  Stimulated Brillouin scattering provides optical gain for efficient and
narrow-linewidth lasers in high-Q microresonator systems. However, the thermal
dependence of the Brillouin process, as well as the microresonator, impose
strict temperature control requirements for long-term frequency-stable
operation. Here, we study Brillouin back action and use it to both measure and
phase-sensitively lock modal temperature to a reference temperature defined by
the Brillouin phase-matching condition. At a specific lasing wavelength, the
reference temperature can be precisely set by adjusting resonator free spectral
range. This backaction control method is demonstrated in a chip-based Brillouin
laser, but can be applied in all Brillouin laser platforms. It offers a new
approach for frequency-stable operation of Brillouin lasers in atomic clock,
frequency metrology, and gyroscope applications.
