Coated optical element laser damage dominant mode judgment and threshold estimation method

Abstract

The invention relates to a method for judging a laser damage dominant mode and estimating a threshold value of a coated optical element, and belongs to the technical field of laser optical element damage evaluation. The implementation method comprises the following steps: calculating thin film light field distribution based on a transmission matrix method or a finite difference time domain method; predicting temperature and stress fields of the film and the substrate by combining finite element analysis; comparing damage threshold values to judge a dominant mode; and the model accuracy is verified through experiments. According to the method, a film electric field-thermal coupling model and a substrate thermal analysis model are respectively constructed, damage contributions of the two models are quantified, and small sample experiment verification is combined, so that the problems that a film-coated optical element film and substrate damage competition mechanism is complex, damage dominant modes are difficult to distinguish, and a traditional experiment method is low in efficiency and high in cost are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser optical element damage assessment, and more specifically relates to a method for determining a dominant mode and a threshold value of laser damage to a coated optical element. Background Art

[0002] Coated optical components can be susceptible to laser-induced damage when exposed to high-power / high-energy lasers. For in-band lasers, the intrinsic absorption of the film and substrate is often low, making it difficult to determine the damage initiation region. Damage may occur at internal defects in the film or at interlayer discontinuities. For out-of-band lasers, intrinsic absorption can be strong, putting both the film and substrate at risk of thermal failure. However, the damage pattern can vary dynamically with parameters such as laser wavelength and pulse width. The competitive mechanism between the film and substrate makes it difficult to accurately predict the damage threshold using a single model, necessitating the separation of the damage contributions of the film and substrate.

[0003] In addition, traditional damage assessment methods directly obtain the laser damage threshold through experiments. For example, the prior art ISO 21254 Lasers and laser-related equipment-Test methods for laser-induced damage threshold still faces problems such as large demand for test samples, long testing cycles, and high costs.

[0004] To address the above problems, the present invention proposes a damage assessment method that integrates physical modeling and experimental verification. It constructs a thin film electric field-thermomechanical coupling model and a substrate macroscopic absorption-thermomechanical model respectively, quantifies the damage contribution of the two, clarifies the damage initiation area, and uses small sample experiments to optimize the model parameters to achieve efficient prediction of the damage threshold and accurate analysis of the damage mechanism. Summary of the Invention

[0005] To clarify the complex damage competition mechanism of coated components under intra- and extra-band lasers and to address the shortcomings of traditional testing methods such as low efficiency and high cost, the present invention provides a method for determining the dominant mode and threshold estimation of laser damage to coated optical components that integrates physical modeling and experimental verification.

[0006] In order to achieve the above object, the present invention is implemented by adopting the following technical solutions: the method comprises: Model the damage mechanisms of the thin film and substrate separately, clarify the laser system, laser wavelength, laser power / energy, laser spot size, laser energy distribution, laser irradiation time, and incident angle laser parameters, calibrate the extinction coefficient, refractive index and film structure parameters of the optical component coating material, build an electric field distribution calculation model for the thin film part, and use the time-domain finite difference method or transfer matrix method to obtain the light field intensity distribution of each layer inside the incident surface film layer; 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 input to obtain the temperature distribution and stress distribution of the incident surface film layer. The film damage threshold is predicted by combining the film failure temperature and strength limit. Based on the actual film structure, the transmittance / reflectivity characteristics of the front and back surfaces, and the substrate material parameters, a thermal finite element analysis of the substrate damage is performed to obtain the substrate temperature and stress distribution. The substrate damage threshold is predicted by combining the substrate failure temperature and strength limit. The film damage threshold is compared with the substrate damage threshold, and the one with the lower threshold is considered the dominant laser damage mode. When the threshold difference is not significant, it is determined to be a mixed damage mode.

[0007] In one solution, during the establishment of the electric field distribution calculation model, the time-domain finite-difference 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 damage of the material, thereby adapting to the modeling needs of different types of damage and improving the accuracy of the prediction.

