further, in step S10, when calculating the local area flux of the corresponding local area, a specific calculation formula is:
wherein ,ET For the total energy of the target plane pulse obtained in step S7, g i，j Is within the outline of the light spotPixel gray scale at ith row and j column position, Σg i，j Sigma is the gray scale of the total pixel in the light spot outline local g i，j Is the total pixel gray scale in the local area of the light spot, N t S is the total number of pixel points in the local area of the light spot pixel The CCD is the area of a single pixel cell.
Further, in step S11, when calculating the damage density of each flux interval, a specific calculation formula is:
wherein ,
for the damage density of a specific flux interval, N m For the number of damage points of a specific flux interval, n f For a specific flux interval of the number of pixels, S pixel Is the CCD pixel area.
In summary, due to the adoption of the technical scheme, the beneficial effects of the invention are as follows:
in the invention, a coordinate mapping transformation relation between the laser irradiation light spot and the damage point of the optical element is established, and the space mapping of the optical element and the laser target surface local light spot is realized; and calculating and recording the local area flux, the number of damage points and the number of pixel points contained in the local area of each local area, dividing flux intervals according to a certain rule, calculating the damage density of each flux interval by using the number of damage points and the number of pixel points corresponding to each flux interval, and creatively replacing the 'average flux of the whole light spot' in the prior art with the 'optical element damage point local flux' in the application, thereby effectively relieving the influence of the non-uniform distribution of the light spot on the measurement precision of the surface damage density.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, it being understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and other related drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic view of the optical path structure of the present invention;
FIG. 2 is a schematic view of dividing a local area of a light spot, wherein a rectangular frame on the outer side represents a certain local area of the light spot, a circular frame represents a light spot contour in the local area, and small dots represent damage points;
wherein, the reference numerals are as follows:
1-laser, 2-second reflecting mirror, 3-first reflecting mirror, 4-half wave plate, 5-energy meter, 6-polaroid, 7-mapping conversion equipment, 8-lens, 9-optical wedge, 10-first three-dimensional translation stage, 11-second three-dimensional translation stage, 12-microscope, 13-absorption trap, 14-photocell-oscilloscope, 15-CCD camera, 16-computer, 17-optical element to be tested.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the particular embodiments described herein are illustrative only and are not intended to limit the invention, i.e., the embodiments described are merely some, but not all, of the embodiments of the invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the invention, as presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be made by a person skilled in the art without making any inventive effort, are intended to be within the scope of the present invention.
It is noted that relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
The features and capabilities of the present invention are described in further detail below in connection with the examples.
The invention provides a high-precision localized mapping measurement system for optical element surface damage density, which is provided by a preferred embodiment of the invention, and comprises a laser 1, a first reflecting mirror 3, a second reflecting mirror 2, an energy meter 5, a half-wave plate 4, a polarizing plate 6, mapping transformation equipment 7, a lens 8, an optical wedge 9, an I three-dimensional translation stage 10, an II three-dimensional translation stage 11, an absorption trap 13, a microscope 12, a photoelectric tube-oscilloscope 14, a CCD camera 15 and a computer 16.
After the propagation azimuth angle of the light beam output by the laser 1 is adjusted through the first reflecting mirror 3 and the second reflecting mirror 2, the irradiation energy of the target surface is monitored by measuring the transmitted light of the first reflecting mirror 3; then, the reflected light of the second reflecting mirror 2 passes through the half-wave plate 4 and the polarizing plate 6 to obtain measurement pulses with higher extinction ratio (about 100:1 orders of magnitude), and then the target light spot near field and time waveform of the laser pulses are sampled and measured through the optical wedge 9 (the light spot sampled by the first reflecting surface of the optical wedge is used for measuring the target light spot near field distribution, and the light spot sampled by the second reflecting surface of the optical wedge is used for measuring the time waveform), wherein the target light spot near field distribution meets the requirement of conjugate measurement. The laser 1 can repeatedly output laser pulses with light beams spatially distributed in a single longitudinal mode of a nearly flat top type or with a preset bandwidth, and the output wavelength of the laser pulses is matched with the working wavelength of a test sample. The pulse energy stability of the light beam output by the laser 1 is less than or equal to 5% RMS, the polarization state of the laser is linear polarization with the polarization degree of 100:1, the pulse time waveform is smooth and stable, and the pulse width instability is less than or equal to 5% RMS.
