A method for judging and estimating threshold of laser-induced damage dominant mode of coated optical elements

Abstract

The present application relates to a kind of coated optical element laser damage dominant mode determination and threshold estimation method, belong to laser optical element damage evaluation technical field.The implementation method of the present application is as follows: based on the calculation of film optical field distribution of transmission matrix method or finite difference time domain method;Temperature and stress field of film and substrate are predicted by combining finite element analysis;Compare damage threshold to determine dominant mode;And verify the accuracy of model through experiment.The present application quantifies the damage contribution of film electric field-thermal coupling model and substrate thermal analysis model by constructing them respectively, and verifies the model through small sample experiment, aims to solve the problems of complex damage competition mechanism of coated optical element film and substrate, difficult to distinguish dominant damage mode, and low efficiency and high cost of traditional experimental method.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser optical element damage evaluation, and more particularly to a method for judging a laser damage dominant mode and estimating a threshold value of a coated optical element. BACKGROUND

[0002] The coated optical element will be damaged by laser-induced damage under the action of high-power / high-energy laser. For in-band laser, the intrinsic absorption rate of the film and the substrate is often low, and it is difficult to determine the damage starting area. The damage may occur in the internal defects of the film or the interlayer discontinuity. For out-of-band laser, due to the possible strong intrinsic absorption, the film and the substrate are simultaneously facing the risk of thermal failure, but the damage mode may dynamically change with the laser wavelength, laser pulse width and other parameters. The competition mechanism of the film and the substrate makes it difficult to accurately predict the damage threshold by a single model, and it is urgent to split the damage contribution of the film and the substrate.

[0003] In addition, the traditional damage evaluation method directly obtains the laser damage threshold through experiments, such as the prior art ISO 21254 Lasers and laser-related equipment-Test methods for laser-induced damage threshold, but still faces problems such as large demand for test samples, long test period, high cost and the like.

[0004] In view of the above problems, the application provides a damage evaluation method combining physical modeling and experimental verification, respectively constructs a film electric field-thermal coupling model and a substrate macroscopic absorption-thermal model, quantifies the damage contribution of both, determines the damage starting area, optimizes the model parameters by small sample experiments, and realizes efficient prediction of the damage threshold and accurate analysis of the damage mechanism. SUMMARY

[0005] In order to clarify the complex damage competition mechanism of the coated element under in-band and out-of-band laser, and solve the problems of low efficiency and high cost of the traditional test method, the application provides a method for judging a laser damage dominant mode and estimating a threshold value of a coated optical element, which combines physical modeling and experimental verification.

[0006] In order to achieve the above purpose, the application is implemented by using the following technical scheme: the method comprises:

[0007] Modeling the damage mechanism of the film and the substrate respectively, determining the laser system, laser wavelength, laser power / energy, laser spot size, laser energy distribution, laser irradiation time, incident angle laser parameters, calibrating the extinction coefficient, refractive index and film structure parameters of the optical element coating material, constructing an electric field distribution calculation model for the film part, and obtaining the light field intensity distribution of each layer inside the incident surface film layer by using the finite difference time domain method or the transfer matrix method.

[0008] Based on the obtained light field intensity distribution, a thin film thermal force analysis finite element model is constructed, laser parameters and light field intensity distribution are taken as inputs, temperature distribution and stress distribution of the incident surface film layer are obtained, and the thin film damage threshold is predicted by combining the thin film failure temperature and strength limit;

[0009] In combination with the actual film structure, the front and back surface film transmittance / reflection characteristics and the substrate material parameters, thermal force finite element analysis is performed on the substrate damage to obtain the temperature distribution and stress distribution of the substrate, and the substrate damage threshold is predicted by combining the substrate failure temperature and strength limit;

[0010] The thin film damage threshold and the substrate damage threshold are compared, and the lower threshold is the dominant laser damage mode. When the threshold difference is not large, it is determined as a mixed damage mode.

[0011] In one scheme, in the process of establishing the electric field distribution calculation model, for defect-induced damage, the finite difference time domain method is used to describe the local electric field enhancement effect, and for material intrinsic damage, the transfer matrix method is used for calculation, thereby adapting to the modeling needs of different types of damage and improving the prediction accuracy.

