Disclosure of Invention
The invention aims to solve the technical problems that:
The existing sampling grating is prepared on the surface of a fused quartz element, and meanwhile, chemical films with different functions are coated on the surface of the grating, so that the residual pollutant on the surface of the sampling grating is easy to cause, and the laser damage resistance of the sampling grating is further reduced.
The invention adopts the technical scheme for solving the technical problems:
the invention provides a preparation method of an in-vivo sampling grating of an optical element, which comprises the following steps:
s1, designing a grating structure, preparing a sampling grating on the surface of an optical element in the element through optimal design, and designing relevant parameters of line pairs, periods and duty ratios of the sampling grating in the body according to parameter requirements of the sampling grating;
S2, cleaning the large-caliber fused quartz optical element and fixing the optical element on a processing platform;
s3, adjusting the wavelength of the femtosecond laser, focusing the light spot into the element, and carrying out internal modification processing according to the designed period and duty ratio;
s4, adjusting the focusing light spot to move in the element, and scanning and modifying the element by a specific line pair in the element to obtain the large-caliber fused quartz element in-vivo sampling grating.
Further, the line pair of the sampling grating in the S1 is 2000l/mm, the duty ratio is 50% -100%, and the period is 1-3 mu m.
Further, the wavelength of the femtosecond laser in S3 is 1030nm and the pulse width is 255fs.
Further, the single pulse energy of the femtosecond laser in S3 is 0.1-1.5 muJ.
The invention provides an optical element internal sampling grating prepared by the method.
Compared with the prior art, the invention has the beneficial effects that:
The invention provides the preparation of the sampling grating in the large-caliber fused quartz element by using the femtosecond laser, and the femtosecond laser is used for modifying the interior of the fused quartz element, so that the diffraction rate of the fused quartz element is changed, and the function of the sampling grating is realized. The method has the advantages that (1) the femtosecond laser can directly focus on the inside of the element and directly act on the material in the fused quartz element to realize the local modification of the fused quartz element, (2) the femtosecond laser modification area is stable, the modification of the femtosecond laser is mainly determined by multiphoton ionization, the power density of the femtosecond laser only reaches an ionization threshold value in an extremely small area around the center of a light spot, so that the irradiation modification range in the acting material is stable, (3) the processing precision is high, the femtosecond laser breaks the diffraction limit by means of ultrashort pulse, the material can be subjected to superfine modification on the submicron or even nanometer size, the important parameters such as the center line density, the uniformity of sampling efficiency, the projection efficiency and the like of the sampled grating can be effectively ensured, (4) the thermal effect is small, the pulse width of the femtosecond laser is far smaller than the energy relaxation time, the crystal lattice is not influenced by electroacoustic interaction, the temperature in the action area is not remarkably increased, the damaged thermal effect can be ignored, the strict environmental factors are not required by the processing technology of the femtosecond laser, chemical reagents and sample pollution are not involved in the preparation process, the process is widely applicable to various dielectric transparent materials, and (6) the surface of the fused quartz element can not be damaged by the surface of the fused grating element due to the fact that the surface of the fused quartz element is prepared in the fused quartz element is not damaged by the surface of the fused quartz element, and the surface of the fused grating is not damaged by the fused laser.
Detailed Description
In order that those skilled in the art will better understand the present invention, exemplary embodiments or examples of the present invention will be described below with reference to the accompanying drawings. It is apparent that the described embodiments or examples are only implementations or examples of a part of the invention, not all. All other embodiments or examples, which may be made by one of ordinary skill in the art without undue burden, are intended to be within the scope of the present invention based on the embodiments or examples herein.
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.
The invention provides a preparation method of an in-vivo sampling grating of an optical element, which comprises the following steps:
S1, designing a grating structure, preparing a sampling grating on the surface of an optical element in the element through optimal design, and designing relevant parameters of line pairs, periods and duty ratios of the sampling grating in the body according to parameter requirements of the sampling grating.
According to the invention, the structural parameters of the in-vivo sampling grating are designed according to the requirements of the actual strong laser device on the parameters of the sampling grating and the requirements of the femtosecond laser modification process parameters, and errors may exist between a theoretical calculated value and an actual processing value in the process of processing the sampling grating.
The parameters of the existing sampling grating are mainly that the center line density is 2000l/mm, the groove depth of the surface grating is tens of nanometers, and the period is 1-3 mu m, so that the invention designs the volume sampling grating according to the grating diffraction theory, and in order to realize the same effect, the center line density of the volume sampling grating is 2000l/mm, the grating groove width is regulated and controlled through the grating duty ratio, the sampling grating with the duty ratio of 50% -100% is prepared, and key parameters such as the first-order transmission efficiency, the sampling efficiency and the like are checked.
