Guided-wave brillouin scattering in air

Photonic-bandgap fibers have had major impact from fundamental studies of photon-atom interactions to new applications in nonlinear optics. While much is known about the optical properties of these fibers, relatively little is known about their optomechanical properties. Here we identify a new form of optomechanical coupling in gas-filled hollow-core fibers. We show that forward Brillouin scattering is produced by air in the core of a photonic bandgap fiber. Asingle Brillouin resonance is identified at 35 MHz, which corresponds to a guided sound wave within the center of an air-filled hollowcore fiber. A simple analytical model, refined by numerical simulations, is developed that accurately predicts the Brillouin coupling strength and frequency from the gas and fiber parameters, revealing that this optomechanical interaction is highly tailorable. This new mechanismcould become the basis for new types of sensing and spectroscopy. Moreover, this previously unknown nonlinearity within hollow core fibers represents a power and noise limitation that requires further consideration.