Recently, fibre optical sensors (FOS) have gained increased popularity and market acceptance. In comparison to conventional sensors they offer a number of distinct advantages which makes them unique for certain types of applications, mainly where conventional sensors are difficult or impossible to deploy or can not provide the same wealth of information.
2. TYPES OF FIBRE OPTICAL SENSORS
According to the spatial distribution of the measurand (the quantity to be measured), FOS can be classified as...
Point sensors: the measurement is carried out at a single point in space, but possibly multiple channels for addressing multiple points.
Examples are Fabry-Perot sensors and single Fibre Bragg Grating (FBG) sensors.
Integrated sensors: the measurement averages a physical parameter over a certain spatial section and provides a single value.
An example is a deformation sensor measuring strain over a long base length.
Quasi-distributed or multiplexed sensors: the measurand is determined at a number of fixed, discrete points along a single fibre optical cable. The most common example are multiplexed FBG's.
Distributed sensor: the parameter of interest is measured with a certain spatial resolution at any point along a single optical cable.
Examples include systems based on Rayleigh, Raman and Brillouin scattering.
3. GENERAL ADVANTAGES OF FIBRE OPTICAL SENSORS
Completely passive: can be used in explosive environment.
Immune to electromagnetic interference: ideal for microwave environment.
Resistant to high temperatures and chemically reactive environment:ideal for harsh and hostile environment.
Small size: ideal for embedding and surface mounting.
High degree of biocompatibility, non-intrusive nature and electromagnetic immune: ideal for medical applications like intra-aortic balloon pumping.
Can monitor a wide range of physical and chemical parameters.
Potential for very high sensitivity, range and resolution.
Complete electrical insulation from high electrostatic potential.
Remote operation over several km lengths without any lead sensitivity: ideal for deployment in boreholes or measurements in hazardous environment.
Multiplexed and distributed sensors are unique in that they provide measurements at a large number of points along a single optical cable: ideal for minimising cable deployment and cable weight, or for monitoring extended structures like pipelines, dams etc.
In what follows we give a brief explanation of the working principles of optical fibres and each type of sensors.
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