阅读说明：本技术 弱荧光成像条件下的熔石英亚表面缺陷高分辨成像方法 (Fused quartz subsurface defect high-resolution imaging method under weak fluorescence imaging condition ) 是由 刘红婕 董瑞 王方 陈元 郑天然 袁晓东 胡东霞 于 2021-07-21 设计创作，主要内容包括：本发明公开了一种弱荧光成像条件下的熔石英亚表面缺陷高分辨成像方法,包括构建一缺陷检测装置；连续激光束60°-70°入射到样品表面,在离轴照明模式下,使样品的表面及亚表面缺陷在EMCCD中清晰成像。本发明针对样品亚表面的缺陷具有极低的荧光量子产额及传统成像CCD无法响应弱荧光的问题,确定其荧光缺陷的激发光谱以及发射光谱特性,在此基础上选择合适的激发波长光源及探测器,采用离轴照明模式,克服了同轴照明方式下杂散光严重干扰目标弱荧光信号,无法高分辨成像的问题；结合所测样品的透反率、菲涅尔反射、激光能量等因素,确定了最佳的入射角度,本发明可实现对熔石英光学元件快速、无损、高分辨、大面积成像检测。(The invention discloses a fused quartz subsurface defect high-resolution imaging method under the weak fluorescence imaging condition, which comprises the steps of constructing a defect detection device; the continuous laser beam is incident to the surface of the sample at 60-70 degrees, and the surface and sub-surface defects of the sample are clearly imaged in the EMCCD under an off-axis illumination mode. Aiming at the problems that the defects of the subsurface of a sample have extremely low fluorescence quantum yield and the traditional imaging CCD cannot respond to weak fluorescence, the excitation spectrum and the emission spectrum characteristics of the fluorescence defects are determined, and on the basis, a light source and a detector with proper excitation wavelength are selected, and an off-axis illumination mode is adopted, so that the problems that stray light seriously interferes with target weak fluorescence signals and high-resolution imaging cannot be realized under a coaxial illumination mode are solved; the method determines the optimal incident angle by combining the factors of the transmittance and the reflectance of the measured sample, Fresnel reflection, laser energy and the like, and can realize rapid, nondestructive, high-resolution and large-area imaging detection on the fused quartz optical element.)

1. A fused quartz subsurface defect high-resolution imaging method under the weak fluorescence imaging condition is characterized in that: comprises the following steps;

(1) constructing a defect detection device, wherein the defect detection device comprises a sample table, a laser and an image acquisition unit;

the sample stage is horizontally arranged and used for placing a sample and driving the sample to move in three dimensions;

the laser is used for emitting continuous laser beams, and forms an excitation light source to the surface and the subsurface of the sample after shaping and focusing;

the image acquisition unit is positioned right above the sample stage, is used for imaging a fluorescence signal excited by the surface of the sample, and comprises an EMCCD, a filter and a variable-magnification microscopic imaging lens which are sequentially arranged from top to bottom;

(2) selecting a sample made of a fused quartz material, mounting the sample on a sample table, enabling a continuous laser beam to be obliquely incident to the surface of the sample, enabling an included angle of 60-70 degrees to be formed between the continuous laser beam and the surface of the sample, enabling a variable power microscopic imaging lens to be positioned right above the sample and vertical to the surface of the sample and be used for obtaining a fluorescence signal generated by the continuous laser beam on the surface of the sample and carrying out fluorescence imaging, wherein the variable power microscopic imaging lens adopts a 5-45 power optical magnification imaging lens, and the numerical aperture of an objective lens in the variable power microscopic imaging lens is 0.4;

(3) under the excitation light source, the height and the variable multiple of the variable-power imaging lens are adjusted, so that the surface of the sample is clearly imaged in the EMCCD.

2. The fused silica subsurface defect high resolution imaging method under weak fluorescence imaging conditions of claim 1, wherein: and (2) performing spectral analysis on the surface defects of the sample by using a fluorescence spectrometer.

