Bidirectional symmetrical 200G simplified coherent PON system with laser-free ONUs

Coherent detection is a highly attractive solution for beyond 200G passive optical network (PON) systems. However, directly migrating the coherent technologies from core networks to PON is excessively expensive. The narrow-linewidth local oscillator (LO) laser is a significant portion of the cost in coherent receivers. This paper proposed a laser-free optical network unit-based 200G symmetric simplified coherent PON architecture. The remote tone (RT) and downstream (DS) signal are transmitted to the optical network unit (ONU) via two fibers. At the ONU side, the RT is amplified and then demultiplexed to serve as both the LO for different ONUs and the light source of the upstream (US) signal, eliminating the need for carrier recovery across the network. In this architecture, two wavelengths are utilized for both US and DS to reduce the bandwidth requirements for the ONU and enhance the rate adjustment capability without increasing the total wavelength occupancy of the network. Alamouti coding is employed to achieve polarization-independent and simplify coherent reception, making the system more practical. Simulation and experimental results show that the performance of the system is very robust to the polarization states of the signal light and the LO light. The scheme reduces the optoelectronic bandwidth requirement of the ONU from >50 GHz to only 13.75 GHz (roll-off = 0.1) for a simplified coherent receiver based on balanced photodetectors in 200G PON. Under band-limited conditions, taking soft-decision forward error correction (SD-FEC) (2e-2) as the threshold, the sensitivity is significantly (∼4 dB) improved compared with heterodyne coherent detection. Consequently, it can achieve a power budget of 37 dB/31.5 dB for DS and US in a 200G bidirectional symmetrical system, which shows potential applicability in very-high-speed PON systems.