He Resonator*

Fiber-optic Fabry-Perot cavities that are filled with superfluid helium offer a number of advantages as quantum optomechanical devices. Previous work on these devices [1] has shown that each optical mode has near-perfect overlap with just the one acoustic mode that has the same transverse mode profile and a longitudinal wavenumber exactly double that of the optical mode. This "single-mode" optomechanical coupling results from the simple geometry of these devices. Here, we provide a detailed characterization of this coupling by measuring the (small) optomechanical interactions with several other paraxial acoustic modes. We find that these interactions can be attributed to the cavity's geometric asymmetry, corrections to the paraxial approximation, and the non-hermiticity arising from optical and acoustic losses. These results open the way to studying quantum optomechanical systems with degenerate mechanical modes, which may be useful for exploring exceptional point physics in the quantum regime.