Contents I. Introduction
Datacom internet traffic is rapidly increasing with the large demand for cloud computing, video streaming, and mobile services. For the standardization of 400 gigabit Ethernet (400 GbE), IEEE is currently discussing its baud rate and modulation format. A pulse amplitude modulation with 4-level amplitude (PAM4) is promising because multi-level modulations can increase bit-rate without increasing baud rate, consequently a practical integrated optical module for 400 GbE can be produced by using mature techniques in current 100-GbE transceivers. To achieve error-free operation, a receiver for the PAM4 signal requires a high signal-to-noise ratio (SNR) compared with that for the non-return-to-zero (NRZ) signal, because the PAM4 signal has a modulation penalty of around 4.8 dB. Moreover, crucial issues in further spreading the 400-GbE transceiver are its size and power consumption. A small, high-sensitivity optical module with low power consumption is indispensable for improving port density on line cards and reducing installation space of equipment in buildings such as data centers. In the optical devices, both the avalanche photodiode (APD) and the semiconductor optical amplifier (SOA) can give a high gain. Especially APD shows promising characteristics for achieving low-power-consumption receiver with a small footprint, unlike SOAs [1], [2].
