Differential fiber optical sensor with interference energy analyzer

This invention relates to fiber optical sensors and, more particularly, to fiber Bragg grating sensors used to measure temperature and strain changes. An area that has been investigated closely is the measurement of longitudinal strain and temperature. M. Xu, H. Gieger and J. P. Dakin in "Multi...

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Bibliographic Details
Main Authors Spirin, Vasilii V, Shlyagin, Mikhail G, Udd, Eric
Format Patent
LanguageEnglish
Published 21.09.2004
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Summary:This invention relates to fiber optical sensors and, more particularly, to fiber Bragg grating sensors used to measure temperature and strain changes. An area that has been investigated closely is the measurement of longitudinal strain and temperature. M. Xu, H. Gieger and J. P. Dakin in "Multiplexed Point and Stepwise Continuous Fiber Grating Based Sensors: Practical Sensor for Structural Monitoring?", Proceedings of SPIE, Vol. 2294, p. 94, 1994 described the usage of dual overlaid fiber gratings to measure strain and temperature. The major difficulty with this approach is that to obtain reasonable accuracy the wavelengths must be widely separated. Fiber etalon based strain sensors based on air cavities that are sold commercially by FISO and Luna offer relative immunity to temperature sensitivity but are difficult to multiplex into a single fiber line. A fiber optic sensor, which includes an interference energy analyzer, is used to measure strain and temperature distribution along a test fiber. The sensor includes the following: a plurality of double-Bragg grating elements positioned along a test fiber, a broadband light source which produces a broadband spectral profile that propagates along the test fiber, an optical filter that is able to change the parameters of the broadband spectral profile, an optical reflection detector, a fiber optic beamsplitter, and an interference energy analyzer. Each double-Bragg grating element consists of two weak Bragg gratings, separated by a distance unique to each element. The interference energy analyzer calculates the energies of the interference patterns, which are created by beams reflected from double-Bragg grating elements. The energy of the interference signal changes when the gratings in one element non-uniformly change its parameters due to non-equal temperature or strain influence on two gratings.