Low-Complexity Integrated Optical-Electrical Quasi-Coherent RX Front End Supporting 5 km SSMF Transmission in C-Band for 50G PON

An integrated balanced quasi-coherent receiver front end for 50G Passive Optical Network (PON) was designed and experimentally demonstrated. The received intensity modulated optical signal is boosted by a local oscillator (LO) laser and downconverted to an electrical intermediate frequency (IF) signal after integrated 2×2 multi-mode interference (MMI) and high-speed germanium photodiode (PD). By adjusting the wavelength offset between LO and transmitter laser, the IF can be chosen near the 3 dB bandwidth of RX front end to suppress the chromatic dispersion (CD) induced distortion in the upper sideband. After amplification by an integrated transimpedance amplifier (TIA), the baseband signal is demodulated via a simple square and low-pass-filter (LPF) function using offline digital signal processing (DSP). In contrast to traditional coherent receivers, no phase locking or carrier phase recovery is required. For 50 Gb/s non-return to zero(NRZ), corresponding to pre-FEC limit at bit error rate (BER) reference level 1e-2 in ITU-T G.9804, the measured optical modulated amplitude (OMA) sensitivity is −25.8 dBm (average received optical power is $-27$ dBm) in back-to-back (B2B) case and $-19.8$ dBm after 5 km standard single-mode-fiber (SSMF) transmission in C-band at 1550 nm, without any CD compensation and equalization in DSP. Measurement results show that total chromatic dispersion of 85 ps/nm can be handled for BER value lower than 1e-2. Compared with direct detection by using the same integrated high-speed germanium photodiode and TIA, the measured OMA sensitivity at BER reference level 1e-2 of the proposed quasi-coherent receiver has 13 dB improvement in B2B case and higher than 16 dB enhancement after 5 km SSMF transmission in C-band for 50 Gb/s NRZ with 10.1 dBm LO power. By reducing LO power for high-power input signals, $>$24 dB dynamic range is covered at BER reference level 1e-2, which satisfies class C+ level in ITU-T 50G PON PMD layer specification, assuming 8.5 dBm transmitter launch power.