Fiber Optic Extrinsic Sensors

About the fiber optic extrinsic sensors

Extrinsic fiber optic sensors use a fiber optic cable, normally a multimode one, to transmit modulated light from a conventional sensor. A major feature of extrinsic sensors, which makes them so useful in such a large number of applications, is their ability to reach places which are otherwise inaccessible. One example of this is the insertion of fiber optic cables into the jet engines of aircraft to measure temperature by transmitting radiation into a radiation pyrometer located remotely from the engine. Fiber optic cable can be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible.

Extrinsic fiber optic sensors provide excellent protection of measurement signals against noise corruption. Unfortunately, the output of many forms of conventional sensor is not in a form which can be transmitted by a fiber optic cable. Conversion into a suitable form must therefore take place prior to transmission. For example, in the case of a platinum resistance thermometer (PRT), the temperature changes are translated into resistance changes. The PRT must therefore have an electrical power supply. The modulated voltage level at the output of the PRT can then be injected into the fiber optic cable via the usual type of transmitter. This complicates the measurement process and means that low-voltage power cables must be routed with the fiber optic cable to the transducer. One particular adverse effect of this is that the advantage of intrinsic safety is lost.

Recent research has been directed to these kinds of problems which beset some extrinsic sensors, and this has resulted in the development of power sources in the form of electronically generated pulses driven by a lithium battery. This avoids having to transmit electrical power to the sensor via cables and provides intrinsically safe operation.

Piezoelectric sensors lend themselves to use in extrinsic sensors because the modulated frequency of a quartz crystal can be readily transmitted into a fiber optic cable by fitting electrodes to the crystal which are connected to a low-power LED. Resonance of the crystal can be created either by electrical means or by optical means using the photothermal effect. The photothermal effect describes the principle where, if light is pulsed at the required oscillation frequency and directed at a quartz crystal, the localized heating and thermal stress caused in the crystal results in it oscillating at the pulse frequency. Piezoelectric extrinsic sensors can be used as part of various pressure, force and displacement sensors. At the other end of the cable, a phase-locked loop is typically used to measure the transmitted frequency.

One extremely accurate form of extrinsic sensor is a device known as the Accufibre temperature sensor. This is a form of radiation pyrometer which has a black-box cavity at the focal point of the lens system. A fiber optic cable is used to transmit radiation from the black-box cavity to a spectrometric device which computes the temperature.

Fiber optic cables are also now commonly included in digital encoders, where the use of fibers to transmit light to and from the disks allows the light source and detectors to be located remotely. This allows the devices to be smaller, which is a great advantage in many applications where space is at a premium.

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