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Sensors based on the Sagnac effect are used for angle measurement[53]. FBGs are very effective intrinsic sensors for measurement of strain or temperature as presented previously. Recently these FBGs have also been developed to measure co-located temperature and strain simultaneously with very high accuracy[]. Other intrinsic sensors were used as hydrophones distributed sensors for seismic, sonar applications[56] and as temper- ature and pressure sensors for oil industry[57]. Hydroph- ones are fiber sensors with more than one hundred sensors per fiber cable.

Depending on the arrangement of sensors in different arrays, hydrophone systems may be named as bottom-mounted or towed streamer systems etc[58]. These systems can work at very high temperatures suitable for semiconductors[56].

Some other intrinsic sensors were used as such as fiber optic gyroscopes for aerospace[59], land transit for navigation purposes and hydrogen sensors. The fiber only carries the light from the source to the sensing part, and from the sensing part to the demodulation system.

Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting[39]. Applications of extrinsic sensors: A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible with excellent protection of measurement signals against any noise[64].

There are many applications such as 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. Micro bend Sensors An intensity modulation multimode sensor, where the modulation caused during measurement of any property like pressure can be reflected in the form of a micro bend loss modulation, moving fiber modulation or an absorbing layer modulation.

The sensor comprises of a certain length of fiber which is placed between two rigid plates having an optimum corrugation profile so that the fiber experiences multiple bends. Due to the micro bending induced losses, the lower order guided modes are converted to higher order modes and are eventually lost by radiation into the outer layers resulting in a reduction of the optical intensity coming out of the fiber.

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A displacement of the plates due to measurable property causes a change in the amplitude of the bends and consequently an intensity modulated light emerges from the fiber core. Such sensors are apt for civil engineering structures especially in embeddable composites like reinforced concrete material where the reinforcing fibers in the composite structure act as natural bend loss sites for the optical fiber. In addition, the micro bend sensitive optical fiber can be jacketed in such a way that periodic micro bending is induced.

When the jacketed fiber is loaded, the overall effect leads to an increase in loss.

They were also used effectively in situ device for salinity measurements chloride detection of ocean surface[65]. The combination of multiple micro bend sensors can form a sensor array for the quasi-distributed sensing application in the monitoring of local strain or deformation along structures, and the OTDR can be conveniently used for interrogation of each sensor unit. The sensor sensitivity can be set at a specific value according to the requirements of the measurement condition. Connected with multiplexed sensing processing schemes, the sensor array may find an application in the real-time monitoring and damage detection of large and critical engineering structures[66].

Inferometric fiber sensor Most of the components of these sensors use either all-fiber or integrated optic material to provide better stability and compactness. An interferometric sensor works on the modulation in the phase of light emerging from a single mode fiber. The variation in phase is converted into an intensity shift using interferometric schemes Sagnac forms, ring resonators, Mach-Zehnder, Michelson, Fabry- Perot or dual mode, polarimetric, grating and Etalon based interferometers.

These sensors can be used as intrinsic or extrinsic sensors and distributed or quazi-distributed sensors depending on the type of applications. The applications are in acoustic e. All these schemes are used to split the light wave and then recombine at a photo-detector[70]. There are many applications of in several sensing systems[] such as sensing vibration signals[75]. Recently a long FBG based Michelson modal interferometer is studied as a sensing structure for measuring environmental refractive index, temperature, and liquid level by Caldas et al[76].

Some Selected Applications of Fiber Sensors The fiber sensors are used in past few decades and can be used in future in many fields, and their applications range from space to undersea. The first theoretical prediction of practical applicability of optical fibers occurred in , system experiments in , and economically practical system deployments in [77].

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In recent years it has become apparent that these light wave systems are steadily replacing copper wire as an appropriate means of communication signal transmission. The only basic difference is that the fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down the copper lines. They span the long distances between local phone systems as well as providing the backbone for many network systems. These systems are also useful in cable television services, university campuses, office buildings, industrial plants, and electric utility companies.

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Knowledge of the hot-spot temperature in a power transformer is a key element for its design and utilization. It enables manufacturers to refine their design, improve the quality and competitiveness, and users to utilize the full overload capacities of transformers and hence meet requirements of their customers without excessively reducing life expectancy of equipment[]. The measurement of hot-spot to allow predictive loading and enhance capacity utilization has increased giving rise to more and more cost- effective fiber-optic temperature measurement systems such as the fluorescent decay time technique has been developed.

