InP-Based Photodetectors Monolithically Integrated with 90° Hybrid toward Over 400Gb/s Coherent Transmission Systems

We present InP-based photodetectors monolithically integrated with a 90◦ hybrid toward over 400 Gb/s coherent transmission systems. To attain a wide 3-dB bandwidth of more than 40 GHz for 400 Gb/s dual-polarization (DP)-16-ary quadrature amplitude modulation (16 QAM) and 600 Gb/s DP-64 QAM through 64 GBaud operation, A p-i-n photodiode structure consisting of a GaInAs thin absorption and low doping ntyped InP buffer layers was introduced to overcome the trade-off between short carrier transit time and low parasitic capacitance. Additionally, this InP buffer layer contributes to the reduction of propagation loss in the 90◦ hybrid waveguide, that is, this approach allows a high responsivity as well as wide 3-dB bandwidth operation. The coherent receiver module for the C-band (1530 nm – 1570 nm) operation indicated the wide 3-dB bandwidth of more than 40 GHz and the high receiver responsivity of more than 0.070 A/W (Chip responsivity within the C-band: 0.130 A/W) thanks to photodetectors with this photodiode design. To expand the usable wavelengths in wavelength-division multiplexing toward large-capacity optical transmission, the photodetector integrated with the 90◦ hybrid optimized for the L-band (1565 nm – 1612 nm) operation was also fabricated, and exhibited the high responsivity of more than 0.120 A/W over the L-band. Finally, the InP-based monolithically integrated photonic device consisting of eight-channel p-i-n photodiodes, two 90◦ hybrids and a beam splitter was realized for the miniaturization of modules and afforded the reduction of the total footprint by 70% in a module compared to photodetectors with the 90◦ hybrid and four-channel p-i-n photodiodes. key words: digital coherent transmission, InP, 90◦ hybrid, multimode interference structure, p-i-n photodiode