Long range extensometer for civil structure monitoring using fibre Bragg gratings

We report on an extensometer based on fibre Bragg gratings (FBG) which offers a relatively large and high dynamic range for the monitoring of the relative displacement between two adjacent buildings with individual foundations located in an earthquake-prone area. The sensor consists of a sensing FBG subjected to strain and a reference FBG. The fibre containing the strain-sensing FBG is directly surface mounted onto the walls of both buildings, with the fixations ?1 m apart. A large range of ±20 mm is achieved by desensitization through a tilt of the fibre axis with respect to the displacement vector (?6° away from the normal). The sensor is illuminated by a broadband source (SLD with ?0 ? 815 nm) and the reflected signal is monitored directly by a CCD-based optical spectrum analyser. The recorded spectra are curve-fitted to two Gaussian distribution functions on a PC. A precision of 36 ?m displacement with an integration time of 3 ms has been achieved