Next-Generation Access Network Based on Coherent Optics With Hybrid Transceivers, Multi Formats, and Flexible Rates

Coherent detection has emerged as a key technology for advancing passive optical networks (PON) beyond 100 Gbps per wavelength, due to its advantages over the intensity and direct-detection (IM/DD) approach, which was previously dominant in PON systems. In coherent PON architectures, the cost of the optical network unit (ONU) is substantial due to the complexity of its transceivers. Consequently, numerous studies have focused on simplifying the transmitters and receivers on the ONU side. Yet, simplifying the ONU transceiver introduces issues like reduced dynamic range and lower data rates. How to make a balance between the performance and the cost, particularly in terms of component complexity, represents a critical challenge for the advancement of coherent PON. Therefore, we propose and demonstrate a hybrid, multi-format, and flexible-rate bidirectional coherent PON system, supporting a compatible OLT and ONUs with different types of transceivers. ONUs are categorized into different tiers based on their performance and cost requirements, with transceivers of varying complexity allocated accordingly. As a demonstration of concept, we have successfully conducted experimental transmissions of 25-GBaud 4/16/64- quadrature-amplitude-modulation signals across 20-km fiber in a bidirectional setup, achieving data rates ranging from 50-Gbps to 300-Gbps. The low-end, middle-end, and high-end ONUs attain power budgets of 39/31/21 dB, 43/36/26 dB, and 40/33/23 dB, respectively. This architecture serves as an effective bridge from the current 50G IM/DD PON to the anticipated 200G coherent PON, meeting the varied requirements of users at different service levels.