High Responsivity Si-Ge Waveguide Avalanche Photodiodes Enhanced by Loop Reflector

We present a loop reflector-assisted silicon-germanium waveguide avalanche photodiode with improved responsivity. Compared to the same APD without the reflector, it has 1.49 times higher responsivity, \sim1.12 A/W, without compromising the speed performance. It exhibits a 3 dB-bandwidth of \sim25 GHz, a build-up time limited gain-bandwidth product of \sim296 GHz, a highest gain-bandwidth product of \sim497 GHz. Clear eye diagrams are measured at both 32 Gbps NRZ and 64 Gbps PAM4 modulation, and a 1 \sim 2 dB better sensitivity up to −15.7 dBm with 32 Gbps NRZ at a BER of 2.4 \times 10^{-4}.