Interferometric visualization and demodulation method for measuring quasi-static strain in fiber Bragg grating sensors by a simple rotating etalon filter

Year: 2004

Authors: Rocco A., Coppola G., Ferraro P., Foti G., Iodice M.

Autors Affiliation: Istituto Nazionale di Ottica Applicata, Sezione di Napoli, Via Campi Flegrei 34 80078 Napoli Italy;
Istituto per la Microelettronica e i Microsistemi (CNR), Via P. Castellino 111, 80131 Napoli, Italy;
Univ. degli Studi Mediterranea di Reggio Calabria, Italy

Abstract: Optical fiber sensors are the ideal system to monitor “smart structures” and on-site/real time stress measurements: they can be in fact easily embedded or attached to the structures under test and are not affected by electro- magnetic noise. In particular a signal from a Fiber Bragg grating sensor (FBG) may be processed such that its information remains immune to optical power fluctuations. Different interrogation methods can be used for reading out Bragg wavelength shifts. In this paper we propose a very simple interferometric method for interrogating FBG sensors, based on bi-polished silicon sample acting like an etalon tuneable filter (ETF). The Bragg wavelength shift can be evaluated by analyzing the spectral response of signal reflected by the FBG sensor and filtered by the ETF that is continuously and rapidly tuned. Tuning was obtained by rotating the ETF. Variation in the strain at the FBG causes a phase shift in the analyzed signal. The overall spectral signal, collected with time, consists in an interferometric figure which finesse and fringe contrast depending on the geometrical sizes and facets reflectivity of the silicon sample. The fringe pattern, expressed by the Airy’s formula, depends on the wavelength λ of the incident radiation and on the angle of incidence. The phase of fringe pattern can be retrieved by a standard FFT method giving quantitative measurements of the quasi-static strain variation sensed by the FBG. In this way, the method allows a valuable visualization of the time-evolution of the incremental strain applied to the FBG.
Principle of functioning of this method is described and first results obtained employing such configuration, are reported.

Journal/Review: PROCEEDINGS OF SPIE

Volume: 5459      Pages from: 128  to: 136

KeyWords: interferometry; sensors; Fiber Bragg Gratings
DOI: 10.1117/12.545483

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