Distributed temperature sensing based on time-correlated two-photon excited fluorescence (TPF) in doped fibre is
described. Counter-propagating laser pulses generate a TPF flash at the position of overlap which is scanned along the
fibre by a variable relative time delay. The flash is detected and analysed at one end. With the fluorescence power
being completely independent of excitation pulse duration and temporal profile, the sensor does not require ultrashort
excitation pulses for operation. There is potential for high spatial resolution as the length of the sensed region depends
only on pulse duration. TPF is reported in bulk glass doped with rare earths and in doped single-mode fibre. The
suitability of fluorescence transitions for sensing is discussed taking into account the temperature dependence of the
decay times, the location of the terminating energy level relative to the ground state, and the option of non-degenerate
TPF.
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.
Distributed temperature sensing based on time-correlated two-photon excited fluorescence (TPF) in doped optical fibre
is described. Counter-propagating laser pulses generate a TPF flash at the position of their overlap which is scanned
along the fibre by a variable relative time delay. The flash is transmitted to one end where it is detected and analysed to
yield the temperature from its thermal dependence. With the fluorescence power being completely independent of
excitation pulse duration and temporal profile, the sensor does not require ultrashort excitation pulses for operation.
There is potential for high spatial resolution as the length of the sensed region depends only on pulse duration.
Preliminary results are presented for praseodymium doped single-mode fibre.
Distributed temperature sensing based on time-correlated two-photon excited fluorescence (TPF) in doped optical fibre
is described. Counter-propagating laser pulses generate a TPF flash at the position of their overlap which is scanned
along the fibre by a variable mutual delay. The flash is transmitted to one end where it is detected and analysed to yield
the temperature from its thermal dependence. As the fluorescence power is shown to be completely independent of
excitation pulse duration and temporal profile, the sensor does not require an ultrashort pulse excitation source for
operation. With the length of the sensed position depending only on pulse duration, there is also potential for high spatial
resolution.
Distributed sensing of temperature using fluorescence can be achieved by using time-correlated two-photon excitation.
To assess the extension of this technique to crystal fibres, the two-photon absorption cross-section of ruby (Cr:sapphire)
was measured by comparing the fluorescence yield for two-photon excitation with that for single-photon excitation of the
same transition. Its wavelength dependence was determined over the 0.8-1.2μm range with a maximum room
temperature value of 5.5 x 10-53cm4.s.ion-1.photon-1 being observed at 840nm for e-polarisation. From room temperature
to 400oC, the two-photon absorption cross-sections at 1.06μm showed small increases for linearly polarised light but an
increase of ~ 300% for circularly polarised light. The feasibility of a distributed optical fibre temperature sensor based on
time-correlated two-photon excited fluorescence in ruby is discussed.
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