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

A light-trapping-structure vertical Ge photodetector(PD)is demonstrated.In the scheme,a 3μm 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 d B bandwidth of the proposed PD is around67 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 n A 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.