CN114324393B - Calculation method for initial energy deposition of laser damage caused by machining surface defect area of fused quartz optical element - Google Patents
Calculation method for initial energy deposition of laser damage caused by machining surface defect area of fused quartz optical element Download PDFInfo
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- 230000007547 defect Effects 0.000 title claims abstract description 142
- 230000003287 optical effect Effects 0.000 title claims abstract description 118
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 title claims abstract description 116
- 239000005350 fused silica glass Substances 0.000 title claims abstract description 116
- 230000006378 damage Effects 0.000 title claims abstract description 57
- 230000008021 deposition Effects 0.000 title claims abstract description 21
- 238000004364 calculation method Methods 0.000 title abstract description 6
- 238000003754 machining Methods 0.000 title description 4
- 238000012545 processing Methods 0.000 claims abstract description 90
- 230000005284 excitation Effects 0.000 claims abstract description 35
- 239000003574 free electron Substances 0.000 claims abstract description 33
- 238000000034 method Methods 0.000 claims abstract description 29
- 238000005137 deposition process Methods 0.000 claims abstract description 27
- 238000002189 fluorescence spectrum Methods 0.000 claims abstract description 16
- 239000000463 material Substances 0.000 claims abstract description 8
- 230000036962 time dependent Effects 0.000 claims abstract description 4
- 230000007704 transition Effects 0.000 claims description 29
- 230000005283 ground state Effects 0.000 claims description 18
- 238000002474 experimental method Methods 0.000 claims description 14
- 238000001917 fluorescence detection Methods 0.000 claims description 13
- 230000002950 deficient Effects 0.000 claims description 8
- 238000003384 imaging method Methods 0.000 claims description 3
- 238000004445 quantitative analysis Methods 0.000 claims description 3
- 238000011160 research Methods 0.000 claims description 3
- 230000008859 change Effects 0.000 claims description 2
- 230000005274 electronic transitions Effects 0.000 claims description 2
- 230000007847 structural defect Effects 0.000 claims description 2
- 230000008569 process Effects 0.000 description 9
- 230000004907 flux Effects 0.000 description 6
- 230000009471 action Effects 0.000 description 5
- 230000005855 radiation Effects 0.000 description 5
- 238000010521 absorption reaction Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 3
- 229910018557 Si O Inorganic materials 0.000 description 2
- 230000004927 fusion Effects 0.000 description 2
- 230000001939 inductive effect Effects 0.000 description 2
- 238000000752 ionisation method Methods 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 230000008832 photodamage Effects 0.000 description 2
- 238000005424 photoluminescence Methods 0.000 description 2
- LIVNPJMFVYWSIS-UHFFFAOYSA-N silicon monoxide Inorganic materials [Si-]#[O+] LIVNPJMFVYWSIS-UHFFFAOYSA-N 0.000 description 2
- 230000002159 abnormal effect Effects 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
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- 230000031700 light absorption Effects 0.000 description 1
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Abstract
The invention discloses a calculation method for initial energy deposition of laser damage caused by a defect area on a processing surface of a fused quartz optical element, belongs to the field of engineering optics, and aims to solve the problem that the prior art lacks an effective method for calculating or characterizing the initial energy deposition of laser damage caused by the defect area on the processing surface of the fused quartz optical element; the invention is carried out according to the following steps: step one, determining a defect energy level structure of a defect area of a processing surface of a fused quartz optical element; step two, obtaining fluorescence intensity of fluorescence emission spectrum generated by excitation of defect areas and defect-free areas of the processing surface of the fused quartz optical element; step three, establishing a nonlinear ionization model of the material of the defect area of the processing surface of the optical element; step four, obtaining the temperature generated in the time-dependent variation curve of electron density of each energy level on the defect-free surface of the fused quartz optical element and the energy deposition process; and fifthly, acquiring the temperature and free electron density of the energy deposition process at the initial stage of laser damage caused by the processing surface of the fused quartz optical element.
Description
Technical Field
The invention belongs to the field of engineering optics, and particularly relates to a calculation method for energy deposition in an initial stage of laser damage caused by a defect area of a machining surface of a fused quartz optical element.
