EEPN-Free Carrier Phase Recovery for Spectral-Efficient Digital Subcarrier Multiplexing Transmissions

Laser phase noise (LPN) is one of the most severe impairments for the digital subcarrier multiplexing (DSM) transmissions with advanced modulation formats. In particular, it becomes detrimental in dispersion-uncompensated fiber optics links, where the interaction between receiver-side (Rx) chromatic dispersion compensation (CDC) and LPN may lead to the occurrence of equalization enhanced phase noise (EEPN). Here, we propose a carrier phase recovery (CPR) method for spectral-efficient DSM transmissions with two interleaved pilot-tones (TIPTs). Relying on the separated reconstruction of both the transmitter-side (Tx) and Rx LPNs with high accuracy, and compensating the Rx and Tx LPNs before and after CDC, respectively, the EEPN effect can be avoided, leading to an enhanced CPR performance. The improved CPR performance from 0.23 dB to 3.7 dB is numerically achieved for a 95.6 GBaud DP-64QAM DSM transmission system, in comparison with conventional pilot-tone aided CPR scheme, depending on the accumulated CD, laser linewidth, and the used modulation format. Finally, the effective function of our proposed CPR scheme is experimentally verified with about 0.56 dB Q-factor gain, under the optimal launch power condition of 1 dBm, when the 40 GBaud DP-16QAM DSM signals are transmitted over 1040 km standard single-mode fiber (SSMF), based on a 1.7 MHz distributed feedback laser (DFB) at Tx and a 100 kHz external cavity laser (ECL) at Rx, respectively.