Chemistry Reference
In-Depth Information
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How sensing is accomplished
. In this category, the sensors can measure
a change in intensity of one or more light beams or phase changes in
the light beam(s) caused by an interaction between these beams. There-
fore, these sensors are categorised as intensity and interferometric
sensors, respectively.
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The physical extent of the sensing.
This category is based on whether
sensors operate only at a single point or over a distribution of points.
Thus, sensors in this category are categorised as either point-measuring
sensors or distribution-measuring sensors. In the case of a point sensor,
the transducer may be at the end of the fibre, where its role is to trans-
port a light beam to and from the transducer. Examples of this type of
sensor are interferometers-immobilised at the tip of fibres to measure
temperature and/or pressure. In the case of a distributed sensor, the
sensing is performed all along the fibre length. Examples of this type of
sensor are fibre Bragg gratings distributed along a fibre length to
measure strain or temperature.
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The role of the optical fibre in the sensing process.
Further distinction is
often made as to whether measurements act externally or internally to
the fibre. Where the transducers are external to the fibre and the fibre
merely registers and transmits the sensed quantity, the sensors are
termed extrinsic sensors. Where the sensors are embedded in, or are part
of, the fibre - and for this type there is often some modification to
the fibre itself - the sensors are termed internal or intrinsic sensors.
Examples of extrinsic sensors are moving gratings to sense strain,
fibre-to-fibre couplers to sense displacement, and absorption cells
to sense chemistry-related effects. Examples of intrinsic sensors are
those that use microbending losses in the fibre to sense strain, modified
fibre claddings to make spectroscopic measurements, and counter-
propagating beams within a fibre coil to measure rotation.
Several recent examples of sensor networks have been reported. Arrays of
fibre Bragg grating sensors have been deployed for sensing strain in bridges
and other structures. It was demonstrated that these sensors can measure
the change in internal strain within the girders associated with both static
and dynamic loading of the bridge with a truck. The sensor arrays are used
to test both steel and carbon composite tendons within the concrete deck
of support girders. This was performed on the first bridge using carbon-fibre
composite tendons.
8.3.3
Specialty fibres for sensors
As a large percentage of current optical sensors involve optical fibres in
certain shapes, it is important to discuss the status of fibre R&D. Glass fibres
