Distributed fibre grating sensor for smart structures applications

Optical fibre sensors have been widely used to measure strain and temperature for smart structures applications. Distributed sensors using fibre Bragg gratings have previously been demonstrated. These sensors have many advantages, including high spatial resolution and no dead-zones, allowing a complete spatial image of a measurand to be obtained. Interrogation systems using a tunable narrowband source have been previously demonstrated, but are limited to monotonically varying wavelength profiles. Volanthen et al. (1996) demonstrated a broadband interrogation system for distributed grating sensors, capable of measuring arbitrary strain fields. Low-coherence interferometry selected the location under interrogation and a tunable filter measured the local wavelength. Further to previously published work, two improved systems are presented. The first system uses electronic processing for rapid and accurate wavelength measurement, whereas the second system uses a commercially available optical coherence domain reflectometer (OCDR) as part of the interrogation system. The latest sensing results from both systems are presented. The ultimate performance limits are examined experimentally and compared to the theoretically derived values. Both thermal and strain measurements have been made using single fibre gratings up to 40 cm in length. Multiple grating sensors have also been combined into a single sensing network, producing a sensor system that is both distributed and multiplexed.