Background
Fused silica optical elements have become the most widely used optical element in laser driven inertial confinement fusion (Inertial Confinement Fusion, ICF) devices due to their excellent optical properties. Fused silica is a typical hard and brittle material and is extremely difficult to process, and surface structure defects such as pits, cracks and scratches are inevitably introduced into the processing surface of the fused silica optical element due to the action of mechanical forces during processing. The surface structure defects can cause the absorption characteristics of the fused quartz material to be changed, and meanwhile, the phenomena of light field modulation and the like can be generated, so that the laser damage resistance of the fused quartz optical element under the high-power laser service condition is seriously damaged, and the service life of the fused quartz optical element and the improvement of the energy flow density output of an ICF device are greatly limited.
At present, it is widely considered that a great number of microscopic point defects such as vacancies, dislocation and the like can be generated on the processing surface of the fused quartz optical element due to the action of mechanical force in the processing process. The micro defects are intensively distributed at the defect positions of the processing surface structure of the fused quartz optical element, so that the light absorption characteristic of the defect region of the processing surface of the fused quartz optical element is greatly changed, and the laser damage resistance of the defect region of the processing surface of the fused quartz is seriously damaged. Therefore, in some reports at home and abroad, local microscopic point defects formed on the processing surface of a fused quartz optical element are even regarded as damage precursors (also called as 'photodamage defects') for inducing strong laser damage and reducing the laser damage threshold of the optical element. The initial energy deposition process of laser damage caused by the fused quartz optical element processing surface defect area is the abnormal absorption process of microscopic photodamage point defects of the fused quartz material processing surface defect area on incident high-power laser. Meanwhile, the initial energy deposition process of laser damage caused by the defect area of the processing surface of the fused quartz optical element is often influenced by phenomena such as light field modulation generated by the defect of the processing surface structure of the fused quartz optical element.
It is widely considered at home and abroad that the initial energy deposition process of the laser damage caused by the processing surface defect area of the fused quartz optical element is the energy source of the laser damage process of the processing surface defect area of the fused quartz optical element, and the laser damage threshold value of the processing surface defect area of the fused quartz optical element and the laser damage degree of the processing surface defect area of the fused quartz optical element under the specific laser flux condition are directly determined. However, there is currently a lack of efficient calculation methods for calculating or characterizing the laser energy deposited during the initial stages of the laser damage process induced by defective areas on the processing surface of fused silica optical elements.
The initial energy deposition of the laser-induced damage of the processing surface of the fused quartz optical element is an energy source for inducing the laser damage, and has very important significance for fundamentally revealing the laser-induced damage mechanism of the processing surface of the fused quartz optical element and improving the laser damage resistance of the processing surface of the fused quartz optical element in engineering.
Disclosure of Invention
The prior art lacks an effective method for calculating or characterizing the problem of energy deposition in the initial stage of laser damage caused by a defective area on the processing surface of a fused quartz optical element, and provides a method for calculating the energy deposition in the initial stage of laser damage caused by the defective area on the processing surface of the fused quartz optical element.
