Improvement of a Billouin scattering-based distributed fiber optic sensor.

摘要

Brillouin scattering-based fiber optic sensors are becoming commercially viable options for distributed sensing applications. One problem limiting the use of these types of sensors is the cross-sensitivity of Brillouin scattering-based sensors to temperature and strain. A major goal of this thesis was to solve this problem. Performance improvement of the Brillouin scattering sensor in general was sought.;A characterization of commercially available optical fibers towards these goals experimentally revealed that tailoring of Brillouin scattering properties, such as frequency, power, and strain and temperature sensitivity, are possible through geometrical and material fiber property design.;The use of dark pulses was found to improve spatial resolution without sacrificing frequency measurement accuracy. Stimulated Brillouin scattering limitations imposed by dark pulse technology were overcome through using concatenated fiber sections, where different sections had different natural Brillouin frequencies.;A commonly cited method used to separate temperature and strain is to use two fibers. It was found that this method can induce measurement errors, in part because the temperature coefficient of a bonded fiber differs from that of a loose fiber. It is also difficult in practice to guarantee a strain-free fiber. Measurement results are presented for a concrete beam and an overpass. These results are the first published results for a Brillouin system used to measure strain and temperature on an unreinforced hydrating concrete beam and in overpass backfill.;Tests were carried out on fibers glued to a series of metal bars, each having a different coefficient of thermal expansion and subjected to a range of temperatures. The tests provided an effective and low cost means of obtaining the strain, temperature, and intrinsic temperature coefficients for an optical fiber.;It was shown that using Raman and Brillouin scattering together allows the measurement of temperature and strain. A Raman scattering sensor was designed, built, calibrated and used with the Brillouin system.