Fiber optic sensor

An Overview Of Fiber Optic Sensor

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“An Overview of Fiber Optic Sensor”

Abstract:
The study of optical fiber sensors has a history of some 30 years. Different ideas have
been proposed and diverse techniques developed for various measures and applications.
Many of the techniques have found commercial success. Optical fiber sensors have
many advantages, such as immunity to electromagnetic interference, lightweight, small
size, high sensitivity, large bandwidth, ease in signal light transmission, and geometric
versatility in that fiber sensors can be configured into arbitrary shapes. However, optical
fiber sensors compete with other more mature technologies, such as electronic
measurements, in many fields of application. In some cases, optical fiber sensors should
compete with their sibling technologies, such as optical bulk sensors, for items such as
optical gyroscopes and optical current sensors. Despite these challenges, significant
progress has been made in the advancement of optical fiber sensors, and many of these
are now quite mature.

Introduction:
The fiber optic sensors are also called optical fiber sensors to use optical fiber or sensing
elements. These sensors are used to sense some quantities like temperature, pressure,
vibrations, displacements, rotations, or concentration of chemical species. Fibers have
so many uses in the field of remote sensing because they require no electrical power at
the remote location and they have tiny sizes.

Fiber optic sensors supreme in sensitive environments such as noise, high vibration,
extreme heat, wet, and unstable environments. These sensors can easily fit in small areas
and can be positioned correctly wherever flexible fibers are needed. A device, optical
frequency-domain reflectometry, can be used to calculate the wavelength shift. The time
delay of the fiber optic sensors can be decided using a device such as an optical time-
domain Reflectometer.

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Working principle:
Fiber optic sensors work on the principle that light from a laser or any super luminescent
source is transmitted through an optical fiber, experiences changes in its parameters due
to changes in the optical fiber or fiber Bragg gratings, and then reaches a detector that
measures these changes.
A fiber optic sensor system typically consists of a fiber-optic cable connected to a
remote sensor or amplifier. A glass or plastic core is surrounded by a layer of cladding
material in a fiber optic cable.
The difference in densities between the core and the layer allows the cables to operate
on the total internal reflection principle, which states that light striking a boundary
between two components is completely reflected with no loss of light energy. The
reflected light is then sent to a sensor or detector, which converts the light energy into
an electrical signal.
Types:-

1. Based on the sensor location, the fiber optic sensors are
classified into two types:

● Intrinsic Fiber-Optic Sensors
The physical parameter in intrinsic sensors changes some characteristics of the
sensed propagating light beam. In this case, the optical fiber itself serves as a
transducer, with only a simple source and detector being used.

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● Extrinsic Fiber-Optic Sensor
Extrinsic fiber sensors are those in which light signal modulation occurs outside
of the optical fiber and are delivered by optical fiber; light transmission is
dependent on the alignment of the fiber cores, i.e. the input and output fiber
cores. A light detector detects the light, and any deviation of the fiber pair from
perfect alignment is immediately detected by the detector.

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2. Based on operating principles, fiber optic sensors are classified into
three types:

● Intensity-based
More light is required for intensity-based fiber optic sensors, which use multi-
mode-large core fibers. The figure shows how light intensity can be used as a
sensing parameter and how this arrangement can make the fiber work as a
vibration sensor.


When there is a vibration, there is a change in light inserted from one end to the
other, and this causes the intelligence to measure the vibration amplitude. The
closer fiber optic and vibration sensors in the figure are affected by the light
intensity in later parts. Because of variable losses in the system that do not occur
in the environment, these sensors have many limitations. Variable losses include
splice losses, micro, and macro bending losses, joint connections, and so on.
Intensity-based sensors, micro bend sensors, and evanescent wave sensors are a
few examples. The advantages of these fiber optic sensors include low cost, the
ability to perform as real distributed sensors, ease of implementation, the ability
to be multiplexed, and so on.

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● Phase based
The external physical phenomenon to be measured in this case changes the phase
shift between two coherent propagating beams of different paths.
Interferometrically, the phase modulation is detected by comparing the phase of
the light in the signal fiber to that of a reference fiber. Light is split into two
beams in an interferometer, one of which is exposed to the sensing environment
and undergoes a phase change, and the other of which is separated from the
sensing environment and serves as a reference. As shown in Figure, when the
beams are recombined, they interact with each other.


