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.