CN111272217B - Method for extracting ultraviolet to infrared laser spots by utilizing fractal structure - Google Patents
Method for extracting ultraviolet to infrared laser spots by utilizing fractal structure Download PDFInfo
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Abstract
The invention discloses a method for extracting ultraviolet to long-wave infrared laser spots by utilizing a fractal structure. The method utilizes the fluctuation of light to distinguish the wavelength: the electromagnetic waves with the same wavelength reflect and transmit different structures with different scales, and the electromagnetic waves with different wavelengths reflect and transmit different structures with a specific scale. The method also utilizes the self-similarity of the fractal structure to construct structures respectively suitable for ultra-wide spectrum, and converts the difference of the reflection of the electromagnetic wave on the structures with different scales into the difference of spatial positions. The data are analyzed by measuring the reflection characteristics of different positions and combining the self-similarity theory of the fractal structure, and the laser spot size information and the laser wavelength information are obtained. By measuring in a plurality of horizontal directions, the spot shape of the laser light can be obtained. The advantage of this patent is simple structure, can extract wavelength and spot size information of wavelength within the range from 70nm to 14 mu m.
Description
Technical Field
The invention relates to a technology for extracting laser wavelength and light spot information, in particular to a method for extracting ultraviolet to infrared laser light spots by utilizing a fractal structure, which is suitable for measuring laser light spot size information and laser wavelength information.
Background
Starting from the theory of the a-B coefficient of radiation proposed by einstein, the theoretical basis of laser technology is continuously advancing. Until 1953, the american physicist charles haddock with his student ather scholl produced the first microwave laser and obtained a microwave laser with higher coherence. Laser technology is then increasingly used in many fields of application. At present, laser technology has been widely applied in various fields such as industrial processing, medicine, civil engineering, aerospace, military, scientific research and the like, and plays an increasingly irreplaceable role.
Laser spot shape information is one of the important parameters of the laser. The laser processing and manufacturing precision in the industrial processing field, the resolution of a micro-area property characterization means based on laser excitation in the scientific research field, the laser positioning precision in military and other information all depend on the size information of laser spots. Therefore, the laser spot information extraction technology has important application value.
At present, methods for measuring the size of a laser beam mainly include methods such as a trepanning method, a knife edge method, a CCD method, a scanning slit method and the like. For the condition that the laser spot is large, the trepanning method, the knife edge method and the scanning slit method are simple in structure, and the measured data have clear physical significance, so that the laser spot measuring instrument has certain advantages. When the size of the laser spot is close to the wavelength, the shape of the laser spot can be measured by selecting a knife edge method, a scanning slit method and a CCD method, so that more accurate laser light intensity morphology information can be obtained. When the light spot is small, the structure of the knife edge method is complex, and the blade is required to be thin enough to meet the requirement of test precision. At present, due to photoelectric conversion, large pixel size and the like, the CCD method is still difficult to effectively measure mid-infrared and far-infrared lasers, so that the application of the CCD method in the infrared field is limited.
The invention provides a fractal structure-based light spot information extraction technology. Due to the self-similarity of the fractal structure, the extraction of laser spot information with the wavelength ranging from 70nm to 14 mu m can be ensured, and the problem that the mid-far infrared cannot be effectively measured by a CCD method is solved by using the HgCdTe single-pixel infrared detector as a photoelectric conversion structure. The simulation result based on wave optics ensures that when the size of a laser spot is close to the wavelength, the fractal structure can still effectively distinguish the size of the laser spot, so that the fractal structure can be suitable for spot detection and other applications in a micro-area property characterization means based on laser excitation.