The technical scheme provided by the invention is as follows: the method for calculating the initial energy deposition of the laser damage caused by the defect area of the processing surface of the fused quartz optical element comprises the following steps:
step one, determining a defect energy level structure of a defect area on the processing surface of a fused quartz optical element based on a fluorescence detection experiment of a variable excitation light wavelength;
carrying out a photoinduced fluorescence detection experiment for changing the excitation light wavelength on the optimal defect position of the processing surface of the fused quartz optical element, determining the number of electronic defect energy levels through the number of photoinduced fluorescence characteristic peaks, determining the energy bandwidth among the energy levels through the change rule of the characteristic peaks along with the excitation light wavelength, and establishing a defect energy level structure of the processing surface of the fused quartz optical element according to an electronic transition theory; calculating the relaxation time of each energy level transition to the self-trapping region;
step two, respectively obtaining fluorescence intensities of fluorescence emission spectrums generated by excitation of a defect area and a defect-free area of the processing surface of the fused quartz optical element through a photoinduced fluorescence detection experiment;
step three, combining the defect energy level structure of the defect area of the processing surface of the fused quartz optical element built in the step one, and building high-power laser based on an electron transition theory and an atomic orbit theory>1J/cm 2 ) Processing a nonlinear ionization model of a surface defect area material by an optical element under an irradiation condition;
step four, obtaining a time-dependent variation curve of electron density of each energy level on the defect-free surface of the fused quartz optical element and the temperature generated in the energy deposition process at the initial stage of laser-induced damage;
obtaining the intensity of a laser light field based on a laser damage threshold value of a defect-free surface of a fused quartz optical element, substituting the intensity of the laser light field into a nonlinear ionization model in the third step, adjusting the density of electrons in a ground state to enable the density of electrons in a conduction band to be equal to the density of electrons in a critical state, obtaining an initial value of the density of electrons in the ground state, substituting the obtained initial value of the density of electrons in the ground state and the intensity of the laser light field into the nonlinear ionization model in the third step, obtaining an evolution curve of the density of electrons in each energy level with time under the laser flux, and obtaining the density of electrons in the steady state moment and the temperature generated in the energy deposition process;
step five, obtaining the temperature and free electron density of the energy deposition process at the initial stage of laser damage caused by the processing surface of the fused quartz optical element;
based on a fluorescence quantitative analysis theory, obtaining an initial value of the ground state electron density and fluorescence intensity according to the fourth step, obtaining a proportional relationship between the initial value and the fluorescence intensity, converting the peak intensity of a fluorescence emission spectrum generated by the surface structural defect and the defect-free surface excitation of the fused quartz optical element obtained in the second step into the initial value of the ground state electron density, inputting the initial value into a nonlinear ionization model established in the third step, and solving the temperature and the free electron density at the steady state moment of the energy deposition process.
In the first step, variable wavelength excitation light with the wavelength ranging from 400nm to 500nm is used for scanning the processing surface of the fused quartz optical element, and the maximum point of the fluorescence intensity of the processing surface of the fused quartz optical element is selected as the optimal defect position.
Further in step one, the determined defect energy level structure is that the energy bandwidth between the valence band and the defect energy level I is 2.64eV; the energy bandwidth between defect level I and defect level II is 2.64eV; the energy bandwidth between defect level II and conduction band is 2.96eV.
Further in the first step, the relaxation time tau from the transition of the defect energy level I to the self-trapping region is obtained by an E-exponential fitting method I =0.25 ns, transition of defect level II to the self-trapping relaxation time τ II ＝0.092ns。
In the second step, detecting the processing surface of the fused quartz optical element by adopting a super-depth three-dimensional imaging system, and arbitrarily selecting a defect area of the processing surface of the fused quartz optical element as a research object; and (3) carrying out a photo-induced fluorescence detection experiment on the selected defect area and the defect-free surface of the processing surface of the fused quartz optical element by adopting excitation light with the wavelength of 440nm to obtain fluorescence emission spectra generated by excitation of the selected defect area and the defect-free surface of the processing surface of the fused quartz optical element.
The nonlinear ionization model in the third step is specifically:
wherein: n is n V -ground state electron density on the valence band;
n I -defect level 1 electron density;
n II -defect level 2 electron density;
n C -free electron density on the conduction band;
N PD -point defect density;
i-light field intensity;
σ A ,σ B photon absorption cross-sectional area;
σ C photon collision cross-sectional area;
τ I -the defect level I transitions to the belonging self-trapping relaxation time;
τ II -transition of defect level II to the belonging self-trapping relaxation time;
τ C -free electron decay time;
omega-laser frequency;
E g -forbidden band width;
c, heat capacity;
kappa-thermal conductivity;
t-temperature;
E BD Si-O bond energy;
V C -electron collision frequency;
n 0 -initial point defect density;
K 0 -maximum point defect density;
k-the gas mole constant;
E 01 -band width of valence band to defect level I;
E 12 -band width of defect level I to defect level II;
E 23 -defect level II to band width of the conduction band;
e-the amount of charge carried by the electrons;
c-speed of light;
ε 0 -vacuum dielectric constant.
