Gas-filling dynamics of a hollow-core photonic bandgap fiber for nonvacuum conditions
This paper introduces an analytical model for the pressure-driven delivery of gas mixtures consisting of multiple components of gases with varied viscosity to hollow-core photonic bandgap fibers. Photonic bandgap fibers have been used for all-fiber spectroscopy configurations for gases like methane and acetylene that are relevant to the development of all-fiber chemical sensors. The unique structure of these fibers permits the formation of a gas cell with micrometer-scaled volumes to promote strong interaction with the guided optical field supported by increased interaction lengths and low optical losses. The influence of the effective viscosity of the delivered gas is significant to the fiber-filling process and is emphasized in this discussion. Unlike previous work, this investigation is not limited to closed-loop configurations with low gas flows, single-gas delivery systems, or vacuum conditions. The findings of this study are relevant to applications employing spectroscopy principles including manufacturing process monitoring for integrated fibers possessing metal–oxide junctions, antiresonant fibers, and suspended-core-type fiber evanescent-field Raman spectroscopy applications. The experimental results agree with the predictions for the case of hydrodynamic flow under nonvacuum conditions. The dynamics of the gas delivery to the perforated optical fiber were predicted and demonstrated for a 0.365 m fiber length with a 12.5 μm core. This core region was occupied by a preliminary gas, which affected the filling rates. The effective viscosities for the pure gas (acetylene) produced filling rates of 6 s (at 10 psi) and 11 s (at 5 psi). However, mixed gases (acetylene balance nitrogen) presented viscosity-dependent delays of 0.3 and 0.5 s.