High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating.

A high-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section (combination of trapezoidal-rectangular grating profile) fused-silica beam combining grating (BCG) used in the -1st order for spectral beam combining (SBC) is designed and fabricated. Exact grating profile parameters are optimized by using the rigorous coupled-wave analysis and simulated annealing algorithm. As a comparison, traditional pure trapezoidal and pure rectangular gratings are also designed. Numerical results show that such a bullet-alike cross-section BCG exhibits wide bandwidth with the lowest maximum electric field enhancement in the grating material, which is greatly beneficial for the promotion of the power scaling level of the grating-based SBC system. A two-step dry-etching procedure is developed to fabricate such a grating. The averaged diffraction efficiency of greater than 91% was experimentally demonstrated.