106-Gbit/s PAM4 40-km Transmission Using an Avalanche Photodiode With 42-GHz Bandwidth

106-Gbit/s PAM4 transmission over 40-km single-mode fiber is demonstrated using an avalanche photodiode (APD) with a gap-grading layer for high-speed operation. The APD shows maximum bandwidth of 42GHz with 0.5-A/W responsivity at unity gain.

1. Introduction

The demand for a (100Gbit/s PAM4) optical interface in simple intensity-modulation direct-detection (IMDD) systems is certainly increasing. Initially, 100-Gbit/s PAM4 will be applied to 100-/400-Gbit/s Ethernet system for only 500-m optical link [1]. In addition, there arise some requirements of the 100-Gbit/s PAM4 for longer distance such as 2 km, 10 km and more to cover the datacenter networks [2].