Fiber optic sensor:
A fiber optic sensor is a sensor that uses
optical fiber either as
the sensing element or as a means of relaying signals from a remote sensor to
the electronics that process the signals. Fibers have many uses in remote
sensing. Depending on the application, fiber may be used because of its small
size, or because no electrical
power is needed at the remote location, or because many
sensors can be multiplexed along the
length of a fiber by using light wavelength shift for each sensor, or by
sensing the time delay as light passes along the fiber through each sensor.
Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be
calculated using an instrument implementing optical frequency domain
optic sensors are also immune to electromagnetic interference, and do not conduct electricity so
they can be used in places where there is high voltage electricity or flammable material such as jet fuel. Fiber
optic sensors can be designed to withstand high temperatures as well
optic sensors are excellent candidates for monitoring environmental changes and
they offer many advantages over conventional electronic sensors as listed
integration into a wide variety of structures, including composite materials,
with little interference due to their small size and cylindrical geometry.
to conduct electric current.
• Immune to
electromagnetic interference and radio frequency interference.
more resistant to harsh environments.
capability to form sensing networks.
sensing capabilities such as strain, pressure, corrosion, temperature and
1. Fiber optic sensor principles
general structure of an optical fiber sensor system. It consists of an optical
source (Laser,LED, Laser diode etc), optical fiber, sensing or modulator
element (which transduces the measurand to an optical signal), an optical
detector and processing electronics (oscilloscope, optical spectrum analyzer
optic sensors can be classified under three categories: The sensing location,
the operating principle, and the application. Based on the sensing location, a
fiber optic sensor can be
as extrinsic or intrinsic. In an extrinsic fiber optic sensor the fiber is
simply used to carry light to and from an external optical device where the
sensing takes place. In this cases, the fiber just acts as a means of getting
the light to the sensing location.
2. Types of fiber optics sensor
1 Intrinsic sensor
fibers can be used as sensors to measure
2.1 Intrinsicsensor -Temperature/ Pressuresensor
based on the principle of Interference between the beams emerging out from the
reference fiber and the fiber kept in the measuring environment.
monochromatic source of light is emitted from the laser source.
consists of a Laser source to emit light. A beam splitter, made of glass plate
is inclined at an angle of 45º used to split the single beam into two beams.
beam passes through the lens L1 and is focused onto the reference fiber which
is isolated from the
to be sensed.
after passing through the reference fiber then falls on the lens L2.
spitted beam passes through the lens L3 and is focused onto the test fiber kept
in the environment to be sensed.
The splitted beam after passing
through the test fiber is made to fall on the lens L2.
beams after passing through the fibers, produces a path difference due to the
change in parameters such as pressure, temperature etc., in the environment.
a path difference is produced between the two beams, causing the interference
change in pressure (or) temperature can be accurately measured with the help of
the interference pattern obtained.
quantities by modifying a fiber so that the quantity to be measured modulates
the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that
vary the intensity of light are the simplest, since only a simple source and
detector are required. A particularly useful feature of intrinsic fiber optic
sensors is that they can, if required, provide distributed sensing over very
can be measured by using a fiber that has evanescent loss that varies with temperature, or by analyzing
scattering of the optical fiber. Electrical voltage can be
sensed by nonlinear
optical effects in specially-doped fiber, which alter the
polarization of light as a function of voltage or electric field. Angle
measurement sensors can be based on the Sagnac effect.
fibers like long-period fiber grating (LPG) optical fibers can be used
for direction recognition. Photonics Research Group of Aston
University in UK has some publications on vectorial bend
fibers are used as hydrophones for
seismic and sonar applications.
Hydrophone systems with more than one hundred sensors per fiber cable have been
developed. Hydrophone sensor systems are used by the oil industry as well as a
few countries' navies.