The most popular interferometers are Mach-Zehnder, Michelson, Fabry-Perot,
Sagnac, polarimetric, and grating interferometers. The fiber must be mono mode,
or each mode will have a different phase shift due to its Eigen propagation
constant. The light source's coherence length must be larger than the optical path
difference between the two arms' maximum potential difference. The phase shift
will be linked to a change in the measurement arm's refractive index. This
refractive index modulation can occur in the optical fiber core or cladding as a
result of a variety of possible phenomena (thermal, mechanical, chemical,
electromagnetic, etc.).

● Polarization based
Polarization-based optical fibers are important for a particular class of sensors.
This property is easily modified by various external variables, allowing these
types of sensors to be used to measure a wide range of parameters. Special fibers
and other components with exact polarisation features have been developed.

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These are typically used in a wide range of measurement, communication, and
signal processing applications. Above is an optical setup for a polarization-based
fiber-optic sensor. It is formed by polarizing light from a light source using a
polarizer. The polarized light is started at a 45o angle to the chosen axes of a
length of birefringent polarization protecting fiber. This section of the fiber is
used for sensing. The phase difference between the two polarization states is then
changed in the presence of any external disturbances such as stress or strain. The
output polarization is then changed in response to external disturbances. External
disturbances can thus be detected by considering the output polarization state at
the next end of the fiber.

3. Based on application, fiber optic sensors are classified into three types
such as

● Chemical Sensor
A chemical sensor is a system that converts chemical information into a
measurable physical signal that corresponds to the concentration of a certain
chemical species. The chemical sensor is a crucial component of an analyzer,
and it may include some instruments that perform signal processing, sampling,
and data processing. An analyzer may be an important part of an automated
system. The working of the analyzer according to a sampling plan as a function
of time acts as a monitor. A receptor and a transducer are two functional units in
these sensors.

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The chemical information is converted into energy that the transducer can
measure in the receptor part. The chemical information is converted into an
analytical signal in the transducer, but there is no sensitivity. Used for pH
measurement, gas analysis, spectroscopic studies, etc.

● Physical Sensor
Used to measure physical properties like temperature, stress, etc. A physical
sensor is a system designed to detect physical effects and natural phenomena.
These sensors are used to provide information about a system's physical
property. Photoelectric sensors, piezoelectric sensors, metal resistance strain
sensors, and semiconductor piezo-resistive sensors are examples of this type of
sensor.

● Biomedical Sensor
Used in biomedical applications such as blood flow measurement, glucose
content measurement, and so on. A biomedical sensor is an electronic system
that converts non-electric quantities in biomedical fields into electrical quantities
that can be easily detected. As a result, these sensors are incorporated into
healthcare research.

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Advantages:
● Non-electrical
● Explosion-proof;
● Often do not require contact
● Small size and lightweight.
● Allow access to places that are usually inaccessible
● It can be easy to set up (EMI)
● Radiofrequency interference (RFI) and electromagnetic interference (EMI)
● Reliability in solid-state
● good accuracy
● Can be interfaced with data communication systems
Disadvantages:
● Variations in the intensity of the light and relative measurements.
● It is very costly.
● Detection systems may be complex.
● Since it is new to the consumer, it takes some practical training before they can
use it.
● It requires precise installation methods or procedures.
● Developing practical measurement systems with fiber optic sensors is difficult.

Applications
● Measuring physical properties like strain, displacement, temperature, pressure,
velocity, and acceleration in structures of any shape or size.
● Real-time monitoring of physical health.
● Buildings and Bridges: Concrete monitoring during setting, crack monitoring
(length and propagation speed), prestressing monitoring, spatial displacement
measurement, neutral axis evolution, long-term deformation (creep and

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shrinkage) monitoring, concrete-steel interaction, and post-seismic damage
evaluation.
● Tunnels: Multipoint optical extensometers, convergence monitoring, shotcrete
/ prefabricated vaults evaluation, and joint monitoring damage detection
● Dams: monitoring of the foundation, joint expansion monitoring, spatial
displacement measurement, and leakage monitoring, and distributed
temperature monitoring.
● Fiber optic sensors can be used in heritage structures to assess post-seismic
damage, analyze cracks, control restoration, and track displacement.
Conclusion:
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"). Fiber-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. Fiber
optic sensors can be used in heritage structures to assess post-seismic damage, analyze
cracks, control restoration, and track displacement.

References
1. Optical Fiber Sensors: Classification & Applications Priyanka Khandelwal
Assistant Professor JECRC UDML College of Engineering, Jaipur
2. Fiber optic sensors and their applications, Fidanboylu, K.a, *, and Efendioğlu,
H. S.b
3. Introduction to Fiber Optic Sensors and their Types with Applications
4. Fiber optic sensors ppt