On the coherent modes of ultranarrowband random lasers with nonresonant feedback

Abstract

We study the variation of the coherent modes in the emission of a coherent random laser based on nonresonant feedback at critical excitation. A baseline-finding algorithm is applied to separate the coherent and incoherent components from the complete spectrum. By applying intensity thresholds to the isolated coherent spectrum, the modal density is measured as a function of mode intensity, which shows an exponential decay, corresponding to the probability of amplified extended modes. We measure the magnitude of the coherent intensity in the maximally coherent modes, whose distribution shows the presence of equal magnitude of coherent and incoherent fraction in any mode. Upon higher-than-critical excitation, instead of increasing the coherent fraction in the modes, the system tends to increase the number of modes that leads to self-averaging of the spectrum, thereby compromising the coherence.