OSNR Margin Assessment for Optical Transmissions in the Nonlinear Regime with Forward Error Correction

We study numerically and experimentally the OSNR margin of high-speed optical transmission systems in the nonlinear regime at FEC thresholds. It is found that the OSNR margin can be significatnly larger than the Q margin, in contrast to ideal linear case. As a result, the FEC coding gains for transmissions in the nonlinear regime can also be larger than the commonly quoted values for the linear case.