Low-loss and broadband silicon mode filter using cascaded plasmonic BSWGs for on-chip mode division multiplexing

A mode splitter is a key device to eliminate undesired modes but allow desired modes go through for an on-chip mode-division multiplexing (MDM) system. Here, we propose a silicon high-order mode (HOM) pass filter based on the cascaded plasmonic bridged subwavelength gratings (BSWGs). A metal bridge is introduced to generate a plasmonic hybrid mode, which has a significant influence on the fundamental mode but a neglected impact on the first-order mode. A silicon HOM-pass filter for handling the TM0 and TM1 modes is optimized by using the 3D full-vectorial finite difference time domain (3D-FV-FDTD) method. The numerically simulated results indicate that the optimized mode filter is with a low loss of 0.63 dB and a mode extinction ratio (ER) of 26.4 dB based on 4-cascaded plasmonic BSWGs. The 3 dB bandwidth is over 493 nm from 1222 nm to 1715 nm. With the mode ER > 15.0 dB, a broad bandwidth of 150 nm can be achieved. The performance of the proposed mode filter is tolerant to the width error of ± 50 nm. The proposed silicon HOM-pass filter can be utilized in on-chip MDM systems for mode controlling.