| In the research on the detection of narrow line width laser light source,when the background light is weak,the receiving system can better identify the detection light,and when the background light is strong,it often cause detection failure problem because the signal light is submerged.Because the noise of sky background light greatly limits the performance of the detection system,how to reduce the influence of background light on the detection performance of receiving system is an urgent problem to be solved.Spectral filtering technique is one of the effective ways to suppress background noise and improve the signal to noise ratio of laser echo detection system.The spectral filter can change the spectral intensity distribution of incident light according to the requirements,After the laser echo pass through the filter system,the background light is weakened,and the signal light is passed,So the filter with a narrow transmission spectrum width can improve the detection capability of the system to a certain extent.Therefore,spectral filtering technology has been widely used in several technical fields of narrow linewidth laser light source detection,such as laser guiding star technology,laser active lighting imaging,laser radar and so on.The research mainly include the following three aspects:The first part,we determine the research emphasis on the technology of the grating spectrum filter,Theoretically study the grating filter system through beam width and efficiency and the system structure and the relationship between the incident light field distribution.Based on the grating equation,Under the condition that the incident light field is the ideal gaussian beam.We derive the diffraction far field intensity distribution of the grating by combining the Fourier transform theory of the lens.and MATLAB software is used as the main simulation modeling tool to model the whole grating filtering system.Through the discretization of light field,the intensity distribution matrix of different wavelengths in different positions is obtained,and the ratio of the energy of transmitted light field in the designated area and the energy of incident light field is obtained by means of statistics,and the width of transmission spectrum is obtained.Based on this,the relationship curve of line width,penetration rate and the size of slit,focal length of the lens,as well as the correlation parameters of the grating are made,and the conclusion is drawn:the size of the slit is smaller,the line width of the system is narrower,but the transmittance is also smaller,and the longer the focal length of the lens can reduce the difficulty in the installation and adjustment of the slit,but it will cause the increase of the system volume,while the decrease of the grating constant d and the increase of the total number of grating grooves N can improve the system performance.At the same time,the experimental system is set up,and in the center wavelength of 1064 nm,success transform the input laser which spectrum FWHM is 0.2789nm into the output laser which spectrum FWHM is 0.0288nm.and the subnanometer filtering results are obtained.The correctness of the theoretical simulation results is verified experimentally,which provides an effective basis for the optimal structure parameter design of the grating spectrum filtering system.The second part,this part focuses on the influence of atmospheric turbulence on the grating spectrum filtering system.Based on the multi-phase screen theory,combined with the spectral inversion method and the fast Fourier transform method,we simulate the transmission of the laser beam in the turbulent atmosphere,obtain the distribution of the light field incident on the surface of the grating,Based on this,and combines with the grating theory in the first part,Using MATLAB as simulation tools,and set the system transmission spectral line width and transmittance as the main performance parameters,We have verified the feasibility and stability of the grating spectral filtering system when the phase of incident field has random distortion,We also analyze the influence of atmospheric parameters on the performance of the system,such as the coherent length of the atmosphere,the scale of turbulence,and the outer scale,It is concluded that with the increase of the coherence length of the atmosphere,the transmittance gradually become narrower and transmittance is gradually increased.At the same time,we also provide the atmospheric applicability of the system.When ro<0.05m,the performance of the system deteriorates rapidly,so the system should be used in the atmospheric conditions of ro>0.05m.In addition,when ro= 0.1m,we obtain the nanoscale spectrum filter line width(FWHM = 0.3 nm),and effective spectral transmittance over 90%.At the same time,we conduct a qualitative comparison experiment in the laboratory,and use the phase plate to simulate the wavefront aberration,After adding the phase plate,the output spectral line width is changed from 0.034nm to 0.046nm when the incident spectral line width is 0.15nm.It is proved that the system still has filtering ability when the phase of incident field has random distortion,but the filtering performance will decrease.This provides a reliable simulation and experimental basis for the application of the system in complex atmospheric conditions.The third part,this part mainly is the structure design of the system.Based on the analysis of the previous two parts,the optimal structure parameters of each device are determined,and we construct a set of spectral filtering system based on double grating.Using a 4f system structure with a combination of lens and grating,and place the slit on the back focal plane of the lens,We have realized the far-field filtering function of the grating in the near field.In addition,By using a double grating structure,the second grating can effectively compensate the dispersion caused by the first grating and improve the optical beam quality of the output beam.And improve on the system structure,the system is designed as a folded structure with one grating,And by using a reflective prism with a right Angle,the light beam that is first diffracted by the grating can return to the surface of the same grating in parallel.That is,we realize the second utilization of the same grating,this can not only compress the system volume but also guarantee the effect of dispersion compensation,and also improve the stability of the system.At the same time,we compare and analyze the effect of single grating structure and double grating structure on optical beam quality in theory and experiment.When there is no grating in the system,the optical beam quality(M2)we measure is 1.683,When the first grating is added,the optical beam quality(M2)is 1.995,and when the second grating is added,the optical beam quality(M2)is 1.875,The beam quality of a single-grating folded structure system is 1.863.It is concluded that the double grating structure can effectively improve the optical beam quality.In addition we conduct an experimental measurement of the wavefront aberrations introduced by double grating system,And We find that the wavefront aberration is relatively small when the system satisfies the 4f structure.The innovation of this article is:Firstly,the research on grating spectrophotometry is mainly focused on grating spectrometer,and the application of grating in spectral filtering field is very rare.In this paper,we build a 4f system by using grating and lens,which realizes the far field filtering function of the grating in the near field,And this can effectively save the system space and realize more accurate filtering result control;Also we design the system to be a folded structure with one grating,we've reached the goal of using the same grating twice by using a reflective prism with a right Angle,It greatly reduce the volume of the system.The second aspect,By using a double grating structure,the second grating can effectively compensate the dispersion caused by the first grating and improve the optical beam quality of the output beam.we have verified the feasibility and stability of the grating filter system for random light field through a large number of simulation experiments.Also,we have verified the feasibility and stability of the grating filter system for random light field through a large number of simulation experiments. |
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