Effect of gain nonlinearities on period doubling and chaos in directly modulated semiconductor lasers

Directly modulated semiconductor lasers have been predicted 1,2 to exhibit period doubling and chaos in some range of the modulation parameters. These predictions are based on a simple set of two rate equations governing the dynamics of the electron and photon populations. We have found that the sequence of period-doubling bifurcations leading to chaos disappears when the rate equations are modified to include a small power-dependent reduction in the mode gain occurring due to nonlinear phenomena such as spectral hole burning. The amount of nonlinear-gain reduction required for the elimination of chaos is ~1 % for typical values of the laser parameters. Since index-guided InGaAsP lasers generally exhibit a gain reduction of 5-10%, the chaotic behavior can be ruled out for such lasers and should be of no concern for current optical communication systems. Our numerical simulations also show that gain nonlinearities affect significantly the width and the shape of picosecond pulses generated by the method of gain switching.