Discrete Cavity Dynamics in Free-Space Brillouin Laser

TL;DR

This paper introduces a discrete-cavity model for free-space Brillouin lasers, explaining their strong noise suppression by separating interaction and propagation stages, and providing predictions aligned with experimental results.

Contribution

The authors develop a novel discrete-cavity model that captures the unique interaction-propagation separation in free-space Brillouin lasers, explaining their noise suppression mechanisms.

Findings

Noise suppression ratio scales with cavity length to nonlinear medium length ratio.

Model accurately predicts lasing threshold, output power, and linewidth.

Physical origin of noise suppression is linked to interaction-propagation separation.

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, which effectively enhances acoustic damping at the cavity level and strongly reduces pump-noise transfer to the Stokes field. If the cavity round-trip time is much longer than the interaction time in the nonlinear medium, the noise-suppression ratio scales with the ratio of the total cavity length to the nonlinear-medium length. Our discrete-cavity model further provides quantitative predictions for the lasing threshold, output power, phase-noise transfer, and fundamental linewidth, in good agreement with experiment. These results identify the discrete interaction-propagation structure as the physical origin of the unusually strong noise suppression in free-space Brillouin lasers systems.