Disclosure of Invention
The invention provides a method for obtaining the wavelength and the spot size of laser by measuring the intensity of reflected, transmitted or scattered waves in a specific direction and processing data based on the interaction of a fractal structure and electromagnetic waves. Due to the wave nature of light, the electromagnetic waves with the same wavelength reflect and transmit different structures with different scales, and the electromagnetic waves with different wavelengths reflect and transmit different structures with a specific scale. And on the basis, obtaining a theoretical model of reflectivity interacting with a plurality of structures with different scales, and fitting the obtained data. The method also utilizes the self-similarity of the fractal structure to construct structures respectively suitable for ultra-wide spectrum, and converts the difference of the reflection of the electromagnetic wave on the structures with different scales into the difference of spatial positions. The data are analyzed by measuring the reflection characteristics of different positions and combining the self-similarity theory of the fractal structure, and the laser spot size information and the laser wavelength information are obtained. By measuring in a plurality of horizontal directions, the spot shape of the laser light can be obtained. The reflection (or transmission, scattering in a specific direction) rates of different spatial positions extracted according to the characteristic that different positions of the fractal structure have different characteristic scales are convolved with the spatial intensity information of the laser spot to obtain the light intensity information detected by the detector. And obtaining the size information of the light spot through data processing.
The testing device is as follows: the device comprises a semi-transparent semi-reflective mirror, a fractal structure, a displacement platform and a detector. The fractal structure is a deformed Cantor set, the dimension of the structure in the graph 2 is D1 + ln2/ln2.5 ≈ 1.7565 according to the fractal theory, and the fractal structure is centrosymmetric to the point x 0. By the fractal theory, the fractal dimension of a centrosymmetric Cantor set uniquely determines a fractal structure. The height information of the fractal structure is shown in fig. 3, the shaded part is an evaporated polycrystalline gold film with the thickness of 10nm, and the rest part is a bare monocrystalline silicon substrate. The structure transverse size of the fractal structure is 19.6 μm. As shown in fig. 2, the fractal structure is centrosymmetric for the x-0 point, so only data starting from 0 μm is recorded,wherein the radius of the beam waist of the fixed Gaussian beam is w02.25 μm and focused on the fractal pattern plane. The method comprises the following operation steps:
1. the laser to be measured is reflected by the semi-transparent semi-reflecting mirror and is incident on the fractal structure along the normal direction of the plane of the fractal structure; the displacement platform of the fractal structure controls the fractal structure to move along the x direction, so that laser spots fall on different positions of the fractal structure, step-by-step scanning is carried out along the x-axis direction of the figure, and reflected waves R at different x positions are obtained0(x) (ii) a The reflected wave is transmitted through the semi-transparent semi-reflecting mirror and incident on the single-pixel long-wave HgCdTe detector to obtain electric signal I proportional to the reflected wave light intensity0(x)；
2. And respectively rotating the displacement platform by 60 degrees and 120 degrees in the xOy plane, and returning to the fractal structure. Repeating the step 2 and the step 3 to obtain reflected wave light intensity signals I in the other two directions1(x) And I2(x)；
3. From the central symmetry of the structure, the measured I is obtained0(x) Obtaining the translation relation between x and the absolute position of the fractal structure;
4. and if the reflectivity of the silicon substrate is R and the reflectivity of the gold film is R ', the reflectivity R of the fractal structure surface is expressed as a piecewise function of R and R'. Introducing point spread functions PSF (point spread function) of the respective portions, the signal of the reflected wave can be expressed as a convolution PSF R between the point spread function PSF and the fractal structure reflectivity R. The method comprises the steps of obtaining an objective function T (x) only containing limited fitting parameters through simplification, and fitting by using a Levenberg-Marquardt optimization algorithm to obtain laser wavelength and Gaussian beam waist width along the x-axis direction;
5. to I1(x) And I2(x) And (5) repeating the step (5) and the step (6) to obtain the width of the waist of the Gaussian beam in the directions of 60 degrees and 120 degrees, and substituting the width into an elliptic Gaussian beam equation to obtain the long axis and the short axis of the laser spot.
The smart point of the patent is that the fluctuation of light is utilized, when the characteristic length of a scattering element (metal film) is far smaller than the wavelength of electromagnetic waves, the interaction of the scattering element and the electromagnetic waves is equivalent to a dielectric material with effective dielectric constant, and the property of the dielectric material can relate fractal dimension and the effective dielectric constant through a fractal theory. The fractal structure is equivalent to a scattering element with a specific dielectric constant, which is the basis of theoretical model establishment and data processing, and if the self-similarity characteristic of the fractal structure is not used, a database needs to be established, so that not only is complex simulation calculation caused, but also the measurement accuracy is influenced. Therefore, the method has the advantages that the fractal theory corresponds to the electromagnetic wave volatility, and the geometric information and the wavelength of the laser spot are skillfully obtained.