The I-th three-dimensional translation stage 10 must satisfy the following requirements: the functions of fixing, three-dimensional translation and one-dimensional rotation of the sample can be realized, and the relative distance between the target plane and the focusing lens is not changed by the realization of the scanning translation and rotation functions; when the horizontal and vertical translation is performed, the horizontal and vertical translation travel meets the requirement of a test sample, and the positioning accuracy is less than or equal to 0.05mm; when the laser beam is rotated, the rotation angle range can enable the laser incident angle to meet the test requirement, and the positioning accuracy is less than or equal to 0.1 degree; the lateral displacement distance must be greater than the lateral geometry of the component to be tested.
The third three-dimensional translation stage 11 must satisfy the following requirements: the microscope focusing and vertical in-plane scanning functions can be realized, the axial movement precision is required to be smaller than the depth of field of the microscope, and the transverse movement distance is required to be larger than the geometric dimension of the target surface light spot.
The mapping transformation device 7 must fulfil the following requirements: the laser comprises at least 3 non-collinear and distinguishable mark points, can generate light intensity modulation effect on the downstream light beam, and is convenient to move in and out of the light path and does not influence the safe operation of the laser after moving in the light path.
The field of view (can splice) of the microscope 12 is required to be larger than the size of the light spot on the optical element 17 to be detected, the transverse resolution of the microscope 12 is less than or equal to 1 mu m, the microscope 12 adopts a broadband light source (such as white light) for bright field illumination, and the purpose of selecting the broadband light source is to avoid the mutual interference of scattered light and detection light induced by a damage point; the maximum output power of the adopted broadband light source is more than or equal to 100mW, and the output power is continuously adjustable.
By adopting the device, the light path shown in figure 1 is constructed.
Before irradiation, moving the optical element 17 to be measured and the microscope 12 through the third-dimensional translation stage 10 and the second-dimensional translation stage 11, selecting a measurement area of the optical element 17 to be measured, and collecting an irradiation front image of the measurement area of the optical element 17 to be measured by the microscope 12; when the laser device is irradiated, a light beam output by the laser device 1 is incident to the first reflecting mirror 3, transmitted light transmitted by the first reflecting mirror 3 is incident to the energy meter 5, reflected light reflected by the first reflecting mirror 3 is sequentially transmitted through the second reflecting mirror 2, the half-wave plate 4, the polarizing plate 6, the mapping conversion device 7 and the lens 8, then is incident to the optical wedge 9, and reflected light reflected by the front surface of the optical wedge 9 is received by the CCD camera 15 and is used for measuring the near-field distribution of target light spots; the reflected light reflected by the rear surface of the optical wedge 9 is received by a photocell-oscilloscope 14 for measuring a time waveform; the transmitted light transmitted through the optical wedge 9 is incident on the optical element 17 to be measured placed on the I-th three-dimensional translation stage 10, and the transmitted light transmitted through the optical element 17 to be measured is incident on the absorption trap 13 on the II-th three-dimensional translation stage 11; after irradiation, the microscope 12 is moved by the II three-dimensional translation stage 11, and the microscope 12 acquires an irradiated image of the measurement region of the optical element 17 to be measured.
When the measuring system is adopted for measurement, the specific measuring method comprises the following steps:
step S1, light path preparation
Selecting a clean measuring environment with humidity less than or equal to 50RH%, building a measuring light path, and jointly adjusting two reflectors to enable measuring pulses output by a laser to be incident to a sample of an optical element to be measured at a proper angle; and adjusting the half wave plate 4 and the polaroid 6 to obtain an extinction ratio reaching 100: a measurement pulse of magnitude 1; the light spot sampled by the front surface of the optical wedge 9 is used for measuring the near-field distribution of the target surface light spot, and the light spot sampled by the rear surface of the optical wedge 9 is used for measuring the time waveform; wherein the near-field distribution of the target surface light spots meets the requirement of conjugate measurement.
