Small responsivity imbalance of InP-based p-i-n photodiode array monolithically integrated with 90° hybrid using asymmetric waveguide phase shifter for coherent detection

For coherent detection using multi-level modulation formats, we demonstrated small responsivity imbalance of the InP-based p-i-n photodiode array monolithically integrated with the 90° hybrid consisting of 2 × 4 and 2 × 2 multimode interference structures and a 45° phase shifter. Responsivity imbalance of the In-phase and Quadrature channels was less than ±0.2 dB over the C-band by the adoption of the asymmetric waveguide phase shifter which functions as the 45° phase shifter, and its good reproducibility was indicated from the standard deviation for each wafer. Furthermore, excellent common mode rejection ratio below -25 dB was also confirmed from the coherent receiver with this integrated device. These results prove that this structure is very useful for stable manufacturability of the 90° hybrid.