Optical fiber long period grating sensors

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Optical fiber long period grating sensors

 

Author: Wong, Chi-too Jeffrey
Title: Optical fiber long period grating sensors
Degree: M.Sc.
Year: 2000
Subject: Optical fiber detectors
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Electrical Engineering
Pages: 116, [34] p. : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1517626
URI: http://theses.lib.polyu.edu.hk/handle/200/4024
Abstract: There has been a remarkable development of fiber optic sensor in recent years. Practical fiber sensors, ready and able to measure such common variables as pressure, temperature, strain, and torque, now focus on largely overlooked area: chemical application. They are namely, pH, humidity, gaseous... etc. Compared with other types of sensor, the advantages of fiber optic sensors are immunity to EMI, and the ability to work at high-temperature. Indeed, the main advantage of fiber optic sensors in chemical application is their remote sensing capabilities. The technology, often accompanied by a reduction in the price of associated components, have been adapted for use in a wide variety of optical fiber sensor systems. These include, for example, the use of photoinduced gratings as fiber sensor components, coupled with the wider availability of shorter wavelength lasers, bright luminescent sources and high-sensitivity detectors which have opened up new possibilities for both novel fiber optic sensor applications and new sensing systems. When coupled with integrated optic miniaturized devices and detectors, real possibilities or systems integration, at lower cost and increased utility, can be offered. The field of photoinduced gratings in optical fibers is one which has seen explosive growth in recent years and which features strongly in the series. Optical fiber sensors are discussed in some detail the underlying problems and issues involved in the use and implementation of such sensors in these varied and complex materials. In this report, novel sensors with the use of different operating principle are briefly discussed and demonstrated and commented with respect to the accuracy, manufacturing cost, operation cost, durability. In this report, optical fiber long-period grating sensors whose operation is based on the principle of evanescent wave coupling will also be investigated. Long-period grating which couples the light from the fundamental guided mode to forward propagating cladding mode. These modes decay rapidly as they propagate along the fiber axis owing to scattering losses at cladding-air interface and bends in the fiber. Any modulation of the core and cladding guiding properties modify the spectral response of long-period gratings, and this phenomenon will be utilized for sensing purposes. An experiment of examining the change of optical properties of an LPG to its surroundings reflective media are conducted and reported in the dissertation. The LPG fiber sensor was fabricated in the Hong Kong Polytechnic University with the use of excimer laser where details are described in the report. The capability of long-period grating fiber as an intrinsic optical fiber sensors for measuring temperature, strain and refractive index was also discussed.

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