Spontaneous-emission-enabled dynamics at the threshold of a directly modulated semiconductor laser

Abstract

Chaos in semiconductor lasers or other optical systems has been intensively studied in the past two decades. However, modulation around threshold has received much less attention, in particular, in gain-modulated semiconductor lasers. In this paper, we investigate the bifurcation sequence that appears with pump modulation in the threshold region with a large amplitude and different values of modulation frequency. Modulation around threshold necessarily includes “below-threshold” dynamics, which can be effectively displayed only through a nonlinear visualization of the oscillations. The irregular temporal behavior is examined at various modulation frequencies and amplitudes, highlighting a possible route to chaos for very large amplitude modulation in the near-threshold region. The addition of (average) spontaneous emission to the lasing mode enables a coupled dynamics between photons and carriers even below threshold, thus extending the pump range in which modulation actively modifies the laser behavior. We also report on the existence of a transition between similar attractors characterized by a temporal transient that depends on the amplitude of the modulation driving the pump.