Fiber Optic Sensors
About the fiber optic sensors
The basis of operation of fiber optic
sensors is the translation of the physical quantity measured into a
change in one or more parameters of a light beam. The light parameters
that can be modulated are one or more of the following:
1. Intensity
2. Phase
3. Polarization
4. Wavelength
5. Transmission time
Fiber optic sensors usually incorporate
either glass/plastic cables or all-plastic cables. All-glass types are
rarely used because of their fragility. Plastic cables have particular
advantages for sensor applications because they are cheap and have a
relatively large diameter of 0.5-1.0 mm, making connection to the
transmitter and receiver easy. However, plastic cables should not be
used in certain hostile environments where they may be severely
damaged. The cost of the fiber optic cable itself is insignificant for
sensing applications, as the total cost of the sensor is dominated by
the cost of the transmitter and receiver.
Fiber optic sensors characteristically enjoy long life. For example,
the life expectancy of reflective fiber optic switches is quoted at 10
million operations. Their accuracy is also good, with for instance ±1%
of full-scale reading being quoted as a typical inaccuracy level for a
fiber optic pressure sensor. Further advantages are their simplicity,
low cost, small size, high reliability and capability of working in
many kinds of hostile environment. However, in spite of these obvious
merits, industrial usage is currently quite low. This may well be an
example of a vicious circle, where new users do not appear because
there is no substantial body of existing users to consult and gain
experience from.
Two major classes of fiber optic sensor exist, intrinsic sensors and
extrinsic sensors. In intrinsic sensors, the fiber optic cable itself
is the sensor, whereas in extrinsic sensors, the fiber optic cable is
only used to guide light to/from a conventional sensor.

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