Compact and broadband mode demultiplexer using a subwavelength grating engineered MMI coupler

Abstract

A compact and broadband mode demultiplexer (DEMUX) using a subwavelength grating (SWG) engineered multimode interference (MMI) coupler is proposed and analyzed in detail. To effectively separate the input T E 0 and T E 1 modes, the input port is formed by cascading two tapers, connected by a silicon wire, in a single SWG MMI coupler. Moreover, by partially tailoring the grating duty cycle of the SWGs, an extra phase shifter is directly integrated in the MMI coupler, parallel to the input port for adjusting the phase in mode conversion. The input T E 0 mode can pass through the first taper and the silicon wire while being cut off and evolve into SWGs in the second taper, so that it is hardly affected by the extra phase shifter and directly mapped into the output port via MMI. On the contrary, the input T E 1 mode is cut off in the first taper and evolves through the sidewall of the first taper into two T E 0 -like components in the SWG region. Then an extra π / 2 phase shift between them is generated by the phase shifter. Finally, they are combined into the output T E 0 mode via MMI. By carefully choosing the structural parameters, the proposed T E 0 / T E 1 mode DEMUX shows a broad bandwidth of 350 nm from 1520 to 1870 nm for insertion loss < 1.0 d B and cross talk < − 10 d B and achieves a low insertion loss of 0.37 dB/0.40 dB and cross talk of − 18.50 d B / − 17.00 d B for T E 0 / T E 1 input at 1550 nm, within a short device length of only 18.6 µm. In addition, the device also shows a good fabrication tolerance to the deviations of structure parameters.