Lossy waveguide design considering polarization dependency to reduce back reflection in 2 × 1 MMI combiners.

We propose a novel structure that includes two compact, simply structured, and lossy waveguides for reducing back reflection in MMI combiners. The preferred lossy waveguide consists of a bend section and a tapered section. Theoretical calculations and 2D FDTD analysis were used to confirm the properties of our proposed structure. Significantly and interestingly, for TE modes, the optimized bend radius is about 7.5 µm and the specific back reflectance depends on taper end width. For TM modes, to achieve a back reflection value smaller than -30 dB, the taper length of 30 µm is desired regardless of bend radius. Moreover, the introduction of the lossy waveguide influences neither the MMI design nor its operation.