Discrete cavity dynamics in free-space Brillouin laser
Phys. Rev. A 114, 013520 – Published 27 July, 2026
DOI: https://doi.org/10.1103/vhpb-kl3z
Abstract
Highly coherent lasers are central to modern photonics. To date, high-coherence operation has been achieved predominantly in microcavity and fiber-based platforms. More recently, free-space Brillouin-laser experiments have revealed unusually strong noise suppression whose physical origin cannot be explained by conventional continuous-medium models developed for those platforms. In conventional continuous-medium models, the optical and acoustic fields are assumed to remain continuously coupled throughout the cavity evolution, whereas in free-space implementations the coupling is confined to the nonlinear medium and interrupted by passive propagation over the rest of the round trip. To describe this interaction-propagation separation, we develop a discrete-cavity model in which the short Brillouin interaction inside the gain medium and the subsequent free-space propagation are treated as two separate stages of the round-trip evolution. This separation introduces a temporal asymmetry between optical storage and acoustic relaxation, effectively enhancing acoustic damping at the cavity level and strongly reducing pump-noise transfer to the Stokes field. When the cavity round-trip time is much longer than the nonlinear interaction time in the gain medium, the predicted noise-suppression ratio is governed by the ratio between these two timescales. These results identify the discrete interaction-propagation structure as the physical origin of the unusually strong noise suppression in free-space Brillouin laser systems.