Step S2, measurement preparation
The visual field (which can be spliced) of the microscope 12 is required to be larger than the geometric dimension of the target surface light spot, and the transverse resolution of the microscope 12 is required to be less than or equal to 1 mu m; a broadband light source (such as white light) is selected for bright field illumination, so that the mutual interference of scattered light and detection light induced by a damage point is avoided, the maximum output power of the broadband light source is more than or equal to 100mW, and the output power is continuously adjustable; the axial movement precision of the third three-dimensional translation stage 11 is required to be smaller than the depth of field of the microscope 12, and the transverse movement distance is required to be larger than the geometric dimension of the target surface light spot; the optical element 17 to be measured is flexibly placed and fixed on the I three-dimensional translation stage 10; the nonlinearity and uniformity of the response of the CCD camera 15 must be calibrated; the energy meter 5 and the photoelectric tube-oscilloscope 14 need to be calibrated by tracing; adjusting the microscope 12 so that the microscope 12 is perpendicular to the side face of the optical element 17 to be measured; repeatedly adjusting the parameters of the illumination source and the parameters of the microscope 12 to obtain the optimal observation effect, and fixing the observation parameters in the follow-up measurement; the following parameters of microscope 12 (or using the data in the microscope factory inspection report) are then calibrated: field of view, resolution, magnification, and depth of field.
It should be noted that, the calibration sampling coefficient of the step S3 and the calibration mapping transformation relation and mapping error of the steps S4-S5 have no sequence requirement, and the calibration sampling coefficient of the step S3 may be calibrated first and then the mapping transformation relation and mapping error of the steps S4-S5 may be calibrated, or the calibration mapping transformation relation and mapping error of the steps S4-S5 may be calibrated first and then the sampling coefficient of the step S3 may be calibrated.
S3, calibrating sampling coefficients
The traceably calibrated energy meter 5 is used for measuring a sampling coefficient, wherein the sampling coefficient gamma refers to the transmission energy E of the transmitted light transmitted by the first reflector 3 w And the transmission energy E of the transmitted light transmitted through the wedge 9 t Ratio of:
wherein the energy E is transmitted w An indication of the energy meter;
meanwhile, the relation curve between the delay and the output energy of each amplifying stage of the laser 1 can be measured.
S4, calibrating the mapping transformation relation
The calibration sample is placed at the position where the optical element 17 to be measured is to be placed, the mapping transformation device 7 is moved into the optical path for laser irradiation, the transverse coordinates and the longitudinal coordinates of the characteristic area, which are generated by the mapping transformation device 7 and are induced to be modulated on the CCD camera 15 and the calibration sample, of the light intensity are obtained, and the coordinate mapping transformation relation between the laser irradiation light spot and the damage point of the optical element is calculated, wherein the specific calculation formula is as follows:
wherein ,
for calibrating the abscissa of the characteristic area on the sample,/->
and />
For the coordinate mapping transformation matrix, < >>
The horizontal and vertical coordinates of the characteristic area on the corresponding calibration sample are arranged on the CCD camera (15).
Step S5, calibrating the mapping error
After the coordinate conversion relation is calculated, the mapping transformation device 7 is translated along the direction vertical to the light path, laser irradiation is carried out, and the abscissa x of the characteristic area, which is generated by the mapping transformation device 7 and is induced to modulate the light intensity on the calibration sample, is obtained CCD_measure Ordinate and ordinate of
And combining the calculation formula in the step S4 to calculate the abscissa x of the characteristic region on the corresponding calibration sample on the corresponding CCD camera 15 CCD_cal Y, ordinate CCD_cal The method comprises the steps of carrying out a first treatment on the surface of the Then, according to the horizontal and vertical coordinates of the characteristic region on the calibration sample and the horizontal and vertical coordinates of the characteristic region on the CCD camera 15 corresponding to the calibration sample, calculating the coordinate transformation error delta between the laser irradiation light spot and the damage point of the calibration sample single The calculation formula is as follows:
repeating the above steps to obtain multiple coordinate transformation errors, performing Gaussian fitting on all the coordinate transformation errors to obtain standard deviation delta of the mapping error st-er . Finish the processAfter the aforementioned operation, the mapping device 7 is moved out of the measuring beam path.
