Observation of coherence resonance in stimulated Brillouin scattering

This study provides first experimental evidence of coherence resonance (CR) in an autonomous noise generating process, namely stimulated Brillouin scattering (SBS). A single mode continuous-wave Nd:YAG laser is used as pump to excite SBS in a single mode optical fibre of length 240 meters with very weak feedback. The scattered signal displays stochastic dynamic behaviour from its onset. On increase of the pump signal a window of periodicity appears, emerging from and subsequently subsiding into the stochastic emission. These features are characteristic of CR. This work also shows the dependence of the measure of coherence β (essentially signal-to-noise ratio) as a function of the strength of stochastic emission for a full range of this data. The authors argue through theory that the source of the observed coherence is associated with a transient relaxation oscillation of SBS due to pump depletion in a medium of finite length. For continuous-wave pumping, as in the case of these experiments, the stochastic SBS emission parametrically modulates the pump thereby inducing this relaxation oscillation, which may be sustained for a particular strength of the stochastic Stokes signal.