3. The fused silica subsurface defect high resolution imaging method under weak fluorescence imaging conditions of claim 1, wherein: the laser is in an off-axis illumination mode, and the excitation light source is polarized light.

4. The fused silica subsurface defect high resolution imaging method under weak fluorescence imaging conditions of claim 1, wherein: the laser is a 355nm continuous laser, and the filter is a 375nm high-pass filter.

5. The fused silica subsurface defect high resolution imaging method under weak fluorescence imaging conditions of claim 1, wherein: also comprises the following steps;

(4) dividing the surface of the sample into a plurality of areas, setting the motion trail of the sample stage, and enabling the sample stage to sequentially move to each area for independent imaging to form a fluorescence image corresponding to each area one by one.

Technical Field

The invention relates to an imaging method, in particular to a fused quartz subsurface defect high-resolution imaging method under the weak fluorescence imaging condition.

Background

Large high power/high energy laser devices are operated at fluxes close to the damage threshold of the optical element in order to achieve maximum output, and therefore optical element damage performance is particularly important and critical in determining the output capability of such laser devices. At present, most of the damage problems of the optical element under high flux can be attributed to various defects on the subsurface of the optical element, the optical element is processed by the processes of cutting, grinding, polishing and the like, and although the surface looks nearly perfect and flawless and the roughness is below 1nm, the surface and the subsurface layer inevitably have microscopic defects such as subsurface microcracks, impurity pollution and the like. These defects are several microns to several hundred microns deep and absorb laser energy when irradiated with laser light resulting in localized high material temperatures and thus damage. Research shows that the subsurface layer defect can be effectively removed by utilizing hydrofluoric acid etching treatment, so that the damage resistance of the optical element is greatly improved, but the surface shape and the surface defect of the optical element are influenced and secondary pollution is caused by deep hydrofluoric acid etching.

The existing fluorescence imaging test technology can realize nondestructive and rapid detection of the subsurface defect of the optical element, but has the problem that the subsurface defect can only be detected on a large scale, and the detection precision is far from enough for the optical element used for a high-power laser device. Due to the extremely low fluorescence quantum yield of the optical element sub-surface in the micron/submicron level, the conventional imaging CCD has difficulty responding to weak fluorescence and has serious noise. Meanwhile, stray fluorescence in the coaxial illumination mode can seriously interfere with clear imaging of defective target fluorescence, and imaging resolution is low.

Disclosure of Invention

The invention aims to solve the problems, is particularly suitable for large-aperture optical elements, and provides a fused quartz subsurface defect high-resolution imaging method under the weak fluorescence imaging condition, which can carry out high-resolution imaging on the subsurface defect of the optical element under the weak fluorescence imaging condition.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows: a fused quartz subsurface defect high-resolution imaging method under the weak fluorescence imaging condition is characterized in that: comprises the following steps;

(1) constructing a defect detection device, wherein the defect detection device comprises a sample table, a laser and an image acquisition unit;

the sample stage is horizontally arranged and used for placing a sample and driving the sample to move in three dimensions;

the laser is used for emitting continuous laser beams, and forms an excitation light source to the surface and the subsurface of the sample after shaping and focusing;

the image acquisition unit is positioned right above the sample stage, is used for imaging a fluorescence signal excited by the surface of the sample, and comprises an EMCCD, a filter and a variable-magnification microscopic imaging lens which are sequentially arranged from top to bottom;

(2) selecting a sample made of fused quartz materials, installing the sample on a sample table, enabling continuous laser beams to be obliquely incident to the surface of the sample, enabling an included angle of 60-70 degrees to the surface of the sample, enabling a variable power microscopic imaging lens to be located right above the sample and vertical to the surface of the sample and being used for obtaining fluorescence signals generated by the continuous laser beams on the surface of the sample and carrying out fluorescence imaging, wherein the variable power microscopic imaging lens adopts a 5-45-power optical magnification imaging lens, and the numerical aperture of an objective lens in the variable power microscopic imaging lens is 0.4.

