Brillouin Scattering in Anti-Resonant Hollow-Core Fibers

We examine optomechanical interactions in anti-resonant hollow-core fibers, for the first time. Measurements of forward Brillouin scattering confirm predictions of weak interactions and a single driven acoustic mode in air confined to the fiber core.

Hollow-core microstructured fibers provide optical guidance without the strong material interactions that come from a solid core. However, a variety of optomechanical interactions can still occur in both the air in the core of the fiber [1] and in the surrounding silica microstructure [2,3], causing power limitations and an undesired source of noise [4]. Anti-resonant hollow core fibers (AR-HCF) are a class of microstructured hollow-core fibers with a simple and flexible structure that is desirable for low attenuation operation at specific optical wavelengths [5,6]. Moreover, these fibers can have a large optical mode-field diameter and a low optical overlap with the silica microstructure, which are ideal conditions to reduce parasitic optomechanical interactions. In addition, the combination of thin and isolated material structures may enable qualitatively new optomechanical phenomena. Here we examine optomechanical interaction in an AR-HCF, for the first time.

We measure the forward Brillouin response of the fiber by recording probe light that scatters from optically excited acoustic modes through a driven four-wave process. The largest optomechanical response (g~2 x 10-4 W-1m-1) is an order of magnitude lower than the response from previous measurements of photonic-bandgap fibers [1-3]. Finite element method simulations based on SEM measurements of the fiber cross-section (Fig. 1(a)) reveal a class of Bessel-like modes in the air in the core of the fiber that is related to the modes of a hollow-cylinder. Calculations of the optomechanical coupling show the strongest Brillouin gain for the fundamental mode in air at ~9.4 MHz. This mode (Fig.1(c)) has the strongest overlap with the forces produced by the optical mode (Fig.1(b)). The scattered light from this resonance is observed only at the same polarization as the probe, in agreement with the theory of optomechanical coupling with gases [1]. The optomechanical coupling to the silica microstructure is observed to be very weak, in agreement with our numerical predictions, owing to the small overlap of the silica with the optical mode in the core. AR-HCFs provide an attractive alternative to conventional hollow core fibers for noise-sensitive applications.

Fig. 1: a) SEM fiber cross-section of the AR-HCF, b) simulated force distribution from the fundamental optical mode, and c) measured forward Brillouin response with the dominant acoustic mode in air and corresponding theoretical fit inset.

References

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