Analogue And Digital Extrinsic Optical Fibre Sensors Based On Spectral Filtering Techniques

Abstract

Analogue and digital extrinsic optical fibre sensors based on spectral filtering techniquesJ.P. DakinPlessey Electronic Systems ResearchRoke ManorRomsey, U.K.AbstractIncoherent optical sensors based simply on the measurement of the amplitude of lighttransmitted by the sensor head are subject to errors unless care is taken to ensure thestability of light sources and detectors, the constancy of launch conditions and freedomfrom unintentional fibre microbending losses and variations in connector coupling.Sensors based on spectral filtering techniques, however, make use of wavelength -dependent variations in transmission, reflection or scattering processes which greatlyexceed the normal spectral variations of transmission which may normally occur inoptical fibre and connectors.This paper reviews the types of spectral filtering sensors which have been developedwith classification into analogue and digital sensors.IntroductionOptical Fibre sensors may conveniently be divided into two main types. The firsttype, the intrinsic sensor, is constructed using the fibre as the sensing element,and arranging for the physical parameter to be monitored to modify the transmissionproperties. The second type, the extrinsic sensor, uses more conventional opticaltechniques to modify coupling between an input and output fibre and therefore merelyuses the fibre as a means of guiding optical energy to and from a remote passive sensingheai.To date all commercial sensors have used the extrinsic sensing approach because of the easeof tailoring the design of the transduction system to best suit the measurement.The main extrinsic methods which have been used to date include the following:-