Design of an encapsulated fused-silica grating for two-port beam splitting

Abstract

An encapsulated rectangular-groove fused-silica grating as a highly efficient two-port beam splitter is presented, which is designed at a central wavelength of 1550nm under Littrow mounting. The design method of such a high-efficiency two-port beam splitter grating is mainly based on the simplified modal method with consideration of two beam interference of the modes excited by the incident wave. Meanwhile, this encapsulated grating can be considered equivalent to an effective Fabry-Perot-resonator, therefore, Fabry-Perot interference of grating modes inside the grating area also plays an innegligible role in achieving high efficiency and equality in the diffracted zeroth and minus-first orders, which also should be considered in the design. The rigorous coupled-wave analysis(RCWA) is used to verify the design method and optimize grating profile. Numerical simulations show that, the RCWA results agree well with the design conditions derived from modal method. And with the optimum grating profile, high total efficiency(99.78%) and equal intensity of the diffracted zeroth and minus-first orders can be obtained at the wavelength of 1550nm. Moreover, the designed encapsulated beam splitter can work well over the C+L band and within 21° angular range. The encapsulated grating can exhibit higher diffraction efficiency, intensity equality and less reflection loss than conventional rectangular-groove grating.