Transition from amplified spontaneous emission to laser action in strongly scattering media

TLDR

Random laser action can arise from multiple scattering and amplification in active media, but distinguishing it from amplified spontaneous emission remains challenging. The transition is identified by measuring the scattering mean free path. Increasing scattering in an amplifying random medium induces a transition from amplified spontaneous emission to coherent laser oscillation, as confirmed experimentally and by Maxwell‑equation simulations.

Abstract

In an active random medium, the combination of multiple scattering with light amplification may lead to random laser action. However, it is crucial but sometimes difficult to distinguish between amplified spontaneous emission and lasing. By varying the amount of scattering in an amplifying random medium, we have observed the transition from amplified spontaneous emission to lasing with coherent feedback. We have found out when the transition occurs through the measurement of the scattering mean free path. Our numerical simulation based on the direct solution to Maxwell equations clearly illustrates the transition from light amplification to laser oscillation due to an increase of the amount of scattering in active random medium.