200G coherent TFDMA PON with flexible subcarrier reception based on a low-cost ONU [Invited]

Coherent passive optical networks (PONs) have emerged as a promising candidate for next-generation access networks due to their superior receiver sensitivity, high data rates, extended reach, and enhanced flexibility. However, their large-scale deployment is hindered by the complexity and cost of coherent receivers, particularly at the optical network unit (ONU). In this work, we propose and experimentally demonstrate a flexible 200 Gb/s coherent time-and-frequency-division multiple access PON for the downstream, leveraging a simplified ONU architecture and a low-cost MHz-linewidth distributed feedback (DFB) laser. A tailored subcarrier allocation scheme is introduced to enable heterodyne detection with polarization diversity, supporting flexible-rate reception of single or multiple subcarriers via local oscillator (LO) wavelength tuning. Efficient carrier recovery is achieved through pilot-tone-aided digital signal processing, eliminating the need for time-domain pilot symbols and significantly reducing frame overhead. We experimentally demonstrate flexible data rate reception from 50 to 200 Gb/s. A power budget exceeding 32.5 dB is achieved after 25 km standard single-mode fiber transmission using a 1 MHz linewidth DFB LO and a 512-symbol preamble, showcasing high sensitivity and robust performance. This work paves the way for the practical deployment of cost-effective coherent PON systems.