Flexible Spectral-Notch Shaping Enabled Pilot-Tone-Aided Carrier Phase Recovery in 200G Coherent PON

A spectrum-notch-coded modulation scheme is proposed for in-band pilot-tone insertion in single carrier coherent optical transmission systems. By employing a symbol-level block-wise coding strategy based on the weighted running digital sum, the proposed scheme can generate a flexible spectral notch at a specified frequency while preserving standard QAM constellation points and the single-carrier spectral characteristic. The generated notch provides spectral space for pilot-tone insertion, enabling frequency domain pilot-tone-aided carrier phase recovery without relying on out-of-band pilot placement or guard-interval allocation. Theoretical analysis and simulations are provided to verify the notch formation mechanism and the transmission performance of the proposed scheme. A 200G coherent passive optical network downstream transmission experiment is further demonstrated using a 35 GBaud 16QAM signal over 40 km standard single-mode fiber. Experimental results show that the proposed scheme achieves a 35.59 dB power budget at the 15% soft-decision forward error correction threshold and provides a 0.83 dB received optical power gain over the digital subcarrier multiplexing (DSCM) reference without blind phase search. Improved robustness against a 2-MHz-linewidth distributed feedback (DFB) laser used as the local oscillator is also experimentally confirmed. In addition, complexity analysis shows that the proposed scheme requires fewer real-valued multiplications than DSCM under the same conditions.