A fiber optic microphone and fiber-optic based headphone
are useful in areas with strong electrical or magnetic fields, such as
communication amongst the team of people working on a patient inside a magnetic
resonance imaging (MRI) machine during MRI-guided surgery.
fiber sensors for temperature and pressure have been developed for down hole
measurement in oil wells. The fiber optic sensor is well suited for this environment
as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).
fibers can be made into Interferometric
sensors such as fiber optic gyroscopes, which are used in the Boeing 767 and in
some car models (for navigation purposes). They are also used to make hydrogen
sensors have been developed to measure co-located temperature and strain
simultaneously with very high accuracy using fiber Bragg
gratings. This is particularly useful when acquiring
information from small complex structures. Brillouin scattering effects can be used to detect
strain and temperature over larger distances (20–30 kilometers).
fiber-optic AC/DC voltage sensor in the middle and high voltage range (100–
2000 V) can be created by inducing measurable amounts of Kerr
nonlinearity in single mode optical fiber by exposing a calculated length of fiber to the
external electric field. The
measurement technique is based on polar metric detection and high accuracy is achieved in a
hostile industrial environment.
frequency (5 MHz–1 GHz) electromagnetic fields can be detected by induced
nonlinear effects in fiber with a suitable structure. The fiber used is
designed such that the Faraday and Kerr effects cause
considerable phase change in the presence of the external field. With
appropriate sensor design, this type of fiber can be used to measure different
electrical and magnetic quantities and different internal parameters of fiber
power can be measured in a fiber by using a structured bulk fiber ampere sensor
coupled with proper signal processing in a polar metric detection scheme.
Experiments have been carried out in support of the technique.
sensors are used in electrical switchgear to
transmit light from an electrical arc flash to a digital protective relay to enable fast tripping of a
breaker to reduce the energy in the arc blast
2.2 Extrinsic sensors
Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature inside aircraftjet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also 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.
fiber optic sensors provide excellent protection of measurement signals against
noise corruption. Unfortunately, many conventional sensors produce electrical
output which must be converted into an optical signal for use with fiber. For
example, in the case of a platinum resistance thermometer, 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 optical fiber via the usual
type of transmitter. This complicates the measurement process and means that
low-voltage power cables must be routed to the transducer.
sensors are used to measure vibration, rotation, displacement, velocity,
acceleration, torque, and twisting.
2.3 Phase Modulated FiberOptic Sensors:
sensitive fiber optic
sensing method is
based on the
optical phase modulation. The
total phase of the light along an optical fiber depends on the properties like
the physical length of the fiber, transverse geometrical dimension of the
guide, refractive index and the index profile of the waveguide. If we assume
that index profile remains constant with environmental variations, then the
depth of phase modulation depends on the other remaining parameters. The total
physical length of an optical fiber may be modulated by the perturbations like
thermal expansion, application of longitudinal strain and application of a
hydrostatic pressure causing expansion via Poisson's ratio. The refractive
index varies with temperature, pressure and longitudinal strain via photo
elastic effect. Waveguide dimensions vary with radial strain in a pressure
field, longitudinal strain in a pressure field and by thermal expansion. The
phase change occurring in an optical fiber is detected using optical
fiberInterferometric techniques that convert phase modulation into intensity
2.4 Displacementsensor (Extrinsic Sensor)
sent through a transmitting fiber and is made to fall on a moving target. The
reflected light from the target is sensed by a detector with respect to
intensity of light reflected and the displacement of the target is measured..
It consists of a bundle of transmitting fibers coupled to the laser source and a bundle of receiving fibers coupled to the detector.
The axis of the transmitting fiber and the receiving fiber with respect to the moving target can be adjusted to increase the sensitivity of the sensor.
from the source is transmitted through the transmitting fiber and is made to
fall on the moving target. The light reflected from the target is made to pass
through the receiving fiber and the same is detected by the detector.
the intensity of light received, the displacement of the target can be
measured, (i.e.) If the received intensity is more, then we can say that the
target is moving towards the sensor and if the intensity is less, we can say
that the target is moving away from the sensor.
Applications of Fiber Optic Sensors
optic sensors are used in several areas. Specifically:
of physical properties such as strain, displacement, temperature, pressure,
in structures of any shape or size.
the physical health of structures in real time.
and Bridges: Concrete monitoring during setting, crack (length, propagation
speed) monitoring, prestressing monitoring, spatial displacement measurement,
neutral axis evolution, long-term deformation (creep and shrinkage) monitoring,
concrete-steel interaction, and post-seismic damage evaluation.
Multipoint optical extensometers, convergence monitoring, shotcrete /
prefabricated vaults evaluation, and joints monitoring damage detection.
Foundation monitoring, joint expansion monitoring, spatial displacement
measurement, leakage monitoring, and distributed temperature monitoring.
Heritage structures: Displacement monitoring, crack opening analysis,
post-seismic damage evaluation, restoration monitoring, and old-new