67 GHz light-trapping-structure germanium photodetector supporting 240 Gb/s PAM-4 transmission

Abstract

A light-trapping-structure vertical Ge photodetector (PD) is demonstrated. In the scheme, a 3 μm radius Ge mesa is fabricated to constrain the optical signal in the circular absorption area. Benefiting from the light-trapping structure, the trade-off between bandwidth and responsivity can be relaxed, and high opto-electrical bandwidth and high responsivity are achieved simultaneously. The measured 3 dB bandwidth of the proposed PD is around 67 GHz, and the responsivity is around 1.05 A/W at wavelengths between 1520 and 1560 nm. At 1580 nm, the responsivity is still over 0.78 A/W. A low dark current of 6.4 nA is also achieved at − 2    V bias voltage. Based on this PD, a clear eye diagram of 100 GBaud four-level pulse amplitude modulation (PAM-4) is obtained. With the aid of digital signal processing, 240 Gb/s PAM-4 signal back-to-back transmission is achieved with a bit error ratio of 1.6 × 10 − 2 . After 1 km and 2 km fiber transmission, the highest bit rates are 230 and 220 Gb/s, respectively.