Step S6, acquiring an image before irradiation
And (3) fixing the observation parameters of the microscope 12 and the illumination parameters of the light source in the step S2, moving the mapping transformation device (7) out of the light path, and replacing the optical element 17 to be measured with the calibration sample. Shooting a certain region to be measured of the optical element 17 to be measured before damage measurement, and if the region has more scattering points, moving the I-th three-dimensional translation stage 10 and selecting a measurement region with better quality; if no obvious scattering points exist in the region to be detected, shooting the region to be detected. During shooting, the microscope 12 is moved through the II three-dimensional translation stage 11, and the region to be detected of the optical element 17 to be detected is shot by the microscope 12, so that an image before irradiation is obtained. After the photographing is completed, the microscope 12 is removed through the II three-dimensional translation stage 11 and the lens of the microscope 12 is protected.
Step S7, obtaining an irradiated image
Delay parameters among amplifying stages of the laser are selected to obtain target output energy; an absorption trap 13 is arranged, the mapping transformation device 7 is moved into a light path to enable the light beam output by the laser 1 to act with an optical element 17 to be tested, and the total energy E of the target plane pulse is calculated according to the indication of the energy and the sampling coefficient gamma calibrated in the step S3 T The method comprises the steps of carrying out a first treatment on the surface of the The target surface light spot area of the measuring pulse is obtained by combining the light beam space distribution measured by the CCD camera 15; the absorption trap 13 is removed, the microscope 12 is moved through the II three-dimensional translation stage 11, so that the microscope 12 can observe the optical element 17 to be detected in front, and then in step S2, the observation parameters of the microscope 12 and the illumination parameters of the light source are unchanged, the microscope 12 is used for shooting the area to be detected of the optical element 17 to be detected, and an irradiated image is obtained. After the photographing is completed, the microscope 12 is removed through the II three-dimensional translation stage 11 and the lens of the microscope 12 is protected.
Step S8, picture processing
Selecting common image processing software to enable the irradiated image obtained in the step S7 to be subtracted from the irradiated image obtained in the step S6, so as to obtain an effect image after the laser pulse action; for the newly obtained picture, firstly, the influence of dust on the surface of a sample on the result is eliminated, then, the background light elimination and binarization are solved according to a related algorithm (the algorithm in the prior art can be used for solving the centroid of each scattering point by adopting an image moment, and the horizontal coordinate and the vertical coordinate of the damage point on the surface of the optical element 17 to be detected are obtained.
Step S9, dividing local areas of the light spots
Binarization processing (1/e) is carried out according to the near-field distribution of the target surface light spots acquired by the CCD camera (15) 2 ) And obtaining the outline of the laser irradiation light spot, and then dividing local areas according to the shape of the outline of the light spot.
If the outline of the light spot is circular, firstly obtaining an circumscribed rectangle of the outline of the light spot, and then dividing a plurality of square local areas in the circumscribed rectangle; if the outline of the light spot is square, a plurality of directional local areas are directly divided inside the outline of the light spot. When dividing, the side length of each local area is the standard deviation delta st-er Is 6 to 10 times as large as the number of the above.
After the partial area is divided, performing coordinate conversion according to the horizontal and vertical coordinates of the surface damage point of the optical element 17 to be detected obtained in the step S8 by using the calculation formula in the step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage point on the CCD camera 15; and counting the number of damage points in each local area by combining the situation of dividing the local areas of the light spots.
Step S10, calculating local area flux
Calculating local area flux of the corresponding local area according to the number of the damaged points in the local area, and recording the local area flux, the number of the damaged points and the number of the pixel points contained in the local area of each local area;
when calculating the local area flux of the corresponding local area, a specific calculation formula is as follows:
wherein ,ET For the total energy of the target plane pulse obtained in step S7, g i，j For the pixel gray scale of the ith row and j column in the light spot contour, sigma g i，j Is a light spotTotal pixel gray scale, Σ within the outline local g i，j Is the total pixel gray scale in the local area of the light spot, N t S is the total number of pixel points in the local area of the light spot pixel The CCD is the area of a single pixel cell.
