Abstract: A substance sensor comprises a single-mode optical fibre having a core and a cladding having a lower refractive index than that of the core. A sensor material extends within the cladding towards the core. A layer of cladding is left between the sensor material and the core. The thickness (t) of the layer is sufficiently small that the sensor material lies within the evanescent wave which occurs within the cladding when optical radiation is conducted through the core. The refractive index of the sensor material varies in the presence of a substance to be sensed over a range which extends above and below the refractive index of the cladding. When its refractive index is lower than that of the cladding the sensor material operates, as far as the radiation is concerned, similarly to the cladding. However, when the refractive index of the sensor material is higher than that of the cladding the sensor material couples out some of the radiation, thereby reducing the transmissivity of the fibre.

Abstract: A differential laser Doppler velocimeter is based on the use of a modified fibre optic Sagnac Interferometer 1,2. The interferometer phase is dependent not on the target displacement, but on its velocity. The output intensity of the interferometer may be modulated by control means PC which controls loop birefringence introducing a phase bias and polarization offset between counter-propagating beams.