High-speed lateral PIN germanium photodetector with 4-directional light input

We experimentally demonstrate a high-speed lateral PIN junction configuration germanium photodetector (Ge-PD) with 4-directional light input. The typical internal responsivity is about 1.23 A/W at 1550 nm with 98% quantum efficiency and dark current 4 nA at 1V reverse-bias voltage. The equivalent circuit model and theoretical 3-dB opto-electrical (OE) bandwidth of Ge-PD are extracted and calculated, respectively. Compared to the conventional lateral PIN Ge-PD with 1-directional light input, our proposed device features uniform optical field distribution in the absorption region, which will be benefit to realize high-power and high-speed operation. In particular, in the condition of 0.8 mA photocurrent, the measured 3-dB OE bandwidth is about 17 GHz at bias voltage of -8 V which is well matched to the theoretical estimated bandwidth. With additional digital pre-compensations provided by the Keysight arbitrary waveform generator (AWG), the root raised cosine (RRC) filter and roll-off factor of 0.65 are employed at transmitter (TX) side without utilizing any offline digital signal processing (DSP) at receiver (RX) side. The 50 Gbit/s, 60 Gbit/s, 70 Gbit/s, and 80 Gbit/s non-return-to-zero (NRZ), and 60 Gbit/s, 70 Gbit/s, 80 Gbit/s, and 90 Gbit/s four-level pulse amplitude modulation (PAM-4) clear opening of eye diagrams are realized. In order to verify the high-power handling performance in high-speed data transmission, we also investigate the 20 Gbit/s NRZ eye diagram variations with the increasing of photocurrent.