The advantage of this patent is simple structure to can extract laser wavelength and the spot size information of the super wide range of from 70nm to 14 mu m through the electromagnetic wave and the mutual action of structure.
Drawings
FIG. 1 is a schematic view of the apparatus. In the figure: 1 is a lens, 2 is a mercury cadmium telluride unit infrared detector, 3 is a semi-transparent semi-reflecting mirror, 4 is a lens, and 5 is a thin-layer metal fractal grating structure.
Fig. 2 is a schematic top view of a thin metal fractal grating structure. 5.1 is a metal structure part of a thin-layer metal fractal grating structure, and 5.2 is a substrate of the thin-layer metal fractal grating structure.
Fig. 3 is a longitudinal sectional view of a thin-layer metal fractal grating structure. The shaded portion is the evaporated polycrystalline gold film.
Fig. 4 is a normalized result of reflected waves obtained by shifting the fractal pattern in the x direction at different wavelengths λ.
FIG. 5 shows the waist radii w at different beam waists0In the case where the fixed laser wavelength is λ 2 μm, the result of normalization of the reflected wave obtained by shifting the fractal pattern in the x direction is obtained.
FIG. 6 shows the variation of the wavelength λ and the beam waist radius w in the vicinity of the limit wavelength of 14 μm in the long-wavelength direction0In the case, the fractal pattern is shifted in the x direction to obtain a normalized result of the reflected wave.
FIG. 7 shows the variation of the wavelength λ and the beam waist radius w in the vicinity of the short-wavelength limit wavelength of 70nm0In the case, the fractal pattern is shifted in the x direction to obtain a normalized result of the reflected wave.
FIG. 8 is a graph comparing the normalized reflectivity of spots at different locations with the normalized reflectivity inferred from fitting results using the method described in this patent.
Fig. 9 shows the calculation result of the deviation degree of norm according to the present patent when the laser wavelength is different estimated values.
FIG. 10 shows the spot shape of an elliptical Gaussian beam that is fit by the method described in this patent after testing in different directions.
Detailed Description
In the following, the detailed description of the present invention is given by taking the case of laser with 70nm wavelength and 116nm spot as an example according to the above discussion:
1. the laser to be measured is reflected by the semi-transparent semi-reflecting mirror and is incident on the fractal structure along the normal direction of the plane of the fractal structure; and the displacement platform of the fractal structure controls the fractal structure to move along the x direction. Since the wavelength is 70nm and is close to the minimum structure size, it is only necessary to scan around the minimum pair of metal structures, and the region from x ' 0nm to x ' 500nm is scanned by taking the position where x is 0.743 μm as the origin of the new coordinate x '. The laser facula falls on different positions of the fractal structure, and step-by-step scanning is carried out along the x-axis direction of the figure to obtain reflected waves R at different x positions0(x) (ii) a The reflected wave is transmitted through the semi-transparent semi-reflecting mirror and incident on the single-pixel long-wave HgCdTe detector to obtain electric signal I proportional to the reflected wave light intensity0(x)；
2. And if the reflectivity of the silicon substrate is R and the reflectivity of the gold film is R ', the reflectivity R of the fractal structure surface is expressed as a piecewise function of R and R'. Introducing point spread functions PSF (point spread function) of the respective portions, the signal of the reflected wave can be expressed as a convolution PSF R between the point spread function PSF and the fractal structure reflectivity R. And obtaining an objective function T (x) only containing limited fitting parameters through simplification, and fitting by using a Levenberg-Marquardt optimization algorithm to obtain the laser wavelength and the width of the Gaussian beam waist along the x-axis direction. The fitting software used Origin 2018, the fitting result is shown in FIG. 8, the radius of the Gaussian beam waist is
The fitted beam waist radii were substituted into COMSOL 5.4, and the reflectivities of different wavelengths in the interval x '0 nm to x' 500nm were calculated using the wave optics module of COMSOL 5.4, using the norm:
the degree of deviation of the data was characterized and the correlation was obtained as shown in fig. 9. The wavelength obtained was 72 nm.
