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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