ON THE PROBABILISTIC CHARACTERIZATION OF SIDE MODE FLUCTUATIONS IN PULSE-MODULATED NEARLY-SINGLE-MODE SEMICONDUCTOR-LASERS

Occasionally, a weak side mode in a pulse-modulated nearly-single-mode semiconductor laser can possess a significant portion of the total laser output power during the leading edge of a pulse. When combined with the group velocity dispersion of single-mode optical fibers, this mode partitioning can influence the bit error ratio performance of high-speed lightwave systems. Simulation-based solutions of stochastic rate equations, which describe the modulation dynamics of the laser, are used to probabilistically characterize the side-mode fluctuations. The extent of the partitioning is influenced by the threshold gain difference between the lasing mode and the dominant side mode, the laser bias current, and the injection current pulse shape. The effect of these three parameters on the mean and standard deviation of the side mode power during a signaling interval is examined. Extensive computer simulations are used to quantify the modulation dynamics of the laser for bit rates of 2.4 Gb/s and 4.8 Gb/s with NRZ and RZ encoding formats. For example, with the laser biased just above threshold and a threshold gain difference of 5 cm-1, the mean and standard deviation of the side mode contribution increase by a factor of about two when the bit rate is doubled. The encoding format does not influence significantly, the probabilistic characterization of the side mode contribution.