Research On Structure And Optical Properties Of Grating Fabricated By Femtosecond Laser Direct Writing

As a core optical device,diffraction grating has important applications in the fields of spectral analysis,solar cells,petrochemicals,and public safety.At present,the traditional grating preparation process has the disadvantages of high cost,low flexibility and complicated process.As an advanced micro-nano processing technology,the femtosecond laser has extremely high processing accuracy due to its short pulse and high peak power,which is very suitable for processing precision structures such as gratings.Among the current common methods for preparing grating structures by femtosecond lasers,the laser direct writing method has the advantages of high flexibility,low cost,and rapid preparation compared with the double beam interference method and the phase mask method.Therefore,the research work of this paper is centered on the preparation of grating structures by femtosecond laser direct writing on the surface of fused silica glass.The main research contents and results are as follows:(1)Experimentally study the influence of various laser processing parameters on the micro-groove surface morphology,depth and width.Studies have shown that the reduction of single pulse energy and the increase of scanning speed will lead to the reduction of material removal,and thus the depth and width of the groove.Multiple scans can effectively solve the problem of poor straightness of the groove under a single scan.Under the same laser process parameters,the increase in the number of repeated scans will also lead to an increase in the amount of material removed.The increase in the number of repeated scans shows an increasing relationship.Selecting the appropriate feed amount under multiple scans can further improve the processing quality of the wire groove.(2)Based on the research of micro-groove structure,the direct writing preparation process of large-scale grating structure is optimized.Studies have shown that: when preparing a grating structure with a small period,the grating period is doubled or multiple times for processing,and after the writing is completed,the initial position of the light spot is moved in the direction perpendicular to the grating line and then written again,by repeating the processing The quality of the grating can be effectively improved,and a grating structure with a smaller feature size can be prepared.When the laser parameters obtained by processing a smaller area grating are used to prepare a large area grating structure,it is necessary to increase the laser energy or reduce the scanning speed to obtain a similar processing effect.In the current preparation process,when the laser repetition frequency is 100 kHz,the preparation of the grating structure with an area of 40 mm × 40 mm and a period of 50 ?m takes a relatively short time,which only takes about five minutes.(3)The effect of grating structure parameters and the incident conditions of the incident light on the diffraction efficiency are investigated through numerical simulation.Taking trapezoidal structure grating as the object,the period is 30 ~ 50?m,the width of the line groove surface is 11 ~ 19?m,and the groove depth is 26 ~ 34?m.Studies have shown that the lower the width of the groove and the perpendicularity of the side walls,the higher the 0th order diffraction efficiency,and the lower the-1 and +1 order diffraction efficiency.At an incident angle of 0 ~ 60°,the smaller the depth of the grating,the higher the diffraction efficiency of the 0th order,and the lower the diffraction efficiency of the-1 and +1 orders;after the incident angle is greater than 65°,the change of the diffraction efficiency with the depth is opposite to the previous.When the incident angle is in the range of 0 ~ 30°,the larger the period of the grating,the higher the 0th order diffraction efficiency,and the lower the-1,+1 order diffraction efficiency.The influence of incident light on diffraction efficiency is mainly as follows: The diffraction efficiency under TM polarization is slightly higher than that under TE polarization,but the overall phase difference is small.Under the same polarization state,when the incident angle is large(greater than 70°),the diffraction efficiency of the-1 order is generally lower than that of the +1 order.