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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