Distributed crystal fibre sensing for extreme environments

Distributed sensing of temperature can be achieved by using time-correlated two-photon excited fluorescence (TPF). To assess the extension of this technique to crystal fibres for high temperature applications, various aspects are considered including the two-photon absorption cross-section (δ), dopant density and the geometry of single crystal fibres. By comparing the fluorescence yield for two-photon excitation with that for single-photon excitation of the same transition, δ for ruby was measured over the 0.8-1.2 μm range with maximum room temperature values of 5.9 × 10-3 GM for epolarisation and 4.6 × 10-3 GM for o-polarisation at 840 nm. It is shown that values of this magnitude are adequate for a practical TPF based crystal fibre sensor to be realised.