Distributed optical fibre sensing of temperature using time-correlated two-photon excited fluorescence: theoretical overview

The general theory of a distributed temperature sensor based on time-correlated two-photon excited fluorescence in doped optical fibre is presented. Counter-propagating excitation pulses generate a two-photon excited fluorescence flash at their overlap which can be scanned along the length of the fibre by means of a variable mutual delay. The temperature at the sensed location is obtained using the decay time of the fluorescence from this position. As the power of the fluorescence flash is shown to be completely independent of excitation pulse duration and temporal profile, the sensor does not require a picosecond or femtosecond excitation source for operation. Background fluorescence may be reduced by optimising pulse shape and duration, or eliminated entirely by suitable combinations of the pulse wavelengths and the absorption spectrum of the doped medium.