SENSOR ARRANGEMENT

摘要

A substance sensor comprises an optical fibre having a core (1) and a cladding (2) having a lower refractive index. A layer (4) of cladding is left between a sensor material (3) and the core (1). The thickness (t) of the layer (4) is sufficiently small that the sensor material (3) lies within the evanescent wave which occurs within the cladding (2) when optical radiation is conducted through the core (1). The refractive index of the sensor material (3) varies in the presence of a substance to be sensed over a range which extends on either side of the refractive index of the cladding (2). When its refractive index is lower than that of the cladding the sensor material operates, as far as the radiation is concerned, similary to the cladding (2). Conversely, when its refractive index is higher than that of the cladding (2) the sensor material (3) couples out some of the radiation, reducing the transmissivity of the fibre. The sensor material may be polyethylene oxide, in which case the arrangement operates as a humidity sensor.