These technologies offer an opportunity for direct hot-spot measurement without any detrimental effects to the dielectric integrity of the transformers. Additionally, the main source of power to industries or households or trains have to be monitored on real time basis, there safety especially during times of cyclone or earthquakes or tornadoes is crucial.


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Any physical problems in the transformers like dis-integraty, cracks in long range power transmission cables. Fractures in joints including direct buried cable, damages in cable tunnels, cracks in cable ducts or ancillary switchgear and other ancillary equipments can cause severe disruption failure in power transmission network to supply as well as reduce equipment lifetime.

Additionally, if there is over head network power cables , can result in sag or sudden short-circuiting. All types of sensors like extrinsic, intrinsic distributed and quazi distributed sensors are adoptable for aerospace structures depending on need, parts of craft and type of monitoring. For example, Lee et al[82] demonstrated the applicability of FBG sensor system for wings quite effectively. Networks of sensors mounted on commercial aircraft might one day check continuously for the formation of structural defects, possibly reducing or eliminating scheduled aircraft inspections[83] like nerve endings in a human body, in-situ sensors offer levels of vigilance and sensitivity to problems that periodic checkups cannot[19,84].

Adoption of effective sensing system can greatly reduce maintenance expenses for commercial aircraft. Those costs are rising as the aircraft age, many well beyond their design lifetimes. Ground crew technicians might plug a laptop or diagnostic station into a central port on the aircraft to download structural health data. Eventually "these sensing systems" fitted with many sensors could self-diagnose and signal an operator when repairs are needed.

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The potential locations on aircraft where the fiber sensors can be placed and diversified monitoring parameters can make aerospace a very challenging field for adaptation of fiber sensors. Thermal loads which act on these structures do not allow the use of standard fiber optic sensors used for classic avionics application. Latini et al[85] used a tunable laser source, to easily measure the spectral response of different fiber sensors intended to monitor thermal process.

This allows performing a multi-sensor interrogation and analyzing many physical parameters, such as: temperature, strain, pressure, etc[20]. They have resulted in a good response in terms of: sensitivity, resolution, repeatability and dynamic range of the measurement were obtained. The NASA contractors and research collaborators in United States are aggressively working to develop newer sensors, and it was reported that the latest fiber structure for use in NASA is of a size of human hair. In developed countries like United States, the authorized security navy uses fiber optics to secure ships in water.

This includes logistical trail stretches back from the battle or disaster to supply bases in the U. Most of those supplies travel by sea on ships under the Military Sealift Command. During a war, more than 95 percent of all equipment and supplies is carried by sea going vessels and thus better access control and perimeter security needs to be provided. The technology includes fiber optic modules from Weed Instrument Co[86].

Even in other countries, the fiber sensors for ship monitoring are increasing, as US type models are adopted in several countries. For the second factor, i. Recently in , embedded FBG technology was adopted for a Norwegian naval vessel by Wang et al for composite ship hull monitoring.

Advances in Biomedical Sensing, Measurements, Instrumentation and Systems

Signal processing is key for real-time structure monitoring, and they have presented the modular signal processing system effectively. The other sensors which can be used are BOTDR strain sensors, as they have better efficiency due to good resolutions in signals[88]. They can be used for defect identification by measuring strains and also for detecting changes in water levels outside the ship. However, inorder to monitor deep sea floor[90], two factors, i. There are two major technologies to understand real-time geophysical monitoring on a deep-sea floor. The tethered buoy-satellite system is one such technology.

Even if a giant buoy system with strong moorings is used to maintain its reliability during extremely rough weather, it may be not strong enough. Other factors such as antenna direction heading to a stationary satellite, communication method between ocean floor and a surface buoy, and power sources for bottom equipment, are also difficult tasks to be overcome.

During few decades, only a weather buoy, which took measurements at only the ocean surface, has been successful[]. Another approach is to use submarine cable systems. The submarine cable system has long technological history and has been proven in the field[93].

Although optical fiber submarine cables are one of the most advanced and reliable technologies, the use of such submarine cables is extremely costly compared to free-fall-pop-up systems. Another kind of submarine cable is an analog coaxial cable, which still provides electrical power and real-time data-telemetry similar to optical fiber systems.