[0008] In one scheme, after the light field intensity distribution calculation 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 the finite element analysis software is used to construct a thermal analysis finite element model of the actual film system to simulate and analyze the temperature field and stress field of the thin film multilayer structure.

[0009] In one embodiment, the damage modeling process of the substrate includes calibrating the transmittance and reflectivity of the front and rear film layers, building a substrate thermal analysis finite element model based on the incident laser parameters, and predicting the temperature distribution and stress distribution of the substrate.

[0010] In one embodiment, the method further includes an experimental verification step, wherein the experimental system includes a laser, a power meter or energy meter, an optical element, a spot analyzer and a microscope. The laser power is gradually increased and the laser power density is adjusted until damage is detected on the surface of the element that is observable under a microscope of not less than 50 times magnification, thereby verifying the effectiveness of the damage dominant mode and threshold prediction.

[0011] In one embodiment, the method is applicable to optical coating elements with different wavelengths, different laser types, different film structures, different coatings, and substrate materials. It can distinguish the dominant mechanism of laser damage and accurately predict the damage threshold.

[0012] In one embodiment, the thermal finite element model takes laser parameters, light field intensity distribution, film structure and material thermal and mechanical parameters as inputs when calculating temperature distribution and stress distribution.

[0013] Beneficial effects of the present invention: The present invention can scientifically and systematically determine the dominant damage mode of coated optical components under laser irradiation, and realize accurate prediction of the damage threshold. By modeling the damage mechanism of the film and substrate respectively, combining the actual laser parameters and material parameters, and adopting a multi-physics field coupling numerical simulation method, it can be compatible with various damage types such as defect-induced and material intrinsic types, and is applicable to optical components of different structures and materials. This method not only significantly improves the accuracy of damage threshold prediction, but also can ensure the reliability of model results through experimental verification, effectively supporting damage failure analysis and optimization design of optical components. The overall process is clear, the parameters are clear, and it is easy to implement in engineering and promote application, providing an important theoretical basis and technical means for improving the performance and extending the life of optical coating components in high-power laser systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Flow chart of the method of the present invention; Figure 2 is the normalized electric field intensity distribution calculated based on the transfer matrix method; Figure 3 This is a simulation result diagram of the present invention; Figure 4 is the transmittance and reflectance diagram of the front surface; Figure 5 The temperature and stress results obtained from the simulation calculation are shown in the figure; Figure 6 Schematic diagram of the optical path of the laser irradiation effect experimental system.

[0015] In the figure, 1-1080nm continuous laser source, 2-collimating mirror, 3-beam splitter, 4-power meter, 5-sample to be measured, 6-spot analyzer, 7-microscope, 8-motion guide. DETAILED DESCRIPTION

[0016] To facilitate understanding of the present invention, the present invention will be described more fully below with reference to the accompanying drawings. The drawings illustrate exemplary embodiments of the present invention. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the present invention.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those understood by those skilled in the art to which the present invention pertains. The terms used in the present specification are for the purpose of describing specific embodiments only and are not intended to limit the present invention. To facilitate understanding of the present invention, a more comprehensive description of the present invention will be provided below with reference to the accompanying drawings. Typical embodiments of the present invention are shown in the drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the present invention.

[0018] The technical solution of the method for determining the dominant mode and threshold value of laser damage to a coated optical component of the present invention is as follows: Step 1: Model the damage mechanism of the film and substrate respectively, determine the laser parameters, build an electric field distribution calculation model for the film part, and further obtain the light field intensity distribution of each layer inside the incident surface film layer; Specifically, the laser parameters should be clarified, including laser system, laser wavelength, laser power / energy, laser spot size, laser energy distribution, laser irradiation time, incident angle, etc.

