A microrod-resonator Brillouin laser with 240 Hz absolute linewidth

TL;DR

This paper reports the development of an ultralow-noise microrod-resonator Brillouin laser with a 240 Hz linewidth, demonstrating the influence of thermal effects on frequency stability and showing noise reduction via feedback stabilization.

Contribution

The study provides experimental evidence linking thermal effects to frequency noise in microrod Brillouin lasers and demonstrates linewidth reduction through intracavity power stabilization.

Findings

Achieved a 240 Hz laser linewidth.

Identified thermal effects as a dominant noise source.

Implemented feedback control to reduce noise.

Abstract

We demonstrate an ultralow-noise microrod-resonator based laser that oscillates on the gain supplied by the stimulated Brillouin scattering optical nonlinearity. Microresonator Brillouin lasers are known to offer an outstanding frequency noise floor, which is limited by fundamental thermal fluctuations. Here, we show experimental evidence that thermal effects also dominate the close-to-carrier frequency fluctuations. The 6-mm diameter microrod resonator used in our experiments has a large optical mode area of ~100 {\mu}m$^2$, and hence its 10 ms thermal time constant filters the close-to-carrier optical frequency noise. The result is an absolute laser linewidth of 240 Hz with a corresponding white-frequency noise floor of 0.1 Hz$^2$/Hz. We explain the steady-state performance of this laser by measurements of its operation state and of its mode detuning and lineshape. Our results highlight a mechanism for noise that is common to many microresonator devices due to the inherent coupling between intracavity power and mode frequency. We demonstrate the ability to reduce this noise through a feedback loop that stabilizes the intracavity power.