Optical 90° Hybrid Based on an InP 4×4 Multimode Interference Coupler for Coherent Receiver Application

An optical 90° hybrid based on an InP 4×4 multimode interference (MMI) coupler is designed and fabricated for the application in a coherent receiver. It reveals that the width of multimode waveguide is the most critical parameter for the devices, and the multimode waveguide width can be accurately controlled within 0.1 μm in our experiments. Across the entire C-band (1527–1561 nm), the maximum imbalance of the devices is less than 0.7 dB, and the common mode rejection ratios for in-phase channels and quadrature channels are better than -24 dB.