Abstract: A distributed optical fibre sensor is described which is arranged to measure one or more parameters as functions of position along a sensing optical fibre that extends along a path through an environment. The sensor includes a first probe light source arranged to generate pulses of first probe light in one or more first wavelength bands, a second probe light source arranged to generate pulses of second probe light in one or more second wavelength bands separate from said first wavelength bands, a wavelength division multiplexer arranged to launch the first probe light pulses and the second probe light pulses into the sensing optical fibre for backscatter within the sensing optical fibre, and a receiver arranged to receive and separately detect both Raman shifted components of the backscattered probe light, and coherent Rayleigh backscattered components of the second probe light.

Abstract: Methods and apparatus are disclosed for detecting a rate of change of temperature in the environment around a sensing optical fibre at a position along the fibre, in which a time varying coherent Rayleigh backscatter interference signal is detected using probe light backscattered within the sensing fibre from the position. A spectral density profile is calculated which represents frequency components in time variations of the interference signal. One or more properties of a feature of the spectral density profile are then used to determine the rate of change of temperature. The feature may be, for example, a decline in the spectral density profile with increasing frequency, or a peak in the spectral density profile.

Abstract: A distributed optical fibre sensor is discussed which is arranged to detect acoustic vibration and at least a first of two or more other measurands, which could be for example two or more of changes in temperature, changes in static pressure, and changes in static strain. At least first and second optical waveguides are arranged to have optical path length response characteristics to at least one of the other measurands which are different from each other, and an analyser is arranged to determine at least said first other measurand using differences between the interference signals from each of the optical waveguides.

Abstract: There is disclosed a distributed optical fibre sensing system in which the sensor fibre comprises at least first and second waveguides used for separate sensing operations. The sensor fibre may be, for example, a double clad fibre having a monomode core and a multimode inner cladding.

Abstract: The disclosure relates to a distributed optical fibre sensor having an optical switch arranged to selectively and simultaneously couple each of a plurality of interrogators to each of a plurality of sensing optical fibres.