[0019] Calibrate parameters such as the extinction coefficient, refractive index, and film structure of the optical component coating material. Select methods such as the finite-difference time-domain method (FDTD) and the transfer matrix method (TMM) to complete the electric field distribution calculation. For damage caused by defects, the finite-difference time-domain method can be used to better describe the local electric field enhancement effect. For intrinsic material damage, the transfer matrix method can be used for simplification. The calculation method of the transfer matrix method is as follows: (1) in 、 is the electric field amplitude of the forward wave and the backward wave on the air side, 、 is the electric field amplitude of the forward wave and the backward wave on the basal side, N is the number of membrane layer structures, and It is m The interface matrix and transmission matrix of the layer. The incident light field intensity distribution is further calculated from the electric field distribution result: (2) in I is the light field intensity, ε 0 is the dielectric constant of vacuum, c The speed of light.

[0020] Step 2: Construct a finite element model for thin film thermal analysis. Using the laser parameters and the aforementioned light field intensity distribution as input, the temperature and stress distribution of the incident film layer are obtained. The film damage threshold is then predicted by combining the film failure temperature and strength limit. Calibrate parameters such as the elastic modulus, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, and specific heat capacity of the film material, and construct a finite element model for film thermal analysis based on the actual film structure. Optionally, simulation software such as Comsol and Ansys can be used. The temperature control equation is as follows: (3) in k i For the i Layer thermal conductivity, T is the temperature, ρ i For the i Layer material density, Q is the heat source term, including laser energy absorption, heat convection and heat radiation.

[0021] Substitute the temperature field into the constitutive relation and calculate the thermal strain: (4) ε thermal is the thermal strain, α is the coefficient of thermal expansion, T 0 is the reference temperature, I is a second-order unit tensor, and then the displacement field and stress field calculation are completed according to the thermal strain: (5) (6) σ is stress, C is the fourth-order elastic stiffness tensor, u is the displacement, ε e is the elastic strain.

[0022] The film failure temperature and strength limit are set, and the power density / energy density corresponding to when the temperature and stress reach the critical value of damage is used as the damage threshold. The film damage threshold is obtained through the above model.

[0023] Step 3: For the substrate, a finite element model for thermal analysis is constructed based on the transmittance and reflectivity of the front and back surface films to obtain the substrate's temperature and stress distribution. The substrate damage threshold is then predicted by combining the substrate's failure temperature and strength limit. Based on the film system design or experimental measurement, the transmittance and reflectance of the front and rear surface films are calibrated. The transmittance and reflectance of the front surface film to the incident laser are recorded as T 1.R 1. The transmittance and reflectance of the rear surface film to the incident laser are expressed as T 2. R 2. Calibrate the elastic modulus, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, specific heat capacity, and other parameters of the substrate material to construct a finite element model for thermal analysis of the substrate. Substitute the transmittance and reflectivity of the front and back surface films into the model. Optionally, use simulation software such as Comsol or Ansys. Similarly, complete the model construction based on Equations (3) to (6).

[0024] The failure temperature and strength limit are set, and the power density / energy density corresponding to when the temperature and stress reach the critical value of damage is used as the damage threshold. The substrate damage threshold is obtained through the above model.

[0025] Step 4: Under the same laser parameter conditions, the predicted damage thresholds of the film and substrate are compared, and the one with the lower threshold is determined to be the damage-dominant mode. When the difference in the predicted damage thresholds of the film and substrate is ≤10%, it is determined to be a mixed damage mode.

[0026] Step 5: Build a laser irradiation effect experimental system to experimentally verify the model's calculated results. With the exception of power density, all other laser parameters remain consistent with the model. Near the calculated damage threshold, lasers with varying power density levels are used to irradiate the coated optical component. After each exposure, the irradiated area is examined using a microscope until damage occurs. If the measured dominant mode matches the model's prediction and the threshold error is ≤20%, the model is considered valid.

[0027] The laser irradiation effect experiment system includes a continuous or pulsed laser source, a power meter or energy meter, optical components such as lenses, collimators, reflectors, and beam splitters, a spot analyzer, a microscope, and the sample to be tested. Damage refers to damage that can be observed under a microscope with a magnification of at least 50 times.

