TABLE OF CONTENTS The field of distributed fibre optic sensing involves measuring the variation of a physical or a chemical parameter along the length of the fibre. In many cases this technology is more attractive than using a large number of separate point sensors (which provide information about the spatial variability of the measurand by means of an interpolation algorithm) to perform the same task and it also allows comparative measurements as a continuous function along the length of the fibre.
An important property of distributed fibre optic sensors is their ability to be multiplexed. Spatially multiplexed sensors are the simplest choice for many applications although the system requires many detectors, which in turn do not allow for inter-channel comparison, as the analogue a signal source can be affected by connector loss. Time-division multiplexing uses a single source and a detector but requires fast electronics or long optical path differences in the sensor cable. Wavelength-division multiplexing does not require high speed electronics but, again, multiple sources and detectors are necessary. In some applications, frequency-division multiplexing of a sub-carrier has been used, showing a high resolution for a DC response.
Of the techniques currently available, optical time domain reflectometry (OTDR) is the most commonly employed technique for distributed sensing. This makes use of the finite propagation time of optical signals in the fibre and can therefore be considered as the optical waveguide analogue of the well-known radar technique. Of the OTDR techniques available at the moment, the Rayleigh backscattering technique is...
