Enabling High-Capacity Long-Reach Direct Detection Transmission With QAM-PAM Stokes Vector Modulation

Abstract

We introduce a three-dimensional modulation format using a Stokes-vector direct-detection system, where a pulse amplitude modulation (PAM) signal is polarization multiplexed with a quadrature amplitude modulation (QAM) signal. The phase reference of the PAM signal enables the retrieval of the phase information of the QAM signal using a Stokes vector receiver (SVR). A dual-polarization in-phase–quadrature modulator is employed to generate QAM-PAM optical signals with chromatic dispersion precompensation. We show direct detection of dual-polarization 16QAM-PAM2 and 16QAM-PAM4 signals delivering 280 and 336 Gb/s (net bit rate 233 and 280 Gb/s) below the soft-decision forward error correction threshold of 2 $\times$ 10 $^{-2}$ at 320 and 80 km, respectively. In addition, we study system performance at different modulation formats and symbol rates. Our findings indicate that increasing the order of modulation formats of the QAM signal is better than increasing that of the PAM signal to achieve the same bit rate. Finally, the performance of a 56 Gbaud 16QAM signal with PAM2 modulation on one polarization and a 56 Gbaud 16QAM signal with a pilot carrier on the orthogonal polarization detected with an SVR is experimentally investigated.