(3) Under the excitation light source, the height and the variable multiple of the variable-power imaging lens are adjusted, so that the surface of the sample is clearly imaged in the EMCCD.

Preferably, the method comprises the following steps: and (2) performing spectral analysis on the surface defects of the sample by using a fluorescence spectrometer.

Preferably, the method comprises the following steps: the laser is in an off-axis illumination mode, and the excitation light source is polarized light.

Preferably, the method comprises the following steps: the laser is a 355nm continuous laser, and the filter is a 375nm high-pass filter.

Preferably, the method comprises the following steps: also comprises the following steps;

(4) dividing the surface of the sample into a plurality of areas, setting the motion trail of the sample stage, and enabling the sample stage to sequentially move to each area for independent imaging to form a fluorescence image corresponding to each area one by one.

In the invention, the selection of the laser is as follows: according to the characteristics of the fused quartz element, a Perkin Elmer LS55 fluorescence spectrometer is mainly used for carrying out spectral analysis on the surface/subsurface defect of the finely polished fused quartz element, and the surface/subsurface defect of the surface/subsurface defect and an emission spectrum under the excitation of 355nm laser are obtained. As can be seen from figures 1 and 2 of the attached drawings in the specification, the wave band with the excitation wavelength below 390nm is suitable, and a 355nm continuous laser is selected as an excitation source in combination with the use wavelength of a fused silica element.

Regarding the selection of the filter segment: in order to avoid the influence of 355nm scattered light and other stray light, a high-cut-off 375nm high-pass filter is selected, and the high-cut-off 375nm high-pass filter is arranged in front of the CCD, so that the influence of 355nm scattered light and other stray light is avoided, and the detection sensitivity of fluorescence imaging is improved.

Regarding selection of EMCCD: the invention adopts a weak light response detector with high quantum efficiency and low noise, is based on the photoluminescence principle, and can excite fluorescence when laser irradiates a subsurface defect of an optical element, and the fluorescence is imaged to a CCD by a high-magnification imaging lens, but the fluorescence quantum yield of the micron/submicron defect of the subsurface is extremely low, so that the traditional imaging CCD can not respond to a weak fluorescence signal, and the EMCCD is selected.

With respect to the angle of incidence: in the invention, the excitation light source is excited by side incidence because the common high-magnification imaging lens can absorb ultraviolet laser to generate fluorescence, and the generated fluorescence signal is far stronger than that generated by the defect of the finely polished fused quartz, the side incidence excitation can reduce background light noise, and the larger the angle is, the better the angle is. According to the invention, through a large number of experiments and analyses, the polarized light with the incidence angle of 60-70 degrees is selected to be incident, because of Fresnel reflection, the transmitted excitation light energy can be greatly reduced when the angle is too large, and after a plurality of experiments, the incident light with the angle is finally selected, and the surface transmittance is higher than 98%.

For off-axis illumination modes: the reason is that in the coaxial illumination mode, the stray fluorescence of the lens can seriously interfere the clear imaging of the target fluorescence, and high-resolution imaging cannot be realized. And the off-axis illumination mode can eliminate the background stray fluorescence.

Compared with the prior art, the invention has the advantages that: the method comprises the steps of determining the excitation spectrum and emission spectrum characteristics of the fluorescence defect of a sample aiming at the problems that the defect of the subsurface of the sample has extremely low fluorescence quantum yield and the traditional imaging CCD cannot respond to weak fluorescence, and selecting a light source and a detector with proper excitation wavelength on the basis; the off-axis illumination mode is adopted, so that the problems that stray light seriously interferes a target weak fluorescence signal and high-resolution imaging cannot be realized in a coaxial illumination mode are solved; according to a measured sample, determining an optimal incident angle and polarized light incidence by combining factors such as the transmittance, the Fresnel reflection and the laser energy of the sample; the adjustable range of optical amplification of 5-45 times and the objective lens imaging with the numerical aperture of 0.4 are met. The resolution can reach 1um level, and the size of a single fluorescence image can reach 600 mu m multiplied by 600 mu m.

