Bidirectional Symmetric 25G Coherent ONU Using a Single Laser, Single-Ended PIN and a 2-bit ADC

We realize a coherent ONU using only a single PIN phododiode, a 2-bit ADC, and an EML shared signal/LO-laser. The receiver sensitivity after 20 km transmission is enhanced by 18.6 dB over direct detection PAM-4, but requires no additional optoelectronic hardware.