Further in the fifth step, the critical free electron density is 8.7X10 27 m -3 。
Further in step five, a steady state free electron density of 1.44X10 at the defect region of the working surface of the fused silica optical element is obtained 28 /m 3 Steady state free electron density of defect free surface of 8.7X10 27 /m 3 The method comprises the steps of carrying out a first treatment on the surface of the The temperature of the energy deposition process for the defect-free surface of the fused silica optical element processing surface was 14200K, and the temperature of the energy deposition process for the defect-free surface was 12200K.
Compared with the prior art, the invention has the advantages that: 1. the invention establishes a model for simulating the initial energy deposition process of the laser-induced damage based on the energy level structure, and characterizes the initial energy deposition process of the laser-induced damage on the processing surface of the fused quartz optical element; 2. the invention characterizes the energy deposited in the energy deposition process at the initial stage of the laser-induced damage by outputting physical quantities such as free electron density, temperature and the like at the steady state moment; 3. the invention reveals the laser induced damage mechanism of the processing surface of the fused quartz optical element fundamentally, and is beneficial to effectively improving the laser damage resistance of the processing surface of the fused quartz optical element in engineering.
Drawings
FIG. 1 is a flow chart of a calculation method for energy deposition at the initial stage of laser damage induced by a defect area on the processing surface of a fused silica optical element;
FIG. 2 is a graph of steady-state fluorescence spectra of optimal defect positions on the processing surface of a fused silica optical element under the action of excitation light with different wavelengths;
FIG. 3 is a graph showing the peak intensity of characteristic peaks of light induced fluorescence as a function of the wavelength of excitation light;
FIG. 4 is a schematic view of the defect energy level structure of the defect region of the processing surface of the fused silica optical element;
FIG. 5 is a surface topography image of a machined surface texture defect of a fused silica optical component;
FIG. 6 is a graph of fluorescence emission spectra from stimulated emission of defect areas and defect free surfaces of a fused silica optical element processing surface;
FIG. 7 is a graph of electron density at each energy level of a finished surface defect region and a non-defective surface of a fused silica optical element as a function of time.
Detailed Description
In the description of the present invention, it should be noted that terms such as "upper", "lower", "front", "rear", "left", "right", and the like in the embodiments indicate terms of orientation, and such terms of orientation do not constitute limitations of the present invention only for the sake of simplifying the description based on the positional relationship of the drawings of the specification.
In the description of the present invention, it should be noted that the terms "first," "second," and "third" mentioned in the embodiments of the present invention are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first", "a second", or a third "may explicitly or implicitly include one or more such feature.
In order that the above objects, features and advantages of the invention will be readily understood, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings.
According to the electron transition theory and the atomic orbit theory, the initial energy deposition process of laser damage caused by the defect area of the processing surface of the fused quartz optical element is reproduced by describing the electron stimulated transition process and the relaxation process of the ground state electrons of the defect area of the processing surface of the fused quartz optical element. The higher the energy deposited during the energy deposition process at the beginning of the laser induced damage to the processing surface of the fused silica optical element, the higher the temperature at the energy deposition zone must be. Meanwhile, under the condition of laser irradiation, the processing surface of the fused quartz optical element can ionize to generate plasma. The higher the energy deposited during the energy deposition process at the beginning of the laser induced damage of the processing surface of the fused silica optical element, the more plasma (free electrons) is generated, i.e. the higher the plasma (free electrons) density is. Therefore, the method finally characterizes the energy deposited in the initial stage of the laser damage process caused by the defect area of the processing surface of the fused quartz optical element by outputting the temperature of the ground state electron transition of the defect area of the processing surface of the fused quartz optical element and the free electron density at the steady state moment.
