A fiber optic sensor
is a sensor
that uses optical fiber either as the sensing element ("intrinsic
sensors"), or as a means of relaying signals from a remote sensor to
the electronics that process the signals ("extrinsic sensors"). 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 different wavelengths
of light 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.
Intrinsic
sensors
Optical fibers can be
used
as sensors to measure strain, temperature, pressure and other
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 large
distances.
Temperature can be measured by using a fiber that has evanescent loss
that varies with temperature, or by analyzing the Raman 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.
Special fibers like long-period fiber grating (LPG) optical fibers can
be used for direction recognition[1]. Photonics Research Group of
Aston University in UK has some publications on vectorial bend sensor
applications.[2][3]
Optical 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. Both
bottom – mounted hydrophone arrays and towed
streamer systems are in use.
The German company Sennheiser developed a laser microphone for use with
optical fibers.
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.
Optical fiber sensors for temperature and pressure have been developed
for downhole measurement in oil wells.[4][5] The fiber optic sensor is
well suited for this environment as it functions at temperatures too
high for semiconductor sensors (distributed temperature sensing).
Optical 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.
Fiber – optic sensors have been developed to
measure co-located
temperature and strain simultaneously with very high accuracy using
fiber Bragg gratings.[6] 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).[7]
Other
examples
A 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.[8] The measurement technique is based on
polarimetric detection and high accuracy is achieved in a hostile
industrial environment.
High 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 material.
Electrical power can be measured in a fiber by using a structured bulk
fiber ampere sensor coupled with proper signal processing in a
polarimetric detection scheme. Experiments have been carried out in
support of the technique.[9]
Fiber-optic 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.[10]
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 aircraft jet 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.
Extrinsic 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.
Extrinsic sensors are used to measure vibration, rotation,
displacement, velocity, acceleration, torque, and twisting.
References:
- «Bend
Sensors with Direction Recognition Based on
Long-Period Gratings Written in D –
Shaped Fiber by D. Zhao
etc».
- «Implementation
of vectorial bend sensors using
long-period gratings UV-inscribed in special shape fibres».
- «Use
of Dual-Grating Sensors Formed by Different
Types of Fiber Bragg Gratings for Simultaneous Temperature and Strain
Measurements».
- Sensornet. «Upstream
oil & gas case
study» (pdf).
Процитовано 2008 – 12 – 19.
- Schlumberger. «Wellwatcher
DTS Fibre Optic
Monitoring product sheet» (pdf).
Процитовано 2010-09-22.
- Trpkovski
S., Wade, S.
A.; Baxter, G. W.; Collins, S. F.. Dual
temperature and strain sensor using a combined fiber Bragg grating
and fluorescence intensity ratio technique in Er3 doped fiber//
Review
of Scientific Instruments. –
74. – (2003) (5).
DOI:10.1063/1.1569406. Переглянуто: 2008-07-04.
- Measures,
Raymond M.
Structural
Monitoring with Fiber Optic
Technology. – С.
Chapter 7. – San Diego,
California, USA : Academic Press, 2001. ISBN
0 –12 – 487430 – 4.
- Ghosh
S.K., Sarkar,
S.K.; Chakraborty, S.. Design and
development of a fiber optic intrinsic voltage sensor// Proceedings of
the 12th IMEKO TC4 international symposium Part 2. – (2002):
415–419.
- Ghosh
S.K., Sarkar,
S.K.; Chakraborty, S.. A proposal for
single mode fiber optic watt measurement scheme// Journal of Optics
(Calcutta). – 35. – (2006) (2): 118 – 124.
- Zeller,
M.; Scheer, G.
(2008). «Add
Trip Security
to Arc – Flash Detection
for Safety and Reliability, Proceedings of the
35rd Annual Western Protective Relay Conference, Spokane, WA»
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