InP-Based p-i-n-Photodiode Array Integrated With 90 $^\circ$ Hybrid Using Butt-Joint Regrowth for Compact 100 Gb/s Coherent Receiver

This paper reports on the performance of an InP-based p-i-n-photodiode array monolithically integrated with a 90 $^\circ$ hybrid consisting of multimode interference structures using the butt-joint regrowth and its application to compact 100 Gb/s coherent receivers. A responsivity including total loss of 8.2 dB in the waveguide was as high as 0.14 A/W, and responsivity imbalance of all channels was less than ±0.5 dB. The wide 3-dB bandwidth of 26 GHz at a low reverse bias voltage of 1.6 V was obtained under high optical input power conditions (photocurrent = 2.5 mA). The stable dark current was achieved in the accelerated aging test (test condition: −5 V at 175 $^\circ$ C) over 2000 h. Finally, the excellent common mode rejection ratio with low loss imbalance of 90 $^\circ$ hybrids (less than −20 dB up to 25 GHz) and demodulation of 128 Gb/s dual polarization quadrature phase-shift keying modulated signals were demonstrated for the compact coherent receiver (package body size: 15 mm × 26 mm × 5.5 mm).