Abstract
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. A single Brillouin resonance is identified at 35 MHz, which corresponds to a guided sound wave within the center of an air-filled hollow-core 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 mechanism could 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.
Highlights
Hollow-core photonic bandgap fibers (HC-PBF) are unique for their ability to guide light in air through Bragg reflection from a periodic silica matrix that forms the waveguide cladding [1,2,3] (Fig. 1(a))
In comparison to conventional silica fibers, Bragg guidance in HC-PBF drastically reduces the nonlinear interactions with silica and increases the power handling to permit new forms of high power laser delivery [4], pulse compression [5], and light sources [6]
Through a combination of theory and experiment, we show that the hollow core of the fiber acts as a conduit for guided acoustic waves in air
Summary
The forward Brillouin gain and resonance frequency of air in a hollow-core fiber can be predicted given the properties of air and the geometry of the fiber. The forward-SBS gain can be calculated if the optical and acoustic eigenmodes can be determined for the hollow-core fiber geometry. The frequencies (Ωi) are determined by requiring the displacement to be zero at the core of the fiber (u = 0 when r = R). The coupling integrals are calculated with this acoustic profile and assuming a Gaussian electric field profile with a mode-field diameter equal to the core radius.
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