  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.
