Review of the present status of optical fiber sensors

This paper provides a thorough review of state-of-the-art current sensing techniques. It catalogues the current sensors according to the underlying physical principle in order to point out their strengths and weaknesses.

Current Sensing Techniques: A Review

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair.

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Interferometric Fiber Optic Sensors

An apparatus and method for diagnosis or treatment of a vessel or organ. The apparatus includes a deformable body such as a catheter having a tissue ablation end effector and an irrigation channel in fluid communication therewith. At least two sensors are disposed within a distal extremity of the deformable body, the sensors being responsive to a wave in a specified range of frequency to detect deformations resulting from a contact force applied to the distal extremity. A microprocessor can be operatively coupled with the sensors to receive outputs therefrom, the microprocessor being configured to resolve a multi-dimensional force vector corresponding to the contact force. In one embodiment, the sensors are fiber Bragg grating sensors, and the wave is injected into the fiber Bragg grating strain sensors from a laser diode.

Medical apparatus system having optical fiber load sensing capability

A robust interferometric fiber-optic current sensor with inherent temperature compensation of the Faraday effect is presented. Sensor configurations based on Sagnac and polarization-rotated reflection interferometers are considered. The sensing fiber is residing and thermally annealed in a coiled capillary of fused silica. The capillary is embedded in silicone within a ring-shaped housing. It is theoretically and experimentally shown that the temperature dependence of the birefringent fiber-optic phase retarders of the interferometers can be employed to balance the temperature dependence of the Faraday effect (0.7/spl times/10/sup -4///spl deg/C). Insensitivity of the sensor to temperature within 0.2% is demonstrated between -35/spl deg/C and 85/spl deg/C. The influence of the phase retarders on the linearity of the sensor is also addressed. Furthermore, the sensitivity to vibration of the two configurations at frequencies up to 500 Hz and accelerations up to 10 g is compared. High immunity of the reflective sensor to mechanical perturbations is verified.

Temperature and vibration insensitive fiber-optic current sensor

A highly accurate fiber-optic current sensor for direct currents up to 500 kA is presented. Applications include the control of the electrolysis process for the production of metals such as aluminum, copper, zinc, magnesium, and others. The sensor offers significant advantages with regard to performance and ease of use compared to state-of-the-art Hall-effect-based current transducers. The sensor makes use of the Faraday effect in an optical fiber loop around the current-carrying bus bars. A novel scheme of a polarization-rotated reflection interferometer and fiber gyroscope technology is used to measure the magneto-optic phase shifts. An appropriate technique has been developed for packaging the sensing fiber in a flexible strip of fiber-reinforced epoxy for loop diameters of up to several meters. Sensor accuracy and repeatability are well within 0.1% over a wide range of currents and temperatures. The sensor calibration is valid, regardless of the given magnetic field distribution, and remains stable under repeated manipulation of the flexible sensing strip.

Fiber-Optic Current Sensor for Electrowinning of Metals

A sensor for accurate and long-term fluid pressure monitoring in oil bore-holes based on optical fiber Bragg gratings (FBGs) is presented The sensor converts fluid pressure into optical fiber strain by means of a mechanical transducer designed to withstand downhole conditions The sensor response is investigated as a function of pressure and temperature Effects of shock and vibration are also considered The expected stability and lifetime of the polyimide-coated gratings in a downhole environment are estimated by modeling and extended accelerated mechanical and thermal aging tests

Reliability of fiber Bragg grating based sensors for downhole applications

The invention relates to a DFB fiber laser sensor (1). The inventive laser-amplifying fiber (2) has a non-rotational-symmetrical structure so that isotropic pressures p, acoustic waves or chemical substances that can be radially attached to the laser-amplifying fiber (2) can be detected.

Anisotropic distributed feedback fiber laser sensor

The invention relates to a wavelength-coded pressure sensor (1) with fibre-integrated Bragg grating, which is especially suitable for use at high pressures and temperatures in oil wells. The sensor principle provided for in the invention is based on the fact that the hydrostatic pressure of a liquid or gaseous medium (11) is converted by means of a transducer (1) into a longitudinal lengthening or compression of the fibre. The transducer (1) comprises a measurement or pressure cylinder (7a) which exchanges pressure with the medium (11), and a reference cylinder (7b) which is shielded from the medium (11) or subjected to a counter pressure. Temperature-compensated transducers (1) with a temperature-independent Bragg wavelength μB can be obtained because a suitable temperature-dependence of the mechanical prestressing of the pressure sensor fibre (3) is achieved through the selection of the materials, lengths and arrangements of the fibre holder support (5a, 5b). A fibre-integrated Bragg grating temperature sensor (19, 20) can also be provided. The invention further relates to transducers (1) with different cylinder arrangements (7a, 7b) and a wavelength multiplex configuration comprising several transducers (1).

Klaus Bohnert

Papers

Temperature and vibration insensitive fiber-optic current sensor

Fiber-Optic Current Sensor for Electrowinning of Metals

Reliability of fiber Bragg grating based sensors for downhole applications

Anisotropic distributed feedback fiber laser sensor

Pressure sensor with fibre-integrated bragg grating, comprising an integrated temperature sensor with fibre-integrated bragg grating