Step S11, repeatedly measuring and calculating the damage density
Adjusting the II three-dimensional translation stage 11, selecting different measurement areas of the optical element 17 to be measured, and repeating the steps S6-S10 for each measurement area until the measurement of the light transmission area of the whole optical element 17 to be measured is completed;
counting the local area flux values of all local areas, carrying out ascending arrangement, positioning the flux starting point by the minimum local area flux value, taking 1 flux step length as 1 flux interval every time after the flux starting point is increased, and so on, classifying the flux completion intervals of all local areas and forming a plurality of flux intervals; and summing the number of the damage points and the number of the pixel points corresponding to each flux interval, and calculating the damage density of each flux interval by taking the average value of all fluxes in each flux interval as the flux value of the flux interval.
In calculating the damage density of each flux interval, a specific calculation formula is as follows:
wherein ,
for the damage density of a specific flux interval, N m For the number of damage points of a specific flux interval, n f For a specific flux interval of the number of pixels, S pixel Is the CCD pixel area.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.
Claims (10)
1. A high-precision localized mapping measurement system for surface damage density of an optical element, comprising: the device comprises a laser (1), a first reflecting mirror (3), a second reflecting mirror (2), an energy meter (5), a half-wave plate (4), a polaroid (6), a mapping conversion device (7), a lens (8), an optical wedge (9), an I three-dimensional translation stage (10), an II three-dimensional translation stage (11), an absorption trap (13), a microscope (12), a photoelectric tube-oscilloscope (14), a CCD camera (15) and a computer (16);
before irradiation, moving an optical element (17) to be measured and a microscope (12) through a third-dimensional translation table (10) and a second-dimensional translation table (11), selecting a measurement area of the optical element (17) to be measured, and collecting an image before irradiation of the measurement area of the optical element (17) to be measured by the microscope (12); when the light beam is irradiated, the light beam output by the laser (1) is incident to the first reflecting mirror (3), the transmitted light transmitted by the first reflecting mirror (3) is incident to the energy meter (5), the reflected light reflected by the first reflecting mirror (3) is sequentially transmitted by the second reflecting mirror (2), the half-wave plate (4), the polarizing plate (6) and the lens (8) and then is incident to the optical wedge (9), and the reflected light reflected by the front surface of the optical wedge (9) is received by the CCD camera (15) and is used for measuring the near-field distribution of the target light spots; the reflected light reflected by the rear surface of the wedge (9) is received by a photocell-oscilloscope (14) for measuring a time waveform; the transmitted light transmitted by the optical wedge (9) is incident to an optical element (17) to be detected which is placed on a third-dimensional translation stage (10), and the transmitted light transmitted by the optical element (17) to be detected is incident to an absorption trap (13) on a fourth-dimensional translation stage (11); after irradiation, the microscope (12) is moved by the II three-dimensional translation stage (11), and the microscope (12) acquires an irradiated image of the measurement region of the optical element (17) to be measured.
2. A system for high-precision localized mapping of optical element surface damage density as recited in claim 1, wherein: the pulse energy stability of the light beam output by the laser (1) is less than or equal to 5% RMS, the polarization state of the laser is linear polarization with the polarization degree of 100:1, the pulse time waveform is smooth and stable, and the pulse width instability is less than or equal to 5% RMS.
3. A system for high-precision localized mapping of optical element surface damage density as recited in claim 1, wherein: after the reflected light reflected by the first reflecting mirror (3) sequentially passes through the second reflecting mirror (2), the half-wave plate (4) and the polaroid (6), the extinction ratio reaches 100: measurement pulses of the order of 1.