[0012] In one scheme, after the calculation of the light field intensity distribution is completed, the elastic modulus, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, specific heat capacity and other parameters of the thin film material are further input, and a thermal force analysis finite element model of the actual film system is constructed by using finite element analysis software to simulate and analyze the temperature field and stress field of the thin film multilayer structure.

[0013] In one scheme, the damage modeling process of the substrate part includes calibrating the transmittance and reflectance of the front and back film layers, constructing a substrate thermal force analysis finite element model combined with the incident laser parameters, and predicting the temperature distribution and stress distribution of the substrate.

[0014] In one scheme, the method further includes an experimental verification step, and the experimental system includes a laser, a power meter or an energy meter, optical elements, a spot analyzer and a microscope. By gradually increasing the laser power and adjusting the laser power density until the surface damage of the element is observed under not less than 50 times of the microscope, the effectiveness of the damage dominant mode and the threshold prediction is verified.

[0015] In one scheme, the method is applicable to optical coated elements with different wavelengths, different laser types, different film system structures, different coatings and different substrate materials, can distinguish the dominant mechanism of laser damage, and accurately predict the damage threshold.

[0016] In one scheme, the thermal finite element model takes laser parameters, light field intensity distribution, film system structure and material thermal and mechanical parameters as inputs when calculating temperature distribution and stress distribution.

[0017] The present application has the following advantages:

[0018] The present application can scientifically and systematically determine the dominant damage mode of the coated optical element under laser irradiation, and realize accurate prediction of the damage threshold. By modeling the damage mechanism of the thin film and the substrate respectively, combining the actual laser parameters and material parameters, and using the multi-physical field coupling numerical simulation method, it can be compatible with multiple damage types such as defect-induced and material intrinsic type, and is suitable for optical elements of different structures and materials. This method not only significantly improves the accuracy of damage threshold prediction, but also ensures the reliability of the model results through experimental verification, effectively supporting the damage failure analysis and optimization design of optical elements. The overall process is clear, the parameters are clear, and it is easy to implement and popularize. It provides an important theoretical basis and technical means for the performance improvement and life extension of optical coated elements in high-power laser systems. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The present application is a method flowchart;

[0020] Figure 2 The normalized electric field intensity distribution is calculated based on the transfer matrix method;

[0021] Figure 3 The present application is a simulation result graph;

[0022] Figure 4 The front surface transmittance and reflectance graph;

[0023] Figure 5 The temperature and stress result graph obtained by simulation calculation;

[0024] Figure 6 The optical path schematic diagram of the laser irradiation effect experiment system.

[0025] In the figure, 1-1080nm continuous laser source, 2-collimating mirror, 3-beam splitter, 4-power meter, 5-sample to be tested, 6-beam profiler, 7-microscope, 8-motion guide rail. DETAILED DESCRIPTION

[0026] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the related drawings. The drawings show typical embodiments of the present application. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the disclosure of the present application more thorough and comprehensive.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. In this detailed description of the application, references are made to the accompanying drawings, which illustrate specific embodiments in which the application can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the application, and it is to be understood that other embodiments can be utilized and that changes can be made without departing from the scope of the present application. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present application is defined only by the appended claims.

[0028] The technical scheme of the method for judging the dominant mode of laser damage of a coated optical element and estimating the threshold value is as follows:

[0029] Step one: model the damage mechanism of the film and the substrate respectively, determine the laser parameters, for the film part, build a calculation model of electric field distribution, and further obtain the light field intensity distribution of each layer inside the incident film layer;

[0030] Specifically, the laser parameters are determined, including the laser system, the laser wavelength, the laser power / energy, the laser spot size, the laser energy distribution, the laser irradiation time, the incident angle, etc.

