Ultralow-loss polycrystalline silicon waveguides and high uniformity 1x12 MMI fanout for 3D photonic integration.

We have investigated the feasibility of multimode polysilicon waveguides to demonstrate the suitability of polysilicon as a candidate for multilayer photonic applications. Solid Phase Crystallization (SPC) with a maximum temperature of 1000°C is used to create polysilicon on thermally grown SiO2. We then measure the propagation losses for various waveguide widths on both polysilicon and crystalline silicon platforms. We find that as the width increases for polysilicon waveguides, the propagation loss decreases similar to crystalline silicon waveguides. At a waveguide width of 10 µm, polysilicon and crystalline silicon waveguides have propagation losses of 0.56 dB/cm and 0.31 dB/cm, respectively, indicating there is little bulk absorption from the polysilicon and is the lowest propagation loss for polysilicon demonstrated to date. In addition, the first 1x12 polysilicon MMI is demonstrated with a low insertion loss of -1.29dB and a high uniformity of 1.07 dB. These results vindicate the use of polysilicon waveguides of varying widths in photonic integrated circuits.