According to the scalar diffraction theory of the grating, the period of the grating is Λ, and assuming that the amplitude transmittance of a single period is a function f (x), the passing rate function of the grating is:
Wherein the method comprises the steps ofRepresenting convolution, comb represents comb function, and fourier transform is performed to obtain:
Wherein the method comprises the steps ofΛ represents the grating constant, and the diffraction efficiency of the sampled grating is analyzed using the above when plane waves of unit amplitude are perpendicularly incident to the grating.
S2, cleaning the large-caliber fused quartz optical element, and fixing the optical element on a processing platform.
The processing device comprises a laser part and a machine tool part, wherein the laser part comprises a laser, a computer, a water cooling device and an infrared femtosecond laser light path, the machine tool part comprises a computer, an XYZ processing platform, a galvanometer system and a CCD camera, and an in-vivo sampling grating is prepared by modifying the inside of a fused quartz element through the laser processing device. Opening a laser, a control computer, a water cooling device and a processing platform, adjusting a laser path to enable infrared laser with 1030nm wavelength to enter the processing platform, adjusting the laser frequency to be 20KHz, and processing a cross on a silicon wafer with single pulse energy of 2 mu J to determine a focus.
S3, adjusting the wavelength of the femtosecond laser, focusing the light spot into the element, and performing internal modification processing according to the designed period and duty ratio.
The selected processing parameters are shown in table 1.
TABLE 1
When an infrared femtosecond laser is applied to a fused silica material, a large amount of free electrons are generated in a very short time by the inside of the photoelectrically applied material, and although the free electron density is far inferior to that of a conductor and a metal, it is enough that free electrons in the skin depth range directly absorb the femtosecond laser energy. After the electrons absorb energy, the energy is transferred to a lattice system for electroacoustic coupling. The photoelectric action phase and electroacoustic action phase take place for a very short time, and double Wen Fangcheng (formulas (3) and (4)) is used for describing the process, and the action time of electrons is far smaller than the action time of electrons and crystal lattices. The major concentration of the dramatic energy change in the material is in the electronic system, so the energy lost in phonon action process is negligible, and the energy coupling model is expressed as:
Wherein Ce is electron heat capacity, Cl is lattice heat capacity, Te is electron temperature, Tl is lattice temperature, Ke is electron heat conductivity, g is electroacoustic coupling coefficient, and Ieff is a laser source term. According to the action mechanism of the femtosecond laser and the fused quartz material in the formula (3) and the formula (4), the single pulse energy parameter range in the femtosecond laser processing modification process is 0.1-1.5 mu J. In order to precisely regulate the modification degree of the femtosecond laser, so that the wide-range rate and the sampling efficiency of the sampling grating are better, critical electron density parameter theory is introduced to calculate parameters of different energies, repetition frequencies, scanning speeds, light spot overlapping rates and track overlapping rates of the femtosecond laser, a multi-factor orthogonal experiment is conducted to explore the optimal technological parameters of the sampling grating of the femtosecond laser modified preparation body, the ultraviolet laser with the wavelength of 355nm and the infrared laser with the wavelength of 1030nm are determined, the ultraviolet laser passing rate is found in the experimental process, the laser energy absorption is too high for the fused quartz material, so that the internal large-range modification of the fused quartz material cannot be caused, the internal grating cannot be prepared, the influence of nonlinear absorption of the infrared femtosecond laser on the free electron density of the fused quartz material is caused under the action of high peak power, the effect of the nonlinear absorption of the infrared femtosecond laser on the fused quartz material is further caused, therefore, the internal sampling grating is prepared by adopting the infrared femtosecond laser with the wavelength of 1030nm, the experimental verification is carried out between 0.1-1.5 mu J of single-pulse energy range, when the single-pulse energy is lower than 1 mu J, the material modification degree is small, and the single-pulse energy can not be controlled between 1 mu J and 1.5 mu-J and the requirement of the single-pulse energy cannot be met.
When the femtosecond laser acts on the material, the focusing plane of the femtosecond laser appears at different depths of the material, and the absorption depth of the material to the laser can be changed. This embodiment assumes that the femtosecond laser is perpendicularly incident to the fused silica element surface, and thus the equivalent laser heat source term Itotal is expressed as:
Itotal＝Ipeak·fspace·ftime (5)
where Ipeak is the peak power density, fspace is the spatial distribution function, and ftime is the temporal distribution function.
