Enhancing Optical Link Performance with Per-Span Nonlinear SNR Estimation Enabled by Longitudinal Power Monitoring

We experimentally demonstrate the extraction of per-span nonlinear interference (NLI) contributions using Linear Least Squares Longitudinal Power Monitoring (LLS-LPM). This receiver-side DSP technique enables accurate estimation of the nonlinear signal-to-noise ratio (SNRNL) for each span in an optical link. By combining these estimates with optical signalto-noise ratio (OSNR) measurements, we propose a simple and effective method to determine the optimal launch power on a per-span, per-channel basis. This approach improves power optimization in wavelength-division multiplexed (WDM) systems, particularly in scenarios with span heterogeneity or channel power imbalance. Experimental validation over a 5x85 km G.652 SMF link confirms the effectiveness of the method and highlights its advantages over cumulative metrics traditionally used at the receiver.