Carrier-free phase retrieval of dual-polarization QAM signals aided by polarization rotation estimation

Recovering polarization-multiplexed QAM signals through carrier-free phase retrieval (CF-PR) remains a significant challenge, as it requires accurate estimation of the 2 × 2 transfer matrix of standard single mode fiber (SSMF) based on the only intensity measurements. In this paper, we propose and experimentally demonstrate a dual-polarization CF-PR (DP-CF-PR) receiver that reconstructs DP-QAM signals based solely on intensity detection. By integrating iterative field reconstruction with training-sequence assisted polarization rotation matrix estimation, the proposed scheme effectively mitigates cross-polarization coupling and enables the full-field recovery of DP-QAM signals. Numerical simulations show the polarization rotation angle and phase difference estimation errors below 0.18 rad under the variable optical signal-to-noise (OSNR) values and various modulation formats. Experimental validation based on 32GBaud DP-QPSK/16QAM signals transmission over 80 km SSMF holds excellent accuracy of phase reconstruction and BER performance, achieving the 20% soft-decision FEC (20% SD-FEC) threshold at OSNR values of 17.4 dB and 27.5 dB, respectively. The proposed DP-CF-PR architecture therefore, offers a practical and cost-efficient solution for dual-polarization optical field recovery based on direct detection, showing strong potential for short-reach and energy-efficient optical interconnects.