[0031] The extinction coefficient, refractive index, film system structure and other parameters of the coated optical element are calibrated. The finite difference time domain (FDTD) method, the transfer matrix method (TMM) and other methods are selected to complete the calculation of the electric field distribution. For defect-induced damage, the finite difference time domain method can better describe the local electric field enhancement effect, and for material intrinsic damage, the transfer matrix method can be used for simplification. The calculation method of the transfer matrix method is as follows:

[0032] (1)

[0033] Wherein , are the forward wave and backward wave electric field amplitudes of the air side, , are the forward wave and backward wave electric field amplitudes of the substrate side, N is the number of film layer structures, and are the interface matrix and transfer matrix of the first m layer. The incident light field intensity distribution is further calculated from the electric field distribution results:

[0034] (2)

[0035] Wherein I is the light field intensity, epsilon 0 is the vacuum permittivity, c is the speed of light.

[0036] Step two: build a thin film thermal analysis finite element model, the laser parameters and the above light intensity distribution as input, to obtain the incident surface film temperature distribution and stress distribution, combined with the film failure temperature and strength limit to predict the film damage threshold;

[0037] Calibration of thin film material modulus of elasticity, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, specific heat capacity and other parameters, based on the actual film structure to build a thin film thermal analysis finite element model. Optionally, using Comsol, Ansys and other simulation software. Temperature control equation as follows:

[0038] (3)

[0039] Wherein k i The thermal conductivity of the first layer is i The temperature is T rho i The density of the first layer material is i The heat source term, including laser energy absorption, heat convection and heat radiation. Q Substitute the temperature field into the constitutive relation to calculate the thermal strain:

[0040]

[0041] (4)

[0042] epsilon thermal The thermal strain is α The thermal expansion coefficient is T 0 is the reference temperature, I The second order unit tensor, then according to the thermal strain to complete the displacement field and stress field calculation:

[0043] (5)

[0044] (6)

[0045] sigma The stress is C The fourth order elastic stiffness tensor is u The displacement is epsilon e The elastic strain is

[0046] Set the film failure temperature and strength limit, when the temperature and stress reach the critical value of damage, the corresponding power density / energy density is the damage threshold, and the film damage threshold is obtained by the above model.

[0047] ​​Step three: for the substrate part, based on the front and back surface film transmittance and reflectance, a finite element model of thermal analysis of the substrate is constructed to obtain the temperature distribution and stress distribution of the substrate, and the substrate damage threshold is predicted by combining the substrate failure temperature and strength limit;

[0048] Based on the film system design or the measured calibration of the front and back surface film transmittance and reflectance, the transmittance and reflectance of the front surface film to the incident laser are denoted as T 1、 R 1, and the transmittance and reflectance of the back surface film to the incident laser are denoted as T 2、 R 2. The parameters of the substrate material such as elastic modulus, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, and specific heat capacity are calibrated, and a finite element model of thermal analysis of the substrate is constructed. The front and back surface film transmittance and reflectance are substituted into the model, and optional simulation software such as Comsol and Ansys is used. The model is constructed based on equations (3) to (6).

[0049] The failure temperature and strength limit are set, and the critical values of temperature and stress to damage are used as the damage threshold. The substrate damage threshold is obtained through the above model.

[0050] Step four: based on the same laser parameter condition, the predicted damage thresholds of the film and the substrate are compared, and the lower one is determined as the damage dominant mode. When the predicted damage thresholds of the film and the substrate differ by ≤10%, it is determined as a mixed damage mode.

[0051] Step five: build a laser irradiation effect experiment system to verify the model calculation results. Except for the power density, the remaining laser parameters are consistent with the model. In the vicinity of the calculated damage threshold, different power density levels of laser are used to irradiate the coated optical element. After each irradiation, the irradiated area is detected by a microscope until the coated optical element is damaged. If the measured dominant mode is consistent with the model determination and the threshold error is ≤20%, the model is determined to be effective.

[0052] The laser irradiation effect experiment system includes a continuous laser or pulsed laser source, a power meter or energy meter, lenses, collimators, mirrors, beam splitters, and other optical elements, a spot analyzer, a microscope, and a sample to be tested. Damage occurs when it can be observed under a microscope of not less than 50 times.

