Subharmonic Bifurcations and Irregular Pulsing Behavior of Modulated Diode Lasers

The problem of existence of lasing regimes characterized by an irregular response of a semiconductor laser to a periodic modulation of the injection current has been of considerable interest recently. Based on coupled nonlinear rate equations for carrier and photon densities, Lee et al (1) predicted a period doubling route to chaos in a directly modulated semiconductor laser as the modulation index increases. Kawaguchi(2) showed a similar route to chaos in the optical response of a directly modulated, self-pulsating semiconductor laser with a saturable absorber. While chaotic behavior has been experimentally observed in other lasers with modulated parameters3,4, chaos in a modulated diode laser still remains to be demonstrated. In this paper we report the observation of a subharmonic bifurcation and erratic behavior in the output of diode lasers subjected to deep modulation of the injection current. The experimentally measured bifurcation sequences differ considerably from the predicted scenarios. Non-deterministic quantum noise sources are believed to play an important role in the observed phenomena.