Example 1: step one, determining a defect energy level structure of a defect area on the processing surface of a fused quartz optical element based on a fluorescence detection experiment of a variable excitation light wavelength;
in order to make the obtained fluorescence intensity data easier to observe and carry out contrast analysis, a photoinduced fluorescence scanning detection method is adopted, the processing surface of the fused quartz optical element is scanned by using variable wavelength excitation light (in the range of 400-500 nm), and the maximum point of the fluorescence intensity of the processing surface of the fused quartz optical element is selected as the optimal defect position.
As shown in FIG. 2, a photo-induced fluorescence detection experiment is performed on the optimal defect position of the processing surface of the fused quartz optical element to obtain a steady-state fluorescence spectrum of the optimal defect position, a photo-induced fluorescence band with a peak value at a wavelength of 525nm (2.29 eV) is detected in a visible light wave band of 350-850nm, a photo-induced fluorescence band with a peak value at a wavelength of 770nm (1.6 eV) is detected in a near infrared wave band of 650-1100nm, and electronic relaxation occurs in the element and a fluorescence signal corresponding to energy of the two peak positions is released. Since the peak intensity of the fluorescence band at 525nm (2.29 eV) is much higher than the peak intensity of the fluorescence band at 770nm (1.6 eV), the sub-band gap of the fluorescence band at 525nm (2.29 eV) is closer to the valence band, the valence band electrons are excited to the first defect level when the electrons are excited by the excitation light, the sub-band gap of the fluorescence band at 770nm (1.6 eV) is relatively far from the valence band, and the second defect level.
As shown in fig. 3, as the excitation light wavelength increases (energy decreases), peak intensities at wavelengths of 525nm (2.29 eV) and 770nm (1.60 eV) tend to decrease, and when the excitation light wavelength reaches 470nm (2.64 eV), photoluminescence peaks at 525nm (2.29 eV) and 770nm (1.6 eV) disappear, indicating that the optimum defect position has two defect levels of 2.64eV, so that the energy bandwidth between the valence band and defect level I is 2.64eV, and the energy bandwidth between defect level II and defect level I is 2.64eV; as the excitation light wavelength increases, when the excitation light wavelength reaches 420nm (2.96 eV), a photoluminescence band having a peak at 770nm (1.60 eV) appears, and thus the energy bandwidth between defect level II and conduction band is 2.96eV.
This is because when excitation light energy exceeds conduction band energy, an electron undergoes a non-radiative transition, and the non-radiative transition and the radiative transition are in a competing relationship in electron relaxation, i.e., when the number of electrons at the time of the non-radiative transition increases, the number of electrons at which the radiative transition occurs will decrease. Therefore, when the laser wavelength is 420nm or less (2.96 eV), that is, when the excitation light energy is equal to or greater than the band width between the conduction band and the level defect, electrons are excited to the conduction band to become free electrons, and the free electrons are mostly transferred to the next level by the non-radiative transition. At this time, the number of electrons in which a radiation transition occurs will appear very small compared to the number of electrons in which a non-radiation transition occurs, and therefore, the fluorescence peak at 770nm (1.60 eV) is small. But electrons cannot be excited to the conduction band as free electrons when the excitation light energy is smaller than the energy band width between the conduction band and the level defect, i.e., when the laser wavelength is greater than 420nm (2.96 eV). At this time, electrons cannot undergo a non-radiative transition, and can only undergo relaxation in the manner of a radiative transition. Thus, when the laser wavelength is gradually increased to 420nm (2.96 eV), a fluorescence band occurs with a peak at 770nm (1.60 eV).
The time for releasing the fluorescence phase in the different radiation transition processes can be obtained by an E index fitting method, and the defect energy level I is transited to the self-trapping region relaxation time tau I =0.25 ns, transition of defect level II to the self-trapping relaxation time τ II =0.092 ns. The as-built fused silica optical element working surface defect level structure is shown in fig. 4.
According to the invention, through measurement and discovery of a plurality of areas, the defect energy level structures of different defect areas are the same, and finally the area is selected for description.
And setting the wavelength range of the fluorescence emission spectrum to be (450-1100 nm) based on the characteristic peak position of the micro sightseeing damage point defect. The wavelength range of the excitation light used in the invention can provide two wave bands (visible light and near infrared wave bands) to cover the range of 450-1100 nm.