The invention can realize the rapid, nondestructive, high-resolution and large-area imaging detection of the fused quartz optical element, and can provide reliable basis for defect control and quality evaluation of the processing procedure of the optical element with high laser damage threshold.

Drawings

FIG. 1 is a plot of the excitation spectrum of a surface/subsurface defect of a finish polished fused silica component;

FIG. 2 is a graph of an emission spectrum of a surface/subsurface defect of a finish-polished fused quartz component;

FIG. 3 is a schematic structural view of the present invention;

FIG. 4 is a graph of the transmittance of the surface of the element at different angles of incidence;

FIG. 5 is an image of a sample taken with a conventional optical microscope;

FIG. 6 is a fluorescence image obtained by the apparatus and method of the present invention at the same location as in FIG. 5.

In the figure: 1. an EMCCD; 2. a filter plate; 3. a variable magnification microscopic imaging lens; 4. a sample; 5. a sample stage; 6. an excitation light source; 7. a fluorescent signal.

Detailed Description

The invention will be further explained with reference to the drawings.

Example 1 referring to fig. 1 to 5, a fused silica subsurface defect high resolution imaging method under weak fluorescence imaging conditions comprises the following steps;

(1) constructing a defect detection device, wherein the defect detection device comprises a sample table 5, a laser and an image acquisition unit;

the sample table 5 is horizontally arranged and used for placing the sample 4 and driving the sample to move three-dimensionally;

the laser is used for emitting continuous laser beams, and an excitation light source 6 is formed to the surface and the sub-surface of the sample 4 after shaping and focusing;

the image acquisition unit is positioned right above the sample stage 5, is used for imaging a fluorescence signal 7 excited by the surface of the sample 4, and comprises an EMCCD1, a filter 2 and a variable-magnification microscopic imaging lens 3 which are sequentially arranged from top to bottom;

(2) selecting a sample 4 made of a fused quartz material, installing the sample 4 on a sample table 5, enabling continuous laser beams to be obliquely incident on the surface of the sample 4, enabling an included angle of 60-70 degrees to be formed between the continuous laser beams and the surface of the sample 4, enabling a variable power microscopic imaging lens 3 to be positioned right above the sample 4 and vertical to the surface of the sample 4 and be used for obtaining a fluorescence signal 7 generated by the continuous laser beams on the surface of the sample 4 and carrying out fluorescence imaging, wherein the variable power microscopic imaging lens 3 adopts a 5-45 power optical amplification imaging lens, and the numerical aperture of an objective lens in the variable power microscopic imaging lens 3 is 0.4;

(3) the height and variable magnification of the variable magnification imaging lens were adjusted under the excitation light source 6 to allow the surface of the sample 4 to be imaged clearly in the EMCCD 1.

In this embodiment, in the step (2), the fluorescence spectrometer is used to perform spectral analysis on the surface defects of the sample 4. The laser is in off-axis illumination mode and the excitation light source 6 is polarized light. In the defect detection device, the laser is a 355nm continuous laser, the filter 2 is a 375nm high-pass filter 2, the EMCCD1 is selected, and the incident angle is 60-70 degrees.

The selection of the laser, filter 2, EMCCD1, angle of incidence, etc. is all selected according to the characteristics of the fused silica component.