3. And respectively rotating the displacement platform by 60 degrees and 120 degrees in the xOy plane, and returning to the fractal structure. Repeating the step 2 and the step 3 to obtain reflected wave light intensity signals I in the other two directions1(x) And I2(x) In that respect To I1(x) And I2(x) Repeating the step 5 and the step 6 to obtain the width of the waist of the Gaussian beam in the directions of 60 degrees and 120 degrees, substituting the width into an elliptical Gaussian beam equation to obtain the major axis and the minor axis of the laser spot, and obtaining the fitting result as shown in fig. 10.
Claims (1)
1. A method for extracting ultraviolet to long-wave infrared laser spots by utilizing a fractal structure is realized on the basis of a testing device of the fractal reflection structure, a semi-transparent semi-reflective mirror and a detector, wherein the testing device comprises a lens, the semi-transparent semi-reflective mirror, the fractal structure, a displacement platform and the detector, and is characterized by comprising the following steps of:
1) laser is reflected by the semi-transparent semi-reflector and is incident on the fractal structure, the beam waist of the Gaussian beam is adjusted to be always focused on the fractal pattern plane, and the fractal structure is controlled by a displacement platform and is scanned in a stepping mode along the x-axis direction to obtain reflected waves at different x positions; the reflected wave is transmitted by the semi-transparent semi-reflecting mirror and is incident on the single-pixel long-wave tellurium-cadmium-mercury detector to obtain an electric signal proportional to the intensity of the reflected wave, and the wavelength information and the size information of the light spot can be obtained through data processing;
2) the method comprises the steps that information extraction is carried out on a laser spot by using a structure with fractal property, size information of the laser spot is obtained through data processing, the extracted information comprises light intensity information data of reflected waves, transmitted waves and scattered waves in a specific direction, and unit size information of the fractal structure is converted into displacement in the horizontal direction by using self-similarity of the fractal structure;
3) utilizing the wave property of the electromagnetic wave, using a plurality of structures with different sizes to interact with the laser, and extracting the light intensity information of the reflected wave, the transmitted wave or the scattered wave in a specific direction; the self-similarity of the fractal structure is utilized to achieve the purpose of being suitable for laser in a wide wavelength range.
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| CN106304847A (en) * | 2014-03-18 | 2017-01-04 | 都灵理工大学 | For strengthening and the device of directional light radiation |
| CN106767431A (en) * | 2016-12-09 | 2017-05-31 | 西安交通大学 | A kind of confocal micro-displacement measuring device of length scanning and method |
| CN107702664A (en) * | 2017-10-24 | 2018-02-16 | 北京信息科技大学 | A kind of reflective system for detecting verticality and method based on semiconductor laser alignment |
| GB2559657A (en) * | 2016-12-16 | 2018-08-15 | Secr Defence | Method and apparatus for detecting a laser |
| CN110411348A (en) * | 2019-08-28 | 2019-11-05 | 中国人民解放军国防科技大学 | Automatic detection and positioning device and method for laser spot focus |
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Patent Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN103940800A (en) * | 2014-03-10 | 2014-07-23 | 北京理工大学 | Laser confocal Brillouin-Raman spectral measurement method and apparatus |
| CN106304847A (en) * | 2014-03-18 | 2017-01-04 | 都灵理工大学 | For strengthening and the device of directional light radiation |
| CN106767431A (en) * | 2016-12-09 | 2017-05-31 | 西安交通大学 | A kind of confocal micro-displacement measuring device of length scanning and method |
| GB2559657A (en) * | 2016-12-16 | 2018-08-15 | Secr Defence | Method and apparatus for detecting a laser |
| CN107702664A (en) * | 2017-10-24 | 2018-02-16 | 北京信息科技大学 | A kind of reflective system for detecting verticality and method based on semiconductor laser alignment |
| CN110411348A (en) * | 2019-08-28 | 2019-11-05 | 中国人民解放军国防科技大学 | Automatic detection and positioning device and method for laser spot focus |
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