Fiber Optic Intrinsic Sensors
About the fiber optic intrinsic sensors
Intrinsic sensors can modulate the
intensity, phase, polarization, wavelength or transit time of light.
Sensors which modulate light intensity tend to use mainly multimode
fibers, but only monomode cables are used to modulate other light
parameters. A particularly useful feature of intrinsic fiber optic
sensors is that they can, if required, provide distributed sensing
over distances of up to 1 meter.
Light intensity is the simplest parameter to manipulate in intrinsic
sensors because only a simple source and detector are required.
Modulation of the intensity of transmitted light takes place in
various simple forms of proximity, displacement, pressure, pH and
smoke sensors. In proximity
and displacement sensors (the latter are often given the special name Fotonic sensors), the amount of reflected light varies with the
distance between the fiber ends and a boundary. In pressure sensors,
the refractive index of the fiber, and hence the intensity of light
transmitted, varies according to the mechanical deformation of the
fibers caused by pressure. In the pH probe, the amount of light
reflected back into the fibers depends on the pH-dependent color of
the chemical indicator in the solution around the probe tip. Finally,
in a form of smoke detector, two fiber optic cables placed either side
of a space detect any reduction in the intensity of light transmission
between them caused by the presence of smoke.
A simple form of accelerometer can be made by placing a mass subject
to the acceleration on a multimode fiber. The force exerted by the
mass on the fiber causes a change in the intensity of light
transmitted, thus allowing the acceleration to be determined. The
typical accuracy quoted for this device is ±0.02 g in the measurement
range of ±5 g and ±2% in the measurement range up to 100 g.
A similar principle is used in probes which measure the internal
diameter of tubes. The probe consists of eight strain-gauged
cantilever beams which track changes in diameter, giving a measurement
resolution of 20 µm.
A slightly more complicated method of effecting light intensity
modulation is the variable shutter sensor. This
consists of two fixed fibers with two collimating lenses and a
variable shutter between. Movement of the shutter changes the
intensity of light transmitted between the fibers. This is used to
measure the displacement of various devices such as Bourdon tubes,
diaphragms and bimetallic thermometers.
Yet another type of intrinsic sensor uses cable where the core and
cladding have similar refractive indices but different temperature
coefficients. This is used as a temperature sensor. Temperature rises
cause the refractive indices to become even closer together and losses
from the core to increase, thus reducing the quantity of light
Refractive index variation is also used in a form of intrinsic sensor
used for cryogenic leak detection. The fiber used for this has a
cladding whose refractive index becomes greater than that of the core
when it is cooled to cryogenic temperatures. The fiber optic cable is
laid in the location where cryogenic leaks might occur. If any leaks
do occur, light traveling in the core is transferred to the cladding,
where it is attenuated. Cryogenic leakage is thus indicated by
monitoring the light transmission characteristics of the fiber.
Yet another use of refractive index variation is found in devices
which detect oil in water. These use a special form of cable where the
cladding used is sensitive to oil. Any oil present diffuses into the
cladding and changes the refractive index, thus increasing light
losses from the core. Unclad fibers are used in a similar way. In
these, any oil present settles on the core and allows light to escape.
The cross-talk sensor measures several different variables by
modulating the intensity of light transmitted. It consists of two
parallel fibers which are close together and where one or more short
lengths of adjacent cladding are removed from the fibers. When it is
immersed in a transparent liquid, there are three different effects
which each cause a variation in the intensity of light transmitted.
Thus, the sensor can perform three separate functions. First, it can
measure temperature according to the temperature-induced variation in
the refractive index of the liquid. Secondly, it can act as a level
detector according to the depth of the liquid which changes the
transmission characteristics between the fibers. Thirdly, it can
measure the refractive index of the liquid itself when used under
controlled temperature conditions.
The refractive index of a liquid can be measured in an alternative way
by using an arrangement where light travels across the liquid between
two cable ends which are fairly close together. The angle of the cone
of light emitted from the source cable, and hence the amount of light
transmitted into the detector, is dependent on the refractive index of
The use of materials where the fluorescence varies according to the
value of the measurand can also be used as part of
intensity-modulating intrinsic sensors. Fluorescence-modulating
sensors can give very high sensitivity and are potentially very
attractive in biomedical applications where requirements to measure
very small quantities such as low oxygen and carbon monoxide
concentrations, low blood pressure levels, etc., exist. Similarly, low
concentrations of hormones, steroids, etc., may also be measured (Grattan
Light phase, polarization, wavelength and
transit time can be modulated as well as intensity in intrinsic
sensors. Monomode cables are used almost exclusively in these types of
Phase modulation normally requires a coherent (laser) light source. It
can provide very high sensitivity in displacement measurement but
cross-sensitivity to temperature and strain degrades its performance.
Maintaining frequency stability of the light source and manufacturing
difficulties in coupling the light source to the fiber are further
problems. Various versions of this class of instrument exist to
measure temperature, pressure, strain, magnetic fields and electric
fields. Field-generated quantities such as electric current and
voltage can also be measured. In each case, the measurand causes a
phase change between a measuring and a reference light beam which is
detected by an interferometer.
The principle of phase modulation has also been used in the fiber
optic accelerometer (where a mass subject to acceleration rests on a
fiber), and in fiber strain gauges (where two fibers are fixed on the
upper and lower surfaces of a bar under strain).
Devices using polarization modulation require special forms of fiber
which maintain polarization. Polarization changes can be effected by
electrical fields, magnetic fields, temperature changes and mechanical
strain. Each of these parameters can therefore be measured by
Various devices which modulate the wavelength of light are used for
special purposes. However, the only
common wavelength-modulating fiber optic device is the form of laser
Doppler flowmeter which uses fiber optic cables.
Fiber optic devices using modulation of the transit time of light are
uncommon because of the speed of light. Measurement of the transit
time for light to travel from a source, be reflected off an object and
travel back to a detector is only viable for extremely large
distances. However, a few special arrangements have evolved which use
transit time modulation. These include
instruments such as the optical resonator, which can measure both
mechanical strain and temperature. Temperature-dependent wavelength
variation also occurs in semi-conductor crystal beads (e.g. aluminium
gallium arsenide). These are bonded to the end of a fiber optic cable
and excited from an LED at the other end of the cable. Light from the
LED is reflected back along the cable by the beads at a different
wavelength. Measurement of the wavelength change allows temperatures
in the range up to 200°C to be measured accurately. A particular
advantage of this sensor is its small size, typically of 0.5 mm
diameter at the sensing tip. Finally, to complete the catalogue of
transit time devices, the frequency modulation in a piezoelectric
quartz crystal used for gas sensing can also be regarded as a form of
time domain modulation.
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