Coherence resonance in a bistable laser system with time delay

Abstract

We have recently unveiled a new dynamics in Vertical-Cavity Surface-Emitting Lasers (VCSELs), induced by a time-delayed optical feedback [M. Sciamanna <i>et al</i>., Opt. Lett. <b>28</b>, 1543-1545 (2003)]. The optical feedback is responsible for multiple polarization switchings as we increase the injection current. If the current is fixed close to a switching point, the VCSEL exhibits a bistable regime: the laser randomly hops between the two VCSEL linearly polarized (LP) modes, this mode hopping being driven by the laser spontaneous emission noise. Each hop is accompanied by delayed feedback induced instabilities such as rapid anticorrelated oscillations in the intensities of the two LP modes at the frequency of the external cavity (EC). These rapid delay-periodic oscillations therefore complement the slow polarization bistable mode-hopping. Here we summarize our previous theoretical and experimental conclusions and we report on further statistical studies of this optical feedback induced polarization mode-hopping in VCSELs. Interestingly, we show that the addition of an optimal amount of noise on the VCSEL injection current may give rise to coherence resonance. When the noise intensity is optimal, the delay-periodic oscillations in the VCSEL mode-hopping regime exhibit a maximal regularity, i.e. the VCSEL exhibits a sequence of regular pulses at the EC frequency. This almost periodic signal is generated by the interaction between the time-delay and the noise driven mode-hopping dynamics, without the need for an external periodic signal. Our results contribute to recent investigations of coherence resonance in non excitable systems and give evidence of coherence resonance in a realistic, optical bistable system with time-delay, that is, a mode-hopping VCSEL subject to optical feedback.