Design of polarization-independent multilayer dielectric grating with broadband

Spectral Beam Combination (SBC) technology is an effective way to obtain high power laser beam. Polarization independent multilayer dielectric grating (MLDG) with high damage threshold, high diffraction efficiency is one of the key elements of SBC. In order to effectively combine more laser beams,the wavelength range of grating needs to be broad. To the best of our knowledge, 1480 line/mm is the highest reported line density to date. But its bandwidth is only 35nm. In this paper, we designed broadband polarization independent multilayer dielectric gratings of 1480 lines/mm for spectral beam combination. And the grating is designed to work at the -1st order Littrow mounting for the central wavelength of 1053nm. Based on the theory of Rigorous Coupled Wave Analysis (RCWA), a multilayer dielectric film grating is established. Ta2O5 (n=2.08) and SiO2 (n=1.46) are used as a high and low refractive index material to form the high reflectivity mirror of (HL)(12) H structure. The influence of the number of MLDG groove layers on the bandwidth is mainly analyzed, including single-layer, double-layer, three-layer, four-layer and five-layer. Then, we find that increase the number of groove layers can increase bandwidth of MLDG. And polarization-averaged diffraction efficiency of the grating with five-layer groove was greater than 95% in the wavelength range of 1003-1083nm (80nm). In addition, the design parameters, tolerance analysis of the grating grooves are described.