Laser linewidth is of central importance in spectroscopy, frequency metrology, and all applications of lasers requiring high coherence. It is also of fundamental importance, because the Schawlow-Townes laser linewidth limit is of quantum origin. Recently, a theory of stimulated Brillouin laser (SBL) linewidth has been reported. While the SBL linewidth formula exhibits power and optical Q factor dependences that are identical to the Schawlow-Townes formula, a source of noise not present in conventional lasers, phonon occupancy of the Brillouin mechanical mode is predicted to be the dominant SBL linewidth contribution. Moreover, the quantum limit of the SBL linewidth is predicted to be twice the Schawlow-Townes limit on account of phonon participation. To help confirm this theory the SBL fundamental linewidth is measured at cryogenic temperatures in a silica microresonator. Its temperature dependence and the SBL linewidth theory are combined to predict the number of thermomechanical quanta at three temperatures. The result agrees with the Bose-Einstein phonon occupancy of the microwave-rate Brillouin mode in support of the SBL linewidth theory prediction. DOI: 10.1103/PhysRevLett.119.143901 Stimulated Brillouin scattering (SBS) is a third-order (χ 3 ) optical nonlinearity that results from the interaction between photons and acoustic phonons in a medium [1][2][3][4]. SBS has practical importance in optical fiber systems [5,6] where it is an important signal impairment mechanism in long-distance transmission systems [7] and makes possible all-fiber lasers [8] as well as tunable, slow-light generation [9]. Power fluctuation resulting from thermal phonons has also been studied in fiber-optic SBS Stokes wave generation [10], and the intensity and phase noise have been measured in narrow-linewidth Brillouin lasers [11]. More recently, the SBS process has attracted considerable interest in microscale and nanoscale devices [12]. Brillouin laser action has been demonstrated in several microcavity resonator systems including silica [13][14][15][16], CaF 2 [17], and silicon [18], and Brillouin amplification has been demonstrated in integrated chalcogenide waveguides [19]. In silicon waveguides, the use of confinement to enhance amplification has been studied [20]. SBS is also a powerful tool for integrated photonics signal processing [21][22][23], and it has been applied to realize a chip-based optical gyroscope [24]. Moreover, at radio-frequency rates, the SBS damping rate is low enough in certain systems to enable cavity optomechanical effects [25] including optomechanical cooling [26] and optomechanical-induced transparency [27].This work studies a recent prediction concerning the fundamental linewidth (i.e., nontechnical noise contribution to linewidth) of the stimulated Brillouin laser (SBL). The analysis of fundamental fluctuations in Brillouin devices falls into a more general category of optomechanical oscillators in which phonons participate in the oscillation process [28,29]. This participation creates a channe...