4. A system for high-precision localized mapping of optical element surface damage density as recited in claim 1, wherein: the relative distance between the target plane and the lens (8) is not changed when the third three-dimensional translation stage (10) translates and rotates; the positioning precision of the I three-dimensional translation table (10) is less than or equal to 0.05mm when the translation table translates in the horizontal and vertical directions; the positioning precision of the first three-dimensional translation stage (10) is less than or equal to 0.1 degree during rotation.
5. A system for high-precision localized mapping of optical element surface damage density as recited in claim 1, wherein: the mapping device (7) is a mapping device (7) which can be moved out of the optical path and which does not affect the operation of the laser (1) after being moved into the optical path, and which comprises at least 3 non-collinear and distinguishable mark points for generating an optical intensity modulation on the downstream optical beam.
6. A system for high-precision localized mapping of optical element surface damage density as recited in claim 1, wherein: the view field of the microscope (12) is larger than the size of a light spot on the optical element (17) to be detected, the transverse resolution of the microscope (12) is less than or equal to 1 mu m, the microscope (12) adopts a broadband light source, the maximum output power of the broadband light source is more than or equal to 100mW, and the output power is continuously adjustable.
7. The high-precision local mapping measurement method for the surface damage density of the optical element is characterized by comprising the following steps of:
step S1, light path preparation
A clean measuring environment with humidity less than or equal to 50RH percent is selected, a measuring light path is built, and a half wave plate (4) and a polaroid (6) are regulated to obtain an extinction ratio reaching 100: a measurement pulse of magnitude 1; the light spot sampled from the front surface of the optical wedge (9) is used for measuring 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 for measuring the time waveform;
step S2, measurement preparation
Adjusting observation parameters and light source illumination parameters of a microscope (12), and calibrating the field of view, magnification, resolution and depth of field of the microscope; the energy meter (5) and the photoelectric tube-oscilloscope (14) are calibrated by tracing, and the nonlinearity and uniformity of the response of the CCD camera (15) are calibrated;
s3, calibrating sampling coefficients
The sampling coefficient is measured by using a traceably calibrated energy meter (5), and the sampling coefficient gamma refers to the transmission energy E of the transmitted light transmitted by the first reflector (3) w And the transmission energy E of the transmitted light transmitted through the optical wedge (9) t Ratio of:
wherein the energy E is transmitted w An indication of the energy meter (5);
s4, calibrating the mapping transformation relation
The mapping transformation device (7) is moved into a light path to perform laser irradiation, the transverse coordinates and the longitudinal coordinates of the light intensity modulation generated by the mapping transformation device (7) on the CCD camera (15) and the characteristic area on the calibration sample are obtained, and the coordinate mapping transformation relation between the laser irradiation light spot and the optical element damage point is calculated;
step S5, calibrating the mapping error
Translating the mapping transformation device (7) perpendicular to the direction of the light path, and carrying out laser irradiation to obtain the abscissa x of the characteristic area of the light intensity modulation on the calibration sample, which is induced by the mapping transformation device (7) CCD_measure Y CCD on ordinate measure And combining the calculation formula in the step S4 to calculate the abscissa x of the characteristic region on the corresponding calibration sample on the corresponding CCD camera (15) CCD_cal Y, ordinate CCD_cal The method comprises the steps of carrying out a first treatment on the surface of the Then calculating the coordinate transformation error delta between the laser irradiation light spot and the damage point of the calibration sample single The calculation formula is as follows:
repeating the above steps to obtain multiple coordinate transformation errors, performing Gaussian fitting on all the coordinate transformation errors to obtain standard deviation delta of the mapping error st-er The method comprises the steps of carrying out a first treatment on the surface of the Moving the mapping transformation device (7) out of the measuring light path;
step S6, acquiring an image before irradiation
Fixing observation parameters of a microscope (12) and illumination parameters of a light source in the step S2, moving the mapping transformation equipment (7) out of a light path, replacing a calibration sample with an optical element (17) to be detected, moving the microscope (12) through a II three-dimensional translation table (11), and shooting a region to be detected of the optical element (17) to be detected by the microscope (12) to obtain an image before irradiation;
step S7, obtaining an irradiated image
