A broadband second-order mode synthesizer based on an 3×1 multimode interference coupler and phase shifters using silicon waveguides

Abstract

In this paper, we present a novel mode synthesizer based on a silicon-on-insulator (SOI) platform with the advantages of broad bandwidth, high mode conversion efficiency and large fabrication tolerance. By using a 3×1 multimode interference (MMI) coupler, a sinusoidal symmetric Y junction coupler and two linear butterfly-shaped phase shifters, mode synthesis from the fundamental mode TE 0 and the first-order mode TE 1 to the second-order mode TE 2 is achieved by means of numerical simulation. Phase variation in this structure is discussed and performance analysis of the mode synthesizer is performed by means of the numerical method. Simulation results obtained by using three dimension beam propagation method (3D-BPM) incorporated with effective index method (EIM) show high performance of the device with mode conversion efficiency up to more larger than 88% (corresponding to insertion loss less than 0.52 dB) in the whole C-band. The width and length tolerances of the multimode interference coupler are quite large, which can be corresponding up to 50 nm and 900 nm, respectively. In addition, the length deviation of the phase shifters can be up to 500 nm. Large tolerances allow to fabricate the device by using current CMOS technology.