Design And Fabrication Of Multilayer Dielectric Grating For Blue-green-band Laser Frequency Selection

Grating is a commonly used frequency selective component of lasers.Metal grating is usually used to select frequency in laser system.Due to the intrinsic absorption of the metal,the damage threshold is low and it is difficult to be used in high power laser.The intrinsic absorption of the dielectric material is almost zero,so it has higher damage threshold.The multilayer dielectric grating is proposed to replace the metal grating for the laser frequency selection.In this paper,the design of multilayer dielectric gratings in blue-green band are carried out.The relevant research work is of great significance to improve the output power of lasers based on frequency selective grating.The main tasks of the paper are as follows:(1)The diffraction efficiency calculation is carried out by rigorous coupling wave method,and the calculating formulas of diffraction efficiency are given.(2)A diffraction efficiency program based on rigorous coupled wave is developed to optimize the micro-structure parameters of multilayer dielectric gratings.(3)The effects of the duty cycle,the depth and the residual thickness on the-1 order diffraction efficiency are studied with different space frequency and film materials.The study on the space frequency 1740line/mm gratings is carried out,and the influence of the matching layer on the diffraction efficiency and bandwidth is studied by the Hf O2/Si O2 film materials.The effects of one matching layer and two matching layers on the diffraction efficiency and bandwidth are analyzed respectively.By introducing two matching layers,the diffraction efficiency of the grating is greater than 97% within the range of 450nm-510 nm.According to the evaluation function,the process tolerance range under different thickness of top layer is determined,and the thickness of multilayer dielectric film and the microstructure of grating are finally obtained.(4)The experiment research of multi-layer dielectric grating fabrication process is performed.Multilayer dielectric film is coated on substrate,then holographic lithography is used to fabricate photoresist grating mask,and the fabrication of 1740line/mm multilayer dielectric grating is accomplished with ion beam etching.Three different grating gratings are fabricated,and the diffraction efficiency of the grating is measured by the super-continuum laser.The experimental results show that the diffraction efficiency is more than 93% in the range of 453nm-493 nm band,and more than 95% at the center wavelength of 473 nm at Littrow angle while the grating groove depth is 360 nm.The theoretical design has been verified by experiments.