Setting up an absorption trap, moving the mapping transformation device (7) into a light path to enable the light beam output by the laser (1) to act with the optical element (17) to be tested, and calculating the total energy E of the target plane pulse according to the indication of the energy and the sampling coefficient gamma calibrated in the step S3 T The method comprises the steps of carrying out a first treatment on the surface of the The light beam space distribution measured by the CCD camera (15) is combined to obtain the target surface light spot area of the measuring pulse; removing the absorption trap, moving a microscope (12) through a II three-dimensional translation stage (11), and shooting a region to be detected of the optical element (17) to be detected by using the microscope (12) to obtain an irradiated image;
step S8, picture processing
Subtracting the pre-irradiation image obtained in the step S6 from the post-irradiation image obtained in the step S7 to obtain an effect image after the laser pulse acts; obtaining the horizontal coordinate and the vertical coordinate of the surface damage point of the optical element (17) to be detected according to the mass center of each scattering point in the effect image;
step S9, dividing local areas of the light spots
Binarizing the near-field distribution of the target surface light spots acquired by the CCD camera (15) to obtain the outline of the laser irradiation light spots; if the outline of the light spot is circular, firstly obtaining the circumscribed rectangle of the outline of the light spot, and then externallyDividing the interior of the rectangle into a plurality of square local areas; if the outline of the light spot is square, directly dividing a plurality of direction local areas inside the outline of the light spot; the side length of each local area is the standard deviation delta st-er 6 to 10 times of the total number of the components;
according to the horizontal and vertical coordinates of the surface damage point of the optical element (17) to be detected obtained in the step S8, performing coordinate conversion by using the calculation formula in the step S4 to obtain the horizontal and vertical coordinates of the corresponding surface damage point on the CCD camera (15); combining with the division of the local areas of the light spots, and counting the number of damage points in each local area;
step S10, calculating local area flux
Calculating local area flux of the corresponding local area according to the number of the damaged points in the local area, and recording the local area flux, the number of the damaged points and the number of the pixel points contained in the local area of each local area;
step S11, repeatedly measuring and calculating the damage density
Adjusting a II three-dimensional translation table (11), selecting different measurement areas of the optical element (17) to be measured, and repeating the steps S6-S10 for each measurement area until the measurement of the light transmission area of the whole optical element (17) to be measured is completed;
counting the local area flux values of all local areas, carrying out ascending arrangement, positioning the flux starting point by the minimum local area flux value, taking 1 flux step length as 1 flux interval every time after the flux starting point is increased, and so on, classifying the flux completion intervals of all local areas and forming a plurality of flux intervals; and summing the number of the damage points and the number of the pixel points corresponding to each flux interval, and calculating the damage density of each flux interval by taking the average value of all fluxes in each flux interval as the flux value of the flux interval.
8. The method for high-precision local mapping measurement of optical element surface damage density according to claim 7, wherein in step S4, when calculating a coordinate mapping transformation relationship between a laser irradiation spot and an optical element damage point, a specific calculation formula is as follows:
wherein ,
for calibrating the abscissa of the characteristic area on the sample,/->
and />
For the coordinate mapping transformation matrix,
the horizontal and vertical coordinates of the characteristic area on the corresponding calibration sample are arranged on the CCD camera (15).
9. The method of claim 7, wherein in step S10, when calculating the local area flux of the corresponding local area, a specific calculation formula is:
wherein ,ET For the total energy of the target plane pulse obtained in step S7, g i，j For the pixel gray scale of the ith row and j column in the light spot contour, Σg i，j Sigma is the total pixel gray within the spot profile local g i，j Is the total pixel gray scale in the local area of the light spot, N t S is the total number of pixel points in the local area of the light spot pixel The CCD is the area of a single pixel cell.
10. The method for high-precision localized mapping measurement of optical element surface damage density according to claim 7, wherein in step S11, when calculating the damage density of each flux interval, a specific calculation formula is:
wherein ,
for the damage density of a specific flux interval, N m For the number of damage points of a specific flux interval, n f For a specific flux interval of the number of pixels, S pixel Is the CCD pixel area. />
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