Laser Cooling of Traveling-Wave Phonons in an Optical Fiber

In recent years, optical control of mechanical oscillators has emerged as a critical tool for everything from information processing to laser cooling. While traditional forms of optomechanical cooling utilize systems comprising discrete optical and mechanical modes, it has recently been shown that cooling can be achieved in a chip-based system that possesses a continuum of modes. Through Brillouin-mediated phonon-photon interactions, cooling of a band of traveling acoustic waves can occur when anti-Stokes scattered photons exit the system more rapidly than the relaxation rate of the mechanical waves, to a degree determined by the acousto-optic coupling. Here, we demonstrate that a continuum of traveling-wave phonons can be cooled within an optical fiber, extending this physics to macroscopic length scales. Leveraging the large acousto-optic coupling permitted within a liquid-core optical fiber, heterodyne spectroscopy reveals power-dependent changes in spontaneous-Brillouin-scattering spectra that indicate a reduction of the thermal phonon population by 21 K using 120 mW of injected laser power. These results provide alternative ways to manipulate phonon populations that could enable acousto-optic applications with reduced noise or provide ways to control traveling-wave phonons at the quantum level.