Research On Polarization-independent Transmission Grating

With the continuous development of laser technology,as the core devices of laser pulse compression,laser frequency selection and spectral beam combination,diffraction gratings with high dispersion and high diffraction efficiency have been widely used in high-power laser systems.Therefore,the polarization-independent gratings with high efficiency have attracted much attention in recent years.The polarization-independence of grating is beneficial to simplify the structure and improve the output capability in laser beam combination system.If the diffraction efficiency of grating is related to polarization,the total diffraction efficiency and polarization state of diffractive light will be affected when the incident light is non-polarized.Fused-silica transmission gratings not only have board band,high diffraction efficiency and high damage threshold,but also have the advantage of light path without shelter comparing to reflective gratings.In this paper,the design of polarization-independent transmission gratings are carried out.The relevant research work has great significance for improving the output power of laser systems,and increasing the operation stability of optical devices when the incident light is non-polarized.The main works of this paper are as follows:?1?Based on the rigorous coupled-wave analysis,the diffraction efficiency formulas for both TE and TM polarizations are given,and a diffraction efficiency program is developed to design the structure of phase relief gratings;?2?The grating material is selected as fused-silica.The incidence for both TE-and TM-polarized waves are comprehensive consideration.The-1 order transmission diffraction efficiencies are calculated at different duty cycles,depths and space frequencies.The suitable space frequencies are selected to carry out research work by analyzing the related structure parameters.Based on the incidence of Littrow angle at the center wavelength,two grating grooves are discussed,including rectangular and trapezoidal grooves.For rectangular gratings,the diffraction efficiencies as functions of duty cycle and depth are studied;for trapezoidal gratings,the diffraction efficiencies as functions of bottom duty cycle and depth are studied at different bottom angles.Finally,the optimal solution of diffraction efficiency is obtained.Based on the optimized structure parameters,the process tolerance of fused-silica grating is analyzed.The limiting condition is defined as the-1 order diffraction efficiency greater than 95%at the center wavelength for both TE-and TM-polarized waves.After searching for the ranges of depth and duty cycle which satisfy the limiting condition,the manufacture parameters of fused-silica grating are determined;?3?A TiO₂/SiO₂ bilayer dielectric transmission grating on quartz substrate is explored.The bilayer grating structures with different space frequencies are optimized,then the appropriate grating ridge depths of TiO₂ and SiO₂ are selected by contrastive analysis,and the related design of bilayer transmission grating is completed.Finally,the main factors that affect the diffraction efficiency of the designed bilayer dielectric grating are analyzed;?4?The experiment research of fused-silica transmission grating is performed.The measurement of 940lines/mm fused-silica grating is completed,and the diffraction efficiency of the grating is consistent with theoretical analysis.