Viscosity of silica optical fibres characterized using regenerated gratings

The viscosity of an optical fibre over 1000–1150 °C is studied by inscribing an optical fibre Bragg grating that can withstand temperatures up to 1200 °C and monitoring fibre elongation under load through the Bragg wavelength shift. This optical interrogation offers high accuracy and reliability compared with 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 E_a = 450 kJ mol^?1 is extracted; addition of an additional temperature-dependent pre-exponential does not change this value. The log plot of viscosity is found overall to be consistent with that reported in the literature for silica measurements on rods and beams, but substantially higher than past work reported for optical fibres. The annealing and strain temperatures for an optical fibre were derived as 1114 °C and 1010 °C. However, it is shown that, because regenerated gratings already involve a post-annealing process at higher temperature, the structures are equilibrated and much more relaxed compared with normal fibres, making viscosity measurements meaningful. 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 >1000 °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.