Step two, respectively obtaining fluorescence intensities of fluorescence emission spectrums generated by excitation of a defect area and a defect-free area of the processing surface of the fused quartz optical element through a photoinduced fluorescence detection experiment;
as shown in fig. 5, detecting the processing surface of the fused quartz optical element by using a super-depth three-dimensional imaging system, and arbitrarily selecting a defect area of the processing surface of the fused quartz optical element as a research object; as shown in FIG. 6, fluorescence detection experiments were conducted on the selected defective region and the defect-free surface of the processed surface of the fused silica optical element using excitation light having a wavelength of 440nm, respectively, to obtain fluorescence emission spectra generated by excitation of the selected defective region and the defect-free surface of the processed surface of the fused silica optical element.
Since the fluorescence intensity of the fluorescence emission spectrum obtained under the action of the excitation light with the wavelength of 440nm is highest, in order to obtain the fluorescence emission spectrum with the highest fluorescence intensity and the most obvious characteristics, the excitation light with the wavelength of 440nm is selected to carry out a photo-induced fluorescence detection experiment.
Step three, combining the fused quartz light established in the step oneThe surface defect energy level structure of the element machining is learned, and a high-power laser is established based on an electron transition theory and an atomic orbit theory>1J/cm 2 ) Processing a nonlinear ionization model of a surface defect area material by an optical element under an irradiation condition;
and (3) combining the defect energy level structure of the processing surface of the fused quartz optical element obtained in the step one, and establishing a nonlinear ionization model of the material of the defect area of the processing surface of the fused quartz optical element, wherein the ionization model totally comprises 4 electron energy levels (valence band, defect energy level I, defect energy level II and conduction band). The electron in the ground state (non-bonded electron) of the defect area of the processing surface of the fused quartz optical element absorbs photon energy and then passes through the defect energy level I and the defect energy level II to finally transition to a conduction band to become free electrons, wherein partial electrons are unstable and generate radiation transition and non-radiation transition.
Wherein: n is n V -ground state electron density on the valence band;
n I ——defect level 1 electron density;
n II -defect level 2 electron density;
n C -free electron density on the conduction band;
N PD -point defect density;
i-light field intensity;
σ A ,σ B photon absorption cross-sectional area;
σ C photon collision cross-sectional area;
τ I -the defect level I transitions to the belonging self-trapping relaxation time;
τ II -transition of defect level II to the belonging self-trapping relaxation time;
τ C -free electron decay time;
-planck constant;
omega-laser frequency;
E g -forbidden band width;
c, heat capacity;
kappa-thermal conductivity;
t-temperature;
t-time;
E BD Si-O bond energy;
V C -electron collision frequency;
n 0 -initial point defect density;
K 0 -maximum point defect density;
k-the gas mole constant;
E 01 -band width of valence band to defect level I;
E 12 -band width of defect level I to defect level II;
E 23 -defect level II to band width of the conduction band;
e-the amount of charge carried by the electrons;
c-speed of light;
ε 0 -vacuum dielectric constant.
Step four, obtaining a time-dependent variation curve of electron density of each energy level on the processing surface of the fused quartz optical element;
performing laser damage threshold test experiments on the defect-free surface of the fused quartz optical element to obtain the laser damage threshold of 52J/cm 2 According to the formulaObtain i=13 GW/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the Wherein the laser pulse width T=4ns, and phi is the molecular laser flux; let i=13 GW/cm 2 Substituting the three-step non-linear ionization model, and adjust n v Let n C ＝n cr Obtaining an initial value n of the ground state electron density V0 ＝7.2×10 26 。
In ICF devices, fused silica optical elements are often used as frequency tripler elements, so that the service laser wavelength of the fused silica optical element is determined to be 355nm, and the critical free electron density n when the fused silica optical element reaches the laser damage threshold under the action of 355nm wavelength excitation light is calculated and obtained according to the service laser wavelength cr 8.7X10 27 /m 3 。
Wherein: epsilon 0 -vacuum dielectric constant;
ω L -laser frequency;
λ L -laser wavelength;
e-the amount of charge carried by the electrons;
m e -electron quality.
