The viscosity of silica fibres

TL;DR

This study measures the viscosity of silica optical fibres at high temperatures using optical fibre Bragg gratings, providing more accurate data and insights into fibre behaviour in harsh environments relevant for advanced optical technologies.

Contribution

It introduces a novel optical interrogation method for measuring silica fibre viscosity at high temperatures, yielding more reliable data than previous techniques.

Findings

Activation energy for viscous flow: 450 kJ/mol

Viscosity measurements align with some literature but are higher than past optical fibre results

Fibre's higher fictive temperature influences viscosity and activation energy

Abstract

The viscosity of an optical fibre over 1000 to 1150 {\deg}C is studied by inscribing an optical fibre Bragg grating that can withstand temperatures up to 1200 {\deg}C and monitoring fibre elongation under load through the Bragg wavelength shift. This optical interrogation offers high accuracy and reliability compared to direct measurements of elongation, particularly at lower temperatures, thus avoiding significant experimental error. An excellent Arrhenius fit is obtained from which an activation energy for viscous flow of Ea = 450 kJ/mol is extracted; addition of an additional temperature dependent pre-exponential does not change this value. This value is less than that idealised by some literature but consistent with other literature. The log plot of viscosity is overall found to be consistent with that reported in the literature for silica measurements on rod and beams, but substantially higher to past work reported for optical fibres. The discrepancy from an idealised activation energy Ea ~ 700 kJ/mol may be explained by noting the higher fictive temperature of the fibre. On the other hand, past optical fibre results obtained by beam bending with much lower values leave questions regarding the method of viscosity measurement and the time taken for structural equilibration. We note that because regenerated gratings already involve post-annealing to stabilise their operation at higher temperature, the structures are much more relaxed compared to normal fibres. This work highlights the need to stabilize components for operation in harsh environments before their application, despite some mechanical compromise. Given the increasing expectation of all-optical waveguide technologies operating above 1000 {\deg}C, the need to study the behaviour of glass over the long term brings added significance to the basic understanding of glass in this regime.