Pilot symbols and amplitude modulation pilot tone aided nonlinear interference monitoring for coherent systems

A nonlinear interference (NLI) monitoring method is proposed for coherent transmission systems by jointly using implementation agreement-defined pilot symbols (PS) and amplitude modulation pilot tone (AMPT). The PS is employed to extract the total noise from the received signal, while the AMPT enables separation of self-channel interference (SCI) and cross-channel interference (XCI) power from amplifier spontaneous emission (ASE) power. In a 60-GBaud dual-polarization quadrature phase-shift keying (DP-QPSK) 1500-km wavelength-division multiplexing (WDM) simulation system, the NLI signal-to-noise ratio (SNR) and NLI-induced optical SNR (OSNR) penalty were estimated with errors less than 0.26/0.07 dB for a single-channel (SCI-only) system and 0.52/0.30 dB for a 5-channel (SCI+XCI) system within a ∼160-µs time window. In a single-channel 60-GBaud DP-QPSK 500-km experimental system, the estimation errors of NLI SNR and OSNR penalty were below 0.45 dB and 0.35 dB, respectively, using ∼ 300-µs data. The proposed PS-AMPT NLI monitoring method exhibits high accuracy and strong compatibility with deployed optical networks.