Demonstration of Pilot-Aided Continuous Downstream Digital Signal Processing for Multi-Format Flexible Coherent TDM-PON

The coherent passive optical network (PON) is a promising solution for the next generation of high-speed optical access network, capable of supporting 100G and beyond data rate. Moreover, the flexibility and adaptability of coherent PON (CPON) have attracted increasing interest. A flexible PON can enhance system throughput through adaptive modulation formats, baud rates, and coding techniques. One critical challenge for adaptive-modulation flexible CPON is the need for fast-tracking of the channel status, particularly for the downstream transmission. This is particularly important as signals with different modulation formats are continuous in downstream transmission. In the present study, we proposed and experimentally demonstrated a pilot-aided digital signal processing (DSP) scheme in a 300 G flexible CPON based on 4/16/64-QAMs, which avoids burst-signal processing in downstream transmission during modulation-format switching. The pilot symbols that we insert enable continuous tracking of the state-of-polarization, finite impulse response (FIR) coefficients for channel equalization, and carrier phase. The feasibility of the proposed pilot-aided DSP scheme was verified experimentally in four transmission cases involving different modulation formats. As a proof of concept, we achieved continuous signal processing in the downstream coherent signal detection, enabling 100-300 G flexible data transmission. We achieved total power budgets of 38, 30, and 20 dB for 25-GBaud polarization-division-multiplexed (PDM) 4/16/64-QAM signals after 20-km single-mode fiber (SMF), respectively.