Thermal and driven noise in Brillouin lasers

Owing to their highly coherent emission and compact form factor, Brillouin lasers have been identified as a valuable asset for applications including portable atomic clocks, precision sensors, coherent microwave synthesis, and energy-efficient approaches to coherent communications. While the fundamental emission linewidth of these lasers can be very narrow, noise within dielectric materials leads to drift in the carrier frequency, posing vexing challenges for applications requiring ultrastable emission. A unified understanding of Brillouin laser performance may provide critical insights to reach new levels of frequency stability; however, existing noise models focus on only one or a few key noise sources, and do not capture the thermo-optic drift in the laser frequency produced by thermal fluctuations or absorbed power. Here, we develop a coupled-mode theory of Brillouin laser dynamics that accounts for dominant forms of noise in noncrystalline systems, capturing the salient features of the frequency and intensity noise for a variety of systems. As a result, theory and experiment can be directly compared to identify key sources of noise and the frequency bands they impact, revealing strategies to improve the performance of Brillouin lasers and pave the way for highly coherent sources of light on a chip.