[0028] Example: like Figure 1 As shown, the present invention aims to propose a method for determining the dominant mode of laser damage and estimating the threshold of a coated optical component. Taking a planar optical component with a beam splitter coating on the front surface and a 1060nn-1070nm bandpass filter coating on the back surface, and a substrate of HWB850 colored glass as an example, the specific method includes the following steps: Step 1: Model the damage mechanism of the film and substrate respectively, determine the laser parameters, build an electric field distribution calculation model for the film part, and further obtain the light field intensity distribution of each layer inside the incident surface film layer; The laser parameters include: the laser system is continuous laser, the laser energy distribution is Gaussian distribution, the laser wavelength is 1080 nm, the laser spot size is 1 mm, the laser irradiation time is 5 s, and the incident angle is normal incidence.

[0029] Optical element coating materials are Ta2O5, SiO2, refractive index n The extinction coefficients are 2.14 and 1.45 respectively. k Both are 10 -5 The front surface beam splitter film structure is Air|HLHL|Sub. λ 0=1064nm. The normalized electric field intensity distribution calculated based on the transfer matrix method is shown in Figure 2 .

[0030] Step 2: Construct a finite element model for thin film thermal analysis. Using the laser parameters and the aforementioned light field intensity distribution as input, the temperature and stress distribution of the incident film layer are obtained. The film damage threshold is then predicted by combining the film failure temperature and strength limit. Here, the thermal and mechanical parameters are set according to the material parameters of Ta2O5 and SiO2, and a thin film thermal analysis finite element model 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 strength limit is selected from the tensile strength limit of Ta2O5 and SiO2 materials. The simulation results are as follows: Figure 3 , laser power density reaches 57295 W / cm 2 When the power density of the film is 10% higher than that of the substrate damage, the internal temperature of the film has not reached the failure temperature and the stress has not reached the strength limit, so it is considered that the film has not been damaged.

[0031] Step 3: For the substrate, a finite element model for thermal analysis is constructed based on the transmittance and reflectivity of the front and back surface films to obtain the substrate's temperature and stress distribution. The substrate damage threshold is then predicted by combining the substrate's failure temperature and strength limit. The incident laser is outside the working wavelength of the rear surface film, so the reflectivity is 1. The transmittance and reflectivity of the front surface are shown in Figure 4 The thermal and mechanical parameters are set according to the HWB850 material parameters. The base failure temperature is the phase transition temperature of the HWB850 material, and the strength limit is the tensile strength limit of the HWB850 material. The temperature and stress results obtained by simulation are as follows: Figure 5 The results showed that the laser power density reached 50930W / cm 2 When the maximum tensile stress is lower than the strength limit, the substrate surface temperature reaches the phase transition temperature, and it is considered that substrate damage occurs.

[0032] Step 4: Based on the same laser parameter conditions, the predicted damage thresholds of the film and substrate are compared, and the one with the lower threshold is determined to be the dominant damage mode. By comparing the thresholds, it can be seen that the substrate damage threshold is lower than the film damage threshold, and the damage mode is determined to be substrate damage.

[0033] Step 5: Build a laser irradiation effect experimental system to experimentally verify the model's calculated results. With the exception of power density, all other laser parameters remain consistent with the model. Near the calculated damage threshold, lasers with varying power density levels are used to irradiate the coated optical component. After each exposure, the irradiated area is examined using a microscope until damage occurs. If the measured dominant mode matches the model's prediction and the threshold error is ≤20%, the model is considered valid.

[0034] The optical path diagram of the laser irradiation effect experimental system is shown in Figure 6 , including a 1080nm continuous laser source 1, a collimating mirror 2, a beam splitter 3, a power meter 4, a sample to be measured 5, a spot analyzer 6, a microscope 7, and a motion guide 8, which are arranged in sequence.

[0035] Near the damage power density estimated by the model, multiple power density levels were set, and at least 5 repeated experiments were performed at each power density level. The damage probability at each power density was calculated, and the damage probability curve was fitted. The power density corresponding to 95% damage probability was used as the damage threshold. The experimental results show that below the damage threshold, no film damage was observed under the microscope after the irradiation, but after the optical element reached the damage threshold, substrate ablation damage was observed. The damage threshold obtained in the experiment was 52241W / cm 2 , the deviation from the simulation calculation result is less than 20%, and the model is judged to be valid.

