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30 March 2009 Temperature-independent Bragg grating-based sensor for monitoring regions of localised strain concentration
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A simple interrogation technique is presented which relies on a characteristic specific to saturated fibre Bragg gratings (i.e. gratings where most of the energy at the Bragg wavelength has been reflected prior to the incident light reaching the far end of the grating). In this regime, when the grating is illuminated by a broadband source a change in pitch within a region of the grating will result in the emergence of reflected energy in other spectral regions without significant loss in power from the main Bragg peak. Hence there will be an increase in the overall integrated power reflected from the grating, which is a function of the degree of strain gradient experienced by the grating. This allows the degree of strain gradient to be directly converted to an intensity measurement without the need for an optical filter. Because environmental temperature effects would generally not be localised along the short physical length of the grating, any temperature changes will typically shift the reflection spectrum in the wavelength domain rather than alter the amount of reflected light, which renders the measurement effectively temperature-insensitive. Experimental data is presented demonstrating the application of this sensing approach to the detection of growth of cracks in metallic structures and disbonds in composite repairs. Some of these experiments were carried out during environmental thermal cycling to demonstrate the temperature independence of the measurement technique.
© (2009) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Claire E. Davis, William G. A. Brown, and Paul R. Stoddart "Temperature-independent Bragg grating-based sensor for monitoring regions of localised strain concentration", Proc. SPIE 7292, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2009, 72923I (30 March 2009);

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