Low loss and high extinction ratio all-silicon TM-pass polarizer with reflection removal enabled by contra-mode conversion Bragg-gratings

Abstract

Bragg-gratings have been frequently used to design compact and high extinction ratio (ER) on-chip polarizers. However, the strong reflection of the unwanted polarization may deteriorate the performance of the light source or cause unwanted interferences. In this paper, we propose a Bragg-grating-based all-silicon TM-pass polarizer with low reflection, low insertion loss (IL) and high ER. Unlike previously reported polarizers based on single mode waveguides, we construct the Bragg grating with a multimode waveguide, which not only acts as a Bragg reflector, but also a mode-order converter to convert the reflected TE light into higher order modes to be eventually filtered out by utilizing a tapered transition. On the other hand, the grating has little adverse influence on the TM input light since it works at sub-wavelength-guided wave propagation regime. Finally, the polarizer obtained has a length of 30µm, an ER of 51.83dB, an IL of 0.08dB, and an operating bandwidth of ∼61nm for ER > 30dB at the wavelength of 1.55µm. More importantly, the reflection of the unwanted polarization is suppressed to −12.6dB, which can be further lowered via additional design optimization. Our work points to a new direction for making better on-chip polarizers.