Employing signal-to-noise-and-distortion ratio for power loading of a directly modulated laser in an FDM system

For the direct modulation of a laser using frequency division multiplexed (FDM) channels, an iterative procedure is presented for power loading based on the signal-to-noise-and-distortion ratio. The power of nonlinear distortion arising from the modulation dynamics of the laser and dispersive transmission combined with direct detection is estimated using a two-tone test with the power being determined within the receiver bandwidth for each FDM channel. This allows the signal distortion to be isolated and provides a generic result that avoids having to specify the modulation format and execute system-level simulations. The noise arising from the receiver front-end is described by an equivalent thermal noise. The power loading procedure is tested numerically with 16 FDM channels using 8-ary phase shift keying (8-PSK), 16-ary quadrature amplitude modulation (16-QAM), or 32-QAM. The resulting FDM signal is transmitted over 30 km of standard single-mode fiber followed by direct detection. In the case of using 16-QAM, 60.8 Gb/s transmission is achieved, and despite the frequency-selective power fading, all channels perform under the target bit error ratio of 3.8x10-3. Moreover, the robustness of the power loading profile is evaluated by changing the transmission distance while using a profile optimized for a specific distance.