Abstract: Methods, and corresponding apparatus, for detecting the occurrence of cracks or fractures are described and claimed. The methods utilize Bragg gratings, and the occurrence of a crack is detected by detecting a change in the reflectance and/or transmittance characteristics of the Bragg grating when the crack is transmitted to it. In addition to merely detecting that a crack has occurred, the methods may also provide information on the position of the fracture along the length of the Bragg grating. This may be achieved by comparing the peak intensities and/or widths of the optical reflectance spectra of the unbroken and broken grating. In certain preferred embodiments the Bragg gratings are interrogated using the combination of a broad band source and an optical spectrum analyser, or a tunable laser source and a broad band detector.

Abstract: A sensor system comprises first and second fibre bragg gratings (FBGs). The two fibre bragg gratings receive light from a common light source and produce respective measurement signals having a spectrum dependent on a measurement signals having a spectrum dependent on a measurand, and a common reference signal. The two measurement signals pass through respective edge filters to respective detectors and a third detector monitors the reference intensity. A processor produces a value for the measurand from the received and detected signals. The system can be used in a temperature compensated strain gauge.

Abstract: Strain sensors, and in particular strain rosettes, are described and claimed, in which the strain sensing elements are fiber Bragg gratings. Strain sensors in accordance with the described invention comprise first and second fiber Bragg gratings, and a length of optical fiber connecting the gratings in series, each grating being substantially straight and the two gratings being non-parallel and having different nominal Bragg wavelengths, the connecting length of optical fiber having a bend, characterised in that a portion of the connecting length is tapered, the tapered portion including an elongate waist portion having a reduced cross sectional area and said bend is formed in the waist portion.