The femtosecond laser is considered to be gaussian in both spatial and temporal distribution, and Ipeak、fspace、ftime is expressed as:
fspace＝exp[-2(ωs/ω)2] (7)
Where F is the energy density, tp is the pulse width, ω is the beam waist radius, ωs is the distance from the focal spot center. When the femtosecond laser interacts with the material, the femtosecond laser photons are firstly interacted with the electrons of the material, the electrons absorb photon energy to form free electrons, when the density of the free electrons breaks through the critical density, the basic properties of the material can be changed, a large amount of plasmas are generated in the material, the density of the free electrons and the density of the plasmas need to be precisely controlled in order to obtain an in-vivo complete sampling grating, otherwise, the plasmas are splashed to cause the damage of the material. And determining an energy transmission process in the process of modifying the fused quartz material by the femtosecond laser based on a double-temperature equation, and further designing an experiment of modifying the fused quartz material by the femtosecond laser.
After the above operation, the laser was adjusted to emit an infrared femtosecond laser having a wavelength of 1030nm, a pulse width of 255fs, a repetition frequency of 300KHz, and a single pulse energy of 1. Mu.J, and a scanning speed was set to 50mm/s. After refocusing, the laser parameters are adjusted to be 300KHz, the femtosecond laser spots are focused inside the element, the element is internally scanned and modified, the diffraction rate of the modified fused quartz element is changed, the laser beams are separated from the main beam, and the off-axis focusing and sampling of the laser are realized, as shown in fig. 2.
In order to achieve the purposes that the first-order transmission efficiency of the sampling grating is 0.1-0.3%, the center line density is 2000l/mm, and the modified line width is in the micron level, the time-space domain precise regulation and control are carried out on the femtosecond laser energy distribution, so that the light spot energy inside the femtosecond laser focusing element is precisely distributed, and the modification range is effectively controlled.
And S4, as shown in FIG. 3, adjusting the focusing light spot to move in the element, and scanning and modifying in the body by using a specific line pair to obtain the in-vivo sampling grating of the large-caliber fused quartz element.
The modified in-vivo sampling grating of the femtosecond laser is influenced by comprehensive factors, the repairing structure is changed from different aspects of microcosmic appearance, width, diameter, shape and the like, in order to achieve the best first-order transmission efficiency and ensure the surface precision of an optical element and the laser damage threshold value, the scanning track in the modification process of the femtosecond laser is precisely controlled, the moving speed and the repetition frequency of the laser in the modification process of the scanning are regulated and controlled by adopting a response surface method, and perfect concentric circle grating processing can be realized by matching the two components, and meanwhile, the surface integrity of the optical element is ensured. The motion parameters have a certain association relationship, and the track overlapping rate Gy and the light spot overlapping rate Gx can be expressed as follows:
Wherein r0 is the spot focusing radius, dx spot overlapping distance, dx＝vs/f,dy track overlapping distance, and in order to better prepare the in-vivo sampling grating, the spot overlapping rate and the track overlapping rate need to be strictly controlled.
The track overlapping rate is required to be adjusted according to the designed sampling grating parameters and is designed mainly according to a formula (10) and the grating parameters, and the track overlapping rate is directly related to the repetition frequency and the scanning speed as shown in a formula (9), so that in order to reduce energy accumulation and improve the processing efficiency, high repetition frequency and high scanning speed are adopted.
The large-caliber sampling grating provides sampling light beams for laser energy measurement on the premise of not influencing a main light beam, and experimental measurement is carried out on the in-vivo sampling grating prepared by the internal modification of the femtosecond laser by using an optical path system for measuring the sampling efficiency. According to the characteristics of the sampling grating, the performance of the sampling grating for preparing different parameters is evaluated by utilizing an experimental system of sampling efficiency and first-order diffraction efficiency, the laser with different wavelengths is adopted for detecting the tested grating, the incident light vertically irradiates the tested grating, and the first-order diffraction light intensity is measured by using a detector. By changing the wavelength of the incident light, the diffraction efficiency of the first order diffracted light at the corresponding wavelength is measured. And then testing the laser damage resistance of the prepared sampling grating by using an independently built laser damage testing system, respectively testing the tested grating by adopting R-on-1, S-on-1 and 1-on-1 testing methods, and counting the laser damage threshold value.
Although the present disclosure is disclosed above, the scope of the present disclosure is not limited thereto. Various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the disclosure, and such changes and modifications would be within the scope of the disclosure.