Transient multimode statistics in nearly single-mode semiconductor lasers

Abstract

Mode partition noise in optical fiber communication systems is still causing an error rate floor, even with distributed-feedback lasers sources having a side-mode suppression ratio larger than 30 dB in stationary conditions. This phenomenon is caused by the laser turning on initially in a side mode having a different wavelength from the principal mode. The transient statistics of nearly single-mode semiconductor lasers are studied for lasers when the injected current is suddenly switched from below to above the threshold current. The side-mode excitation probability is obtained analytically and numerically for different excitation current and gain differences. In this way the probability of having a side mode larger than the main mode during the leading edge of the optical pulse (rare events) is obtained. This is analogous to an error rate in optical communication systems. We have studied in detail the rare events. For these events a linear relation is found between the value of the main mode intensity during the leading edge of the optical pulse and the logarithm of the relation between the intensities of the side and main modes both averaged over the first relaxation peak.