Let i=13 GW/cm 2 And an initial value n of valence band electron density V0 ＝7.2×10 26 Substituting the laser flux into the model established in the third step to obtain the laser flux of 52J/cm 2 Electron density curves (n) I 、n II 、n V And n C Evolution over time) and the temperature T generated during the deposition of the energy at the initial stage of the laser-induced damage (electron nonlinear ionization process). The results show that n C The evolution over time eventually tends to stabilize. Therefore, the laser flux can be output at 52J/cm 2 Free electron density n at steady-state time C And the temperature T generated during the energy deposition process (electron nonlinear ionization process) at the initial stage of the laser-induced damage.
Step five, obtaining the temperature and free electron density of the energy deposition process at the initial stage of laser damage caused by the processing surface of the fused quartz optical element;
according to the fluorescence quantitative analysis theory, the initial value n of the ground state electron density on the valence band V0 In direct proportion to the fluorescence intensity, the fluorescence intensity obtained in the fourth step and the initial value n of the ground state electron density on the valence band are utilized V0 The ratio can be obtained, and thus the initial value n of the ground state electron density on the valence band can be established V0 The relation between the fluorescence intensity and the peak intensity of the fluorescence emission spectrum generated by the stimulated surface of the defect and the defect-free surface of the fused quartz optical element obtained in the second step is converted into the initial ground state electron density n V0 And will transform n V0 And (3) inputting the nonlinear ionization model established in the step (III), and solving the temperature and the free electron density at the steady-state moment of the energy deposition process. The temperature during energy deposition and the free electron density generated during nonlinear ionization can be used to characterize the energy deposited during energy deposition at the beginning of laser damage.
As shown in FIG. 7, the electron density of each energy level corresponding to the defect region and the defect-free surface of the processed surface of the fused silica optical element was plotted against time to obtain a steady-state free electron density of 1.44X10 at the defect region of the processed surface of the fused silica optical element 28 /m 3 Steady state free electron density of defect free surface of 8.7X10 27 /m 3 The method comprises the steps of carrying out a first treatment on the surface of the The electron ionization region temperature of the defect region of the processing surface of the fused silica optical element is 14200K, the defect-free surface energy deposition process temperature is 12200K, and the value is equal to the reported defect-free surface of the fused silicaThe temperature value 11800K of the induced laser damage is similar, and the accuracy of the method provided by the invention is further verified.
Claims (7)
1. The method for calculating the initial energy deposition of the laser damage caused by the defect area of the processing surface of the fused quartz optical element is characterized by comprising the following steps:
step one, determining a defect energy level structure of a defect area on the processing surface of a fused quartz optical element based on a fluorescence detection experiment of a variable excitation light wavelength;
carrying out a photoinduced fluorescence detection experiment for changing the excitation light wavelength on the optimal defect position of the processing surface of the fused quartz optical element, determining the number of electronic defect energy levels through the number of photoinduced fluorescence characteristic peaks, determining the energy bandwidth among the energy levels through the change rule of the characteristic peaks along with the excitation light wavelength, and establishing a defect energy level structure of the processing surface of the fused quartz optical element according to an electronic transition theory; calculating the relaxation time of each energy level transition to the self-trapping region;
step two, respectively obtaining fluorescence intensities of fluorescence emission spectrums generated by excitation of a defect area and a defect-free area of the processing surface of the fused quartz optical element through a photoinduced fluorescence detection experiment;
step three, combining the defect energy level structure of the defect area of the processing surface of the fused quartz optical element, which is established in the step one, and establishing a nonlinear ionization model of the material of the defect area of the processing surface of the optical element under the irradiation condition of high-power laser based on an electron transition theory and an atomic orbit theory, wherein the irradiation intensity of the high-power laser is more than 1J/cm 2 ；
Step four, obtaining a time-dependent variation curve of electron density of each energy level on the defect-free surface of the fused quartz optical element and the temperature generated in the energy deposition process at the initial stage of laser-induced damage;
obtaining the intensity of a light field based on a laser damage threshold value of a defect-free surface of a fused quartz optical element, substituting the intensity of the light field into a nonlinear ionization model in the third step, adjusting the density of ground state electrons on a valence band to enable the density of free electrons on a conduction band to be equal to the critical free electron density, obtaining an initial value of the density of the ground state electrons on the valence band, substituting the obtained initial value of the density of the ground state electrons on the valence band and the intensity of the light field into the nonlinear ionization model in the third step, obtaining an evolution curve of the density of electrons of each energy level in the intensity of the light field along with time, and obtaining the density of the free electrons at a steady state moment and the temperature generated in an energy deposition process;
step five, obtaining the temperature and free electron density of the energy deposition process at the initial stage of laser damage caused by the processing surface of the fused quartz optical element;
based on a fluorescence quantitative analysis theory, according to the initial value and the fluorescence intensity of the ground state electron density on the valence band obtained in the step four, obtaining the proportional relation of the initial value and the fluorescence intensity, converting the fluorescence intensity of a fluorescence emission spectrum generated by exciting the surface structural defects of the fused quartz optical element obtained in the step two into the initial value of the ground state electron density on the valence band, inputting the initial value into the nonlinear ionization model established in the step three, and solving the temperature and the free electron density at the steady state moment of the energy deposition process;
the nonlinear ionization model in the third step is specifically:
2. the method for calculating the initial energy deposition of the laser damage caused by the defect area of the processing surface of the fused silica optical element according to claim 1, wherein in the first step, the processing surface of the fused silica optical element is scanned by using variable wavelength excitation light with the wavelength ranging from 400nm to 500nm, and the maximum point of the fluorescence intensity of the processing surface of the fused silica optical element is selected as the optimal defect position.
3. The method for calculating the initial energy deposition of laser damage induced by a defect area on the processing surface of a fused silica optical element according to claim 2, wherein in the first step, the determined defect level structure is that the energy bandwidth between the valence band and the defect level I is 2.64eV; the energy bandwidth between defect level I and defect level II is 2.64eV; the energy bandwidth between defect level II and conduction band is 2.96eV.
4. A method for calculating the initial energy deposition of laser damage induced by a defect region on the processing surface of a fused silica optical element according to claim 3, wherein in step one, the relaxation time τ from the transition of the defect energy level I to the self-trapping region is obtained by an E-exponential fitting method I =0.25 ns, transition of defect level II to the self-trapping relaxation time τ II =0.092ns。
5. The method for calculating the initial energy deposition of the laser damage caused by the defect area of the processing surface of the fused quartz optical element according to claim 4 is characterized in that in the second step, the processing surface of the fused quartz optical element is detected by adopting a super-depth three-dimensional imaging system, and the defect area of the processing surface of the fused quartz optical element is arbitrarily selected as a research object; and (3) carrying out a photo-induced fluorescence detection experiment on the selected defect area and the defect-free surface of the processing surface of the fused quartz optical element by adopting excitation light with the wavelength of 440nm to obtain fluorescence emission spectra generated by excitation of the selected defect area and the defect-free surface of the processing surface of the fused quartz optical element.
6. The method for calculating the initial energy deposition of laser induced damage in a defect area of a working surface of a fused silica optical element according to claim 1, wherein in the fifth step, the critical free electron density is。
7. A method for calculating the initial energy deposition of laser induced damage in a defective region of a working surface of a fused silica optical element according to claim 6, wherein in the fifth step, a steady-state free electron density of 1.44×10 in the defective region of the working surface of the fused silica optical element is obtained 28 /m 3 Steady state free electron density of defect free surface of 8.7X10 27 /m 3 The method comprises the steps of carrying out a first treatment on the surface of the The temperature of the energy deposition process for the defect-free surface of the fused silica optical element processing surface was 14200K, and the temperature of the energy deposition process for the defect-free surface was 12200K.
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