[0053] Embodiment:

[0054] For example Figure 1As shown, this invention aims to propose a method for determining the dominant laser damage mode and predicting the threshold of coated optical elements. Taking a planar optical element with a beam-splitting film on the front surface, a 1060nm-1070nm bandpass filter on the rear surface, and HWB850 colored glass as an example, the specific method includes the following steps:

[0055] Step 1: Model the damage mechanism of the thin film and the substrate respectively, determine the laser parameters, construct an electric field distribution calculation model for the thin film, and further obtain the light field intensity distribution of each layer inside the incident film layer;

[0056] The laser parameters include: continuous laser system, Gaussian laser energy distribution, 1080nm laser wavelength, 1mm laser spot size, 5s laser irradiation time, and normal incidence angle.

[0057] The optical element coating materials are Ta2O5 and SiO2, with a refractive index of n The extinction coefficients are 2.14 and 1.45 respectively. k All are 10 -5 The front surface beam splitting membrane structure is Air|HLHL|Sub. lambda 0 = 1064 nm. The normalized electric field intensity distribution calculated based on the transfer matrix method is shown below. Figure 2 .

[0058] Step 2: Construct a finite element model for thin film thermal analysis. Using the laser parameters and the above-mentioned light field intensity distribution as input, obtain the temperature distribution and stress distribution of the incident film layer. Combine the thin film failure temperature and strength limit to predict the thin film damage threshold.

[0059] The thermal and mechanical parameters here are set according to the material parameters of Ta2O5 and SiO2, and a finite element model for thin film thermodynamic analysis is constructed based on the actual film structure. The film failure temperature is selected from the melting point of Ta2O5 and SiO2 materials, and the ultimate tensile strength is selected from the ultimate tensile strength of Ta2O5 and SiO2 materials. The simulation results are as follows. Figure 3 The laser power density reaches 57295 W / cm². 2 When the power density of the film is 10% higher than that of the substrate damage, the internal temperature of the film does not reach the failure temperature and the stress does not reach the strength limit, so the film is considered to be undamaged.

[0060] Step 3: For the substrate, based on the transmittance and reflectance of the thin film on the front and back surfaces, construct a finite element model for substrate thermal analysis to obtain the temperature distribution and stress distribution of the substrate. Combine the substrate failure temperature and strength limit to predict the substrate damage threshold.

[0061] The incident laser beam is outside the operating wavelength range of the rear surface thin film, therefore the reflectivity is 1. The transmittance and reflectivity of the front surface are shown below. Figure 4, thermal parameters and mechanical parameters are set according to the material parameters of HWB850, the base failure temperature is selected as the phase transition temperature of HWB850 material, and the strength limit is selected as the tensile strength limit of HWB850 material. The temperature and stress results obtained by simulation calculation are as follows Figure 5 , the results show that when the laser power density reaches 50930W / cm 2 , the maximum tensile stress is lower than the strength limit, but the surface temperature of the substrate reaches the phase transition temperature, and it is considered that the substrate damage occurs.

[0062] Step four: under the same laser parameter condition, compare the predicted damage threshold of the thin film and the substrate, and determine the lower one as the damage dominant mode; by comparing the threshold values, it is determined that the damage mode is substrate damage because the substrate damage threshold is lower than the thin film damage threshold.

[0063] Step five: build a laser irradiation effect experiment system to verify the model calculation results. Except for the power density, the remaining laser parameters are consistent with the model, and in the vicinity of the calculated damage threshold, different power density levels of laser are used to irradiate the coated optical element, and after each irradiation, the irradiated area is detected by a microscope until the coated optical element is damaged. If the measured dominant mode is consistent with the model determination and the threshold error is less than or equal to 20%, the model is determined to be effective.

[0064] The optical path schematic diagram of the laser irradiation effect experiment system is shown in Figure 6 , which includes 1080nm continuous laser source 1, collimating mirror 2, beam splitter 3, power meter 4, sample to be tested 5, spot analyzer 6, microscope 7 and motion guide 8 arranged in sequence.

[0065] In the vicinity of the damage power density estimated by the model, multiple power density levels are set, and each power density level is repeated at least 5 times. The damage probability under each power density is calculated, and the damage probability curve is fitted. The power density corresponding to the 95% damage probability is taken as the damage threshold. The experimental results show that below the damage threshold, no thin film damage is observed under the microscope after irradiation, but when the optical element reaches the damage threshold, substrate ablation damage can be observed. The damage threshold obtained by experiment is 52241W / cm 2 , and the deviation of the simulation calculation result is less than 20%, and the model is determined to be effective.

