ABSTRACT
The interrogation can be particularly simple, for example, a simple power measurement for an alarm without location or an optical distributed optical fibre sensor (OTDR) to also provide location of the closest point threshold. The concept was made truly distributed by Alan Harmer, who proposed and demonstrated a fibre onto which a plastic helical filament was wound at a pitch suitable for causing the light to be attenuated when side pressure is applied. Most optical linear heat detectors, as used for fire and overheat alarming, are based on Raman distributed temperature sensor (DTS). In the case of a single-mode fibre, the local Numerical aperture (NA) maps directly on to the local capture fraction and, hence, on the strength of the local backscatter signal. The polarisation of the light travelling in optical cables is influenced by external disturbances and the use of polarisation in OTDR (POTDR) in distributed sensing has been demonstrated in the laboratory and in field trials.
