  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 how to realize a
laser source that can reach the extraordinary levels of narrow linewidth
required and yet 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, which acts to suppress the resonator's fast thermal
fluctuations, together with the stimulated Brillouin scattering (SBS) optical
nonlinearity presents a powerful combination that enables the ability to lase
with an ultra-narrow linewidth of 20 Hz. To address the laser's long-term
temperature drift, we apply 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.
