Ryuji Masuyama

High-efficient InP-based balanced photodiodes integrated with 90° hybrid MMI for compact 100 Gb/s coherent receiver

InP-based balanced photodiodes monolithically integrated with the 90° hybrid consisting of multimode interference structures (MMI) by the butt-joint process were demonstrated for the first time. The high responsivity of 0.14 A/W and wide 3-dB bandwidth of 22 GHz at a low reverse bias voltage of 1.6 V were successfully achieved.

InP-Based p-i-n-Photodiode Array Integrated With 90

This paper reports on the performance of an InP-based p-i-n-photodiode array monolithically integrated with a 90° 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 °C) over 2000 h. Finally, the excellent common mode rejection ratio with low loss imbalance of 90° 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).

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We have demonstrated monolithic integration of InP-based waveguide photodiodes (WGPDs) with metal-insulator-metal (MIM) capacitors using the 3-inch diameter wafer process. The uniformity of MIM capacitance and the dielectric breakdown voltage of the capacitor obtained within +/-2 % and over 100 V, respectively. The dark current of WGPDs was less than 3 nA at a reverse voltage of 1.6 V, owing to the InP passivation structure formed on WGPD surface. The fabricated integrated chip size was 2.0 mm × 5.1 mm. These results indicate that integrated WGPDs can provide both smaller and easier assembly for the compact coherent receiver.

Hybrid Using Butt-Joint Regrowth for Compact 100 Gb/s Coherent Receiver

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.

Monolithic integration of InP-based waveguide photodiodes with MIM capacitors for compact coherent receiver

The InP-based pin-photodiode array monolithically integrated with a 90° hybrid and spot-size converters was realized using selective embedding regrowth for compact 100Gb/s coherent receivers. The low dark current of less than 500 pA up to 85 °C was attained by InP passivation effect in four-channel pin-photodiodes. As a receiver using these photodiode arrays, a receiver responsivity including total loss of 6.7 dB in the 90° hybrid and intrinsic loss of 3 dB in the polarization beam splitter was higher than 0.060A/W between −5 °C and 85 °C through the integration of the spot-size converter. Responsivity imbalance of the In-phase and Quadrature channels was also less than 0.5 dB over the C-band.