Hybrid optical fibre sensors

Hybrid sensing systems are slowly becoming available on the market occupying a position between conventional electronic sensors and all-optical instruments. They combine the benefits of established sensors, such as reliability, cheapness and high performance, with the advantages of optical transmission, intrinsic safety and freedom from electromagnetic interference; a digital output improves the transmission accuracy. Non-resonant and resonant sensors are reported; the latter give high dynamic range and good resolution and avoid inaccuracies which arise in the generation of an analogue output and its conversion to digital form. The signal format can simplify multiplexing if, for instance, frequency is used to identify each transmitter or serial operation is adopted. Hybrids may be powered by local long-life batteries or by photo-electric conversion of optical power, normally at levels low enough to comply with intrinsic safety regulations. If batteries are used, some form of planned maintenance is necessary to ensure reliability. Further engineering is necessary before all the potential advantages of the hybrid configuration are exploited in an industrial version, but this intermediate development is considered to be the most appropriate interim step until an all-optical version, matching conventional sensors in price and performance, becomes available