Design And Manufacture Of Polarization-independent Multilayer Dielectric Grating With High Efficiency And Broadband

Grating has important application value in laser pulse compression and laser beam combination.Laser pulse compression and spectral beam combination have special requirements for the dispersion,polarization,damage threshold,efficiency and bandwidth of gratings.In this paper,the polarization independent grating with large bandwidth and high efficiency is studied.The specific work content is as follows:(1)Based on the rigorous coupled wave analysis theory(RCWA),the formula for calculating the diffraction efficiency when TE and TM polarization light incident on multilayer dielectric grating is introduced.And the program for calculating the diffraction efficiency of grating is written.(2)The polarization-independent MLDGs of multi-layer teeth were studied.The grating with 1300 line/mm and the central wavelength of incident light is 1064 nm.Ta2O5 and SiO2 are selected as grating materials.A film stack with large bandwidth,high reflectance at the used wavelength and high transmittance at the recorded wavelength is designed.The RCWA program is used to search and optimize the parameters of MLDGs with single-layer,double-layer,three-layer,four-layer and five-layer.It is found that increasing the number of grating layers can increase the wavelength bandwidth and polychromatic integral diffraction efficiency(PIDE),at the same time the tolerance range is larger than that of single-layer grating.However,with the increasing number of grating layers,the parameters to be controlled during grating fabrication increase greatly,and the total depth of the grating teeth is too large,which is difficult to manufacture.(3)The polarization-independent MLDGs of laminated teeth were studied.The grating main teeth line is 1300 line/mm,auxiliary teeth line is 1740 line/mm,and the central wavelength of incident is 1064 nm.Firstly,the trapezoidal one-dimensional MLDG with side wall angles of 90°,88°,84°,80°,76° and 72° was designed and analyzed.In order to achieve the goal of high efficiency and broadband grating,the MLDG with laminated teeth structure is proposed,which is realized by orthogonally superimposing a high-frequency grating structure on the basis of one-dimensional grating.Research shows the wavelength bandwidth which TE and TM diffraction efficiency is greater than 95%of MLDG with laminated teeth,whether rectangular or trapezoidal,is generally about 30 nm higher than that of one-dimensional MLDG.And the PIDE in the range of 1010-1115 nm is about 2.5%more common than one-dimensional MLDG.Among them,the theoretical value of wavelength bandwidth is up to 97 nm,and the PIDE reaches 98.5055%of MLDG with trapezoidal bottom angle of 80°,and has a good tolerance to manufacture.So,the design scheme of grating with laminated teeth has obvious effects on improving the bandwidth and diffraction efficiency of grating.(4)An experimental study on polarization-independent MLDG with laminated teeth was carried out.SiO2 and Ta2O5 films were plated on quartz substrate,and masks were prepared by holographic exposure,development,cadmium plating,degumming and other steps.The Ta2Os and SiO2 on the top of multilayer dielectric films were etched by ICP technology for the first time.Then the mask with high density is prepared repeatedly,and the Ta2O5 grating tooth is etched twice to complete the grating manufacture.Finally,the morphology and diffraction efficiency of the grating were measured,and the results were consistent with the theoretical analysis.Near Littrow angle incident,the-1st order diffraction efficiency of TE and TM is greater than 95%in the range of 1020-1094 nm.