Ultra-narrow linewidth Brillouin laser with nanokelvin temperature self-referencing

Ultrastable lasers serve as the backbone for some of the most advanced scientific experiments today and enable the ability to perform atomic spectroscopy and laser interferometry at the highest levels of precision possible. With the recent and increasing interest in applying these systems outside of the laboratory, it remains an open question as to how to realize a laser source that can reach the extraordinary levels of narrow linewidth required and still remain sufficiently compact and portable for field use. Critical to the development of this ideal laser source is the necessity for the laser to be insensitive to both short- and long-term fluctuations in temperature, which ultimately broaden the laser linewidth and cause drift in the laser's center frequency. We show here that the use of a large mode-volume optical resonator with 2 m of optical fiber, which acts to suppress the resonator's fast thermal fluctuations, together with stimulated Brillouin scattering optical nonlinearity presents a powerful combination that enables lasing with an ultra-narrow linewidth of 20 Hz. To address the laser's long-term temperature drift, we apply two orthogonal polarizations of the narrow Brillouin line as a metrological tool that precisely senses a minute change in the resonator's temperature at the level of 85 nK. The precision afforded by this temperature measurement enables new possibilities for the stabilization of resonators against environmental perturbation. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement