Controlling the spectral persistence of a random laser

Abstract

Random lasers represent a relatively undemanding technology for generating laser radiation that displays unique characteristics of interest in sensing and imaging. Furthermore, they combine the classical laser’s nonlinear response with a naturally occurring multimode character and easy fabrication, explaining why they have been recently proposed as ideal elements for complex networks. The typical configuration of a random laser consists of a disordered distribution of scattering centers spatially mixed into the gain medium. When optically pumped, these devices exhibit spectral fluctuations from pulse to pulse or constant spectra, depending on the pumping conditions and sample properties. Here, we show clear experimental evidence of the transition from fluctuating (uncorrelated) to persistent random laser spectra, in devices in which the gain material is spatially separated from the scattering centers. We interpret these two regimes of operation in terms of the number of cavity round trips fitting in the pulse duration. Only if the cavity round-trip time is much smaller than the pulse duration are modes allowed to interact, compete for gain, and build a persisting spectrum. Surprisingly this persistence is achieved if the pumping pulse is long enough for radiation in the cavity to perform some 10 round trips. Coupled-mode theory simulations support the hypothesis. These results suggest an easy yet robust way to control mode stability in random lasers and open the pathway for miniaturized systems, as, for example, signal processing in complex random laser networks.