Bifurcation in 20-GHz gain-switched 1.55-μm MQW lasers and its control by CW injection seeding

A gain-switched laser operating at a 20-GHz repetition rate exhibited chaos and period-doubling depending on the bias condition, which was the first observation for 1.55 mu m strain-compensated InGaAlAs-InGaAsP multiple-quantum-well (MQW) lasers, Unlike the case previously reported for bulk lasers, the irregular behavior was set over wide bias current conditions for the MQW laser. We show that the instability is attributable to a small gain compression factor of the MQW laser and a high-repetition frequency of the gain-switching operation. Further, we report that the irregular behavior can be successfully suppressed by seeding the laser with external coherent-light injection. The numerical simulations of a set of rate equations using parameters extracted from small-signal modulation characteristics were carried out in order to clarify the conditions for the stabilization of the gain-switching operation, The scenario behind the inhibition of irregular behavior was due to the enhanced damping phenomena of a nonlinear oscillator induced by the injection seeding light. The influence of the alpha parameter on the optical pulse distortion under the external seeding and the parametric dependence for a single-period operation were investigated based on numerical simulation.