[0066] The above specific embodiments are only specific cases of the present application, and the patent protection scope of the present application includes but is not limited to the above specific embodiments. Any other embodiments obtained by any person skilled in the art without creative labor, which meet the claims of the present application, shall fall within the patent protection scope of the present application.

[0067] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by a computer program instructing relevant hardware, and the program can be stored in a computer readable storage medium. When the program is executed, the program can include the processes of the above-mentioned embodiment methods. The storage medium can be a magnetic disc, an optical disc, a read-only memory (ROM) or a random access memory (RAM), etc.

[0068] It should be understood that the above detailed description of the technical solutions of the present application by means of the preferred embodiments is illustrative rather than limiting. Those skilled in the art can modify the technical solutions recorded in each embodiment or make equivalent replacement for part of the technical features on the basis of the description of the present application; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of each embodiment of the present application.

Claims

1. A method for determining the dominant laser damage mode and predicting the threshold of coated optical elements, characterized in that: The method includes: Damage mechanisms of thin films and substrates were modeled separately. Laser parameters such as laser system, laser wavelength, laser power / energy, laser spot size, laser energy distribution, laser irradiation time, and incident angle were defined. Extinction coefficient, refractive index, and film structure parameters of the optical element coating material were calibrated. An electric field distribution calculation model was constructed for the thin film. The light field intensity distribution of each layer inside the incident film layer was obtained by using the finite difference time-domain method or the transfer matrix method. After the light field intensity distribution is calculated, the elastic modulus, Poisson's ratio, density, thermal conductivity, coefficient of thermal expansion, and specific heat capacity of the thin film material are further input. A thermal analysis finite element model is constructed using finite element analysis software to simulate and analyze the temperature field and stress field of the multilayer thin film structure. Based on the obtained light field intensity distribution, a finite element model for thin film thermal analysis is constructed. The laser parameters and light field intensity distribution are used as inputs to obtain the temperature distribution and stress distribution of the incident film layer. The film failure temperature and strength limit are combined to predict the film damage threshold. By combining the actual membrane structure, the transmittance / reflectance characteristics of the front and rear surface films and the substrate material parameters, a thermo-finite element analysis of substrate damage is performed to obtain the temperature distribution and stress distribution of the substrate. The substrate damage threshold is predicted by combining the substrate failure temperature and strength limit. The thin film damage threshold is compared with the substrate damage threshold, and the laser damage mode with the lower threshold is identified as the dominant laser damage mode. When the threshold difference is not significant, it is determined to be a mixed damage mode.

2. The method for determining the dominant laser damage mode and predicting the threshold of a coated optical element according to claim 1, characterized in that: In the process of establishing the electric field distribution calculation model, the finite-difference time-domain method is used to describe the local electric field enhancement effect for damage caused by defects, and the transfer matrix method is used to calculate the intrinsic material damage.

3. The method for determining the dominant laser damage mode and predicting the threshold of a coated optical element according to claim 1, characterized in that: The substrate damage mechanism modeling includes calibrating the transmittance and reflectance of the pre- and post-film layers, constructing a finite element model of substrate thermal analysis based on laser parameters, and predicting the temperature and stress distribution of the substrate.

4. The method for determining the dominant laser damage mode and predicting the threshold of a coated optical element according to claim 1, characterized in that: The method further includes an experimental verification step. The system used in the experiment includes a laser, a power meter or energy meter, optical elements, a spot analyzer, and a microscope. By gradually increasing the laser power and adjusting the laser power density, until damage that can be observed on the surface of the element under a microscope of not less than 50x is detected, the effectiveness of the damage-dominant mode and threshold prediction is verified.

5. The method for determining the dominant laser damage mode and predicting the threshold of a coated optical element according to claim 1, characterized in that: The thin film thermal analysis finite element model takes laser parameters, light field intensity distribution, film structure, and material thermal and mechanical parameters as inputs when calculating temperature and stress distribution.

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