Nonlinear interference noise in 100-Gbit s-1 communication lines with the DP-QPSK modulation format

The nonlinear interference noise power was measured experimentally as a function of the signal power and the length of the fibre coherent 100-Gbit s^-1 communication line, in which chromatic dispersion is not optically compensated. An expression was derived for the optimal signal power corresponding to a maximal excess of the measured optical signal-to-noise ratio (OSNR) over the OSNR required for conditionally error-free operation. The dependence of signal optimal power measured experimentally well agrees with the theoretical formula derived.