Pressure-induced enhanced guiding properties of gas-filled hollow-core antiresonant fibres

Abstract

Tubular antiresonant fibres (ARFs), as a subset of hollow-core fibres (HCFs), appeared in 2011. In a little over a decade, they have already been proven as a unique and valid alternative to standard single-mode fibres (SMFs). ARFs have been reported with a loss comparable to or even lower than the loss of SMF. Since light is transmitted through the hollow regions of the fibre, weakly interacting with silica, ARFs possess additional advantages over SMFs, such as low nonlinearity, low dispersion, and near-vacuum speed of light, making them great candidates for high-power and telecom applications. In addition, ARFs can be used as a vessel for long-length gas-light interactions with applications in nonlinear optics and gas fibre lasers. Gas-based applications need careful consideration, as it was recently discovered that the difference in gas pressure between the core and the cladding of the fibre creates a gas-induced differential refractive index (GDRI) that alters the guiding properties of the fibre, increasing or decreasing the loss of modes of the fibre.
In this PhD project, I advance the study of mechanics and benefits of the GDRI phenomena in ARFs using a nested nodeless antiresonant fibre (NANF). In the first half, I investigate the form and time constants of the evolution of the pressure distributions in the fibre created by pressurising one of the ends of the fibre. Two numerical models are used, one of which is an existing simplified tube model. The second is a finite volume approach of solving Navier-Stokes equations considering the complex geometry of the NANF cross-section developed in OpenFOAM. Experimentally, the evolution of the pressure distributions along the length of NANF is studied via optical time-domain reflectometry. In the second half, I investigate the reduction and increase in fibre optical loss due to the formed pressure distributions. I simulate NANF's optical properties using numerical mode analysis via COMSOL software and measure fibre throughput power attenuation for experimental confirmation of the change in optical loss.
I have found that the evolution of the pressure distribution can be simulated using a tube model but with an amended (effective) fibre core diameter value. For the case of the particular NANF geometry considered, the effective diameter is 0.8 of the diameter of the core of the fibre, which was confirmed both in simulations and experiments, where the fibre was pressurised (filled) at 3, 4.5, and 6 bar at its proximal end and sequentially vented. During NANF pressurisation at 6 bar, the transient pressure difference between the fibre core and the cladding created a transient loss reduction of ~0.53 dB/km at ~23.5 hours of filling. The absolute value of the loss reduction was found in numerical simulations, and the relative loss reduction was confirmed experimentally.

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Identifiers

ORCID for Natalie Wheeler: orcid.org/0000-0002-1265-9510

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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