A microrod-resonator Brillouin laser with 240 Hz absolute linewidth

Abstract

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