Compact Optical 90° Hybrid based on a Wedge-shaped 2 × 4 MMI Coupler and a 2 × 2 MMI Coupler on Thin-film Lithium Niobate Platform

We conducted an experimental demonstration of a wedge-shaped optical 90-degree hybrid coupler on the thin film lithium niobate (TFLN) platform, utilizing a paired-interference-based 2 × 4 multimode interference (MMI) coupler and a general-interference-based 2 × 2 MMI coupler. The fabricated optical 90-degree hybrid coupler has a compact footprint with a length of 134 μm. In a coherent receiving system, the demonstrated hybrid coupler directly connects to the balanced photodiode array without waveguide crossing or cascaded phase shifters. The device exhibits an excess loss of less than 1.1 dB and a phase deviation of less than ±5 degree over a spectral range of 1530nm-1560nm, which is promising to enable a compact heterogeneously integrated coherent receiving system on the thin film lithium niobate platform.