[0036] The above-mentioned specific embodiments are only specific cases of the present invention. The patent protection scope of the present invention includes but is not limited to the above-mentioned specific embodiments. Any other embodiments that comply with the claims of the present invention and are obtained by any ordinary technician in the relevant technical field without making any creative work should fall within the patent protection scope of the present invention.

[0037] Those skilled in the art will appreciate that all or part of the processes in the above-described method embodiments can be implemented by instructing related hardware through a computer program. The program can be stored in a computer-readable storage medium, and when executed, the program can include the processes in the above-described method embodiments. The storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM), or a random access memory (RAM).

[0038] It should be understood that the detailed description of the technical solutions of the present invention using the preferred embodiments above is illustrative and not restrictive. A person skilled in the art, after reading the present specification, may modify the technical solutions described in the embodiments or replace some of the technical features therein with equivalents; such modifications or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the dominant mode and threshold estimation of laser damage in coated optical components, characterized by: The method includes: Model the damage mechanisms of the thin film and substrate separately, clarify the laser system, laser wavelength, laser power / energy, laser spot size, laser energy distribution, laser irradiation time, and incident angle laser parameters, calibrate the extinction coefficient, refractive index and film structure parameters of the optical component coating material, build an electric field distribution calculation model for the thin film part, and use the time-domain finite difference method or transfer matrix method to obtain the light field intensity distribution of each layer inside the incident surface film layer; 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 input to obtain the temperature distribution and stress distribution of the incident surface film layer. The film damage threshold is predicted by combining the film failure temperature and strength limit. Based on the actual film structure, the transmittance / reflectivity characteristics of the front and back surfaces, and the substrate material parameters, a thermal finite element analysis of the substrate damage is performed to obtain the substrate temperature and stress distribution. The substrate damage threshold is predicted by combining the substrate failure temperature and strength limit. The film damage threshold is compared with the substrate damage threshold, and the one with the lower threshold is considered 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 mode and threshold value of laser damage to a coated optical component according to claim 1, characterized in that: In the process of establishing the electric field distribution calculation model, the time-domain finite difference method is used to describe the local electric field enhancement effect for the damage caused by defects, and the transfer matrix method is used to calculate the intrinsic damage of the material, thereby adapting to the modeling needs of different types of damage and improving the accuracy of prediction.

3. The method for determining the dominant mode and threshold value of laser damage to a coated optical component according to claim 1, characterized in that: After the light field intensity distribution calculation is completed, the elastic modulus, Poisson's ratio, density, thermal conductivity, thermal expansion coefficient, and specific heat capacity parameters of the thin film material are further input, and 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 thin film multilayer structure.

4. The method for determining the dominant mode and threshold value of laser damage to a coated optical component according to claim 1, characterized in that: The substrate damage mechanism modeling includes calibrating the transmittance and reflectivity of the film layer before and after calibration, building a substrate thermal analysis finite element model in combination with laser parameters, and predicting the temperature distribution and stress distribution of the substrate.

5. The method for determining the dominant mode and threshold value of laser damage to a coated optical component 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, an optical element, a spot analyzer and a microscope. The laser power is gradually increased and the laser power density is adjusted until damage is detected on the component surface that is observable under a microscope with a magnification of not less than 50 times, thereby verifying the effectiveness of the damage dominant mode and threshold prediction.

6. The method for determining the dominant mode and threshold value of laser damage to a coated optical component according to claim 1, characterized in that: The method is applicable to optical coating elements with different wavelengths, different laser types, different film structures, different coatings, and different substrate materials. It can distinguish the dominant mechanism of laser damage and accurately predict the damage threshold.

7. The method for determining the dominant mode and threshold value of laser damage to a coated optical component 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 performing temperature distribution and stress distribution calculations.

Images