An Ultra-Short Silicon Polarization Beam Splitter Based on Asymmetrical Directional Coupler

Abstract

Polarization beam splitter is a key device in the combined multiplexing system and polarization diversity system, which can realize the splitting between two orthogonal polarization modes. In this paper, an ultra-short (~ 5 μm long) polarization beam splitter based on asymmetrical directional coupler is proposed by using silicon-on-insulator nanowires. The design of polarization beam splitter is according to mode-coupling theory. By choosing the suitable width and height of waveguide, the effective indices of TM polarization mode in waveguide 1 and 2 are equal, which satisfy phase matching condition. So the TM polarization mode in the waveguide 1 can couple into the waveguide 2. Meanwhile the TE polarization mode can only transmit in the waveguide 1 for phase mismatch, and the function of polarization splitting is achieved. The dependence of the extinction ratio and loss on several parameters (coupling length $L$ and spacing between waveguides G) are investigated with a finite-difference time-domain (FDTD) method. The results show its coupling length $L$ is only 1.78 μm; the extinction ratio for TM and TE polarization modes are 22.4 dB and 28.9 dB respectively; and the loss of TE and TM polarization modes are both less than 0.43 dB. In addition, compared with other polarization beam splitter based on silicon waveguide, it has the advantages of smaller size, simpler structure, easier implementation and integration.