Optical fibers have many uses in remote sensing. Either the sensor is itself can be an optical fiber or optical fiber can be used to connect a non fiber optic sensor to a measurement system. Optical fibers are good choices for sensors because of many factors. An optical fiber may be used because of its small size. There are areas where a dielectric communication channel is required or no electrical power is needed at the remote location. Many optical sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor.
Optical fibers are used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the property to measure modulates the intensity, phase, polarization, wavelength, or transit time of light in the fiber. For sensors that vary the intensity of light, only a simple source and detector are required. Such sensors can provide distributed sensing over distances of up to one meter. In contrast, highly localized measurements can be provided by integrating miniaturized sensing elements with the tip of the fiber. These can be implemented by various micro and nano fabrication technologies, such that they do not exceed the microscopic boundary of the fiber tip, allowing such applications as insertion into blood vessels via hypodermic needle.
Extrinsic fiber optic sensors use an optical fiber cable, usually a multimode fiber to transmit modulated light from either a non-fiber optic sensor or an electronic sensor connected to an optical transmitter. One of the attractive features of extrinsic sensors is their ability to reach inaccessible areas. An example is the measurement of temperature inside aircraft jet engines where an optical fiber is used to transmit radiation into a radiation pyrometer outside the engine.
Extrinsic fiber optic sensors can be used in the same way to measure the internal temperature of electrical transformers. Electrical transformers have extreme electromagnetic fields that make other measurement techniques practically impossible. Extrinsic fiber optic sensors measure acceleration, vibration, rotation, displacement, velocity, torque, and twisting. By utilizing the interference nature of light, a solid state version of the gyroscope has been developed. The fiber optic gyroscope (FOG) has no moving parts, and exploits the Sagnac effect to detect mechanical rotation.
Fiber optic sensors are commonly used in advanced intrusion detection security systems. Optical signal is transmitted along a fiber optic sensor cable placed on a fence (recently we published a news about Saudi Arabia’s use of fiber optic sensors along the borders to monitor intrusion of terrorists and illegal immigrants), pipeline, or communication cables. The returned optical signal is monitored and analyzed for disturbances and situation at remote locations. In Oil and Gas pipelines, this return signal is digitally processed to detect disturbances and trip an alarm if an intrusion has occurred.
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