Angled Bent Multimode Interferometer for Wavelength Division Multiplexing

We propose a novel angled bent multimode interferometer (ABMMI) and demonstrate both numerically and experimentally the advantages of the new structure in reducing the crosstalk (XT), insertion loss (IL) and size of wide channel-spacings wavelength filters. The newly proposed ABMMI utilizes a bent multimode waveguide for the multimode interference instead of the conventional straight multimode waveguide commonly used in MMI splitters or previous implementations of angled MMI (AMMI) based wavelength filters. We also introduce a new relaxed phase error optimization design method for the new structure. With the new structure and new design method, the phase error of the ABMMI can be engineered to be lower than AMMI-based wavelength filters. We implement a four-channel coarse wavelength division multiplexing (CWDM) filter using the ABMMI and achieve low ILs of 0.25 dB to 0.71 dB across all channels and wavelength crosstalk of -19.1 dB to -27.8 dB at the center wavelengths of each channel. The optical bandwidth for XT below -15 dB ranges from 10.9 nm to 12.6 nm for the four-channel ABMMI CWDM filter. Besides, the device can be fabricated by easily using a single-step full etching and the device footprint is relatively compact compared to previous AMMI-based designs.