Interferometric fiber-optic sensing based on the modulation of group delay and first order dispersion: application to strain-temperature measurand

We describe an extension of low coherence interferometry which uses the complete interferogram rather than just the interferogram envelope. We measure the phase across the entire source spectrum using dispersive Fourier transform spectroscopy, and obtain group delay and dispersion from the phase curve. We apply the technique to a simultaneous quasi-static strain-temperature measurand in which strain and temperature are recovered from the modulation of group delay and first-order dispersion. We show these parameters have a negligible cross-term, yield a particularly robust transformation to strain and temperature, are immune to 2 π phase ambiguity and insensitive to fiber geometry. With an ultra broadband thermal source we achieved simultaneous resolution of strain acid temperature to accuracies of 5 μstrain.m and 0.35 K over strain and temperature ranges of 1200 μstrain.m and 22 K. We present detail of a diffraction grating-coupled multiple SELED source and its operation in optical fiber DFTS. With this source we achieved resolutions of <1 μstrain in single parameter measurement of strain and a temperature insensitive measurement of strain to within 15 μstrain.m over a strain range of 1200 μstrain.m in the presence of a temperature fluctuation of 20 K