Wherein, the laser and the filter 2 are analyzed by a fluorescence spectrometer for the surface/subsurface defect of the fine polishing fused quartz element to obtain the graph shown in figure 1 and figure 2. FIG. 1 is an excitation spectrum showing the response of the material, i.e., the material at the defect, to the external excitation light, and showing the relationship between the self-emission wavelength and the excitation wavelength, and it can be seen that the response of the excitation wavelength in the wavelength band below 390nm is high; fig. 2 shows the spectrum of the fused silica element generated by 355nm laser excitation, and it can be seen that the fused silica element satisfies several conditions of higher emission spectrum intensity, higher response below 390nm, and the fused silica use band when excited by 355nm laser.

For the EMCCD1, because the quantum yield of the defect in the invention is very low, the emitted fluorescence is very weak, the high-resolution precision imaging is difficult to realize by the common detector, and the weak light response detector with high quantum efficiency and low noise needs to be used in cooperation to realize the detection of the weak fluorescence signal 7. FIG. 2 shows that the peak has a quantum efficiency of about 90% in the range of 500 to 700nm, and is near 525 nm.

For the incident angle, in order to improve the imaging resolution and the fluorescence collection efficiency, an objective lens with a larger numerical aperture needs to be selected, meanwhile, in order to avoid the excitation light entering the imaging objective lens, the excitation light needs to enter the surface of the sample 4 from the outer side, the larger the incident angle is, the better the incident angle is, but due to fresnel reflection, the energy of the excitation light transmitted by the angle with a larger angle is greatly reduced, and therefore comprehensive consideration is needed to obtain the optimized result. Referring to fig. 4, fig. 4 shows the transmittance and reflectance of the surface of the element at different incident angles, and we finally choose the incident light to be incident at the incident angle P of 60 ° to 70 ° when the surface transmittance is greater than 98% and the imaging objective lens with the numerical aperture of 0.4 can image the surface of the sample 4. In the graph of fig. 4, R is reflectance, Rs is reflectance of S-polarized light, Rp is reflectance of p-polarized light, T is transmittance, Ts is transmittance of S-polarized light, and Tp is transmittance of p-polarized light. In the present invention, the continuous laser beam is obliquely incident on the surface of the sample 4 and forms an angle of 60 ° to 70 ° with the surface of the sample 4, which is also referred to as an incident light angle.

Example 2: referring to fig. 1 to 5, in this embodiment, the continuous laser beam is obliquely incident on the surface of the sample 4 and forms an angle of 65 ° with the surface of the sample 4, and the rest is the same as in embodiment 1. The device of the invention basically ensures that the transmittance of the element surface reaches 98% under the angle and can meet the requirement that the imaging objective with the numerical aperture of 0.4 images the surface of the sample 4. To illustrate the effect of the present invention, we captured the image of sample 4 taken with a normal optical microscope, and the fluorescence image obtained with the apparatus and method of the present invention, as shown in FIGS. 5 and 6, respectively, both of which have image sizes of 600 μm 600. mu.m. As can be seen by comparing FIG. 5 with FIG. 6, the fluorescence imaging of the present invention is very clear.

According to the invention, the maximum output power of the laser is 500mW, during actual operation, the incident angle and the light spot area of a light beam to a test position of a sample 4 can be controlled, the laser power can be controlled according to the strength of a signal of the test sample 4, the numerical aperture of the imaging objective lens is 0.4, the imaging lens can realize 5-45 times of optical amplification, and the optical resolution is superior to 1 um.

Example 3: referring to fig. 1 to 5, the present invention further includes a step (4) of dividing the surface of the sample 4 into a plurality of regions, setting a motion trajectory of the sample stage 5, and sequentially moving the sample stage 5 to each region for individual imaging to form a fluorescence image corresponding to each region.

In fact, the sample stage 5 and the image acquisition unit are both connected with the computer, and the acquired fluorescence image is sent into the computer, so that the fluorescence image can be analyzed by utilizing the prior art, the fused quartz optical element can be rapidly and nondestructively detected in a high-resolution and large-area imaging manner, and a reliable basis can be provided for defect control and quality judgment of a processing procedure of the high-laser-damage-threshold optical element.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.