Optical fiber sensor for simultaneous measurement of distributed strain and temperature

Optical fiber sensors play an important role in health monitoring of space shuttles, aviation vehicles, and civil structures to issue early warnings. In this dissertation, a novel fiber sensor configuration using first and second order fiber Bragg gratings, coupled with Fourier demodulation technique, is investigated for simultaneously measuring strain and temperature. A detailed theoretical analysis for measurement resolution with regard to the sensor system matrix and the wavelength detection resolution is carried out for six different fiber grating based sensor configurations. Experimental methods to induce first and second order Bragg resonances in regular fiber gratings in single mode fibers are investigated. Far field (Fraunhofer field) diffraction intensity pattern in phase masks (dielectric surface relief gratings) is studied using simple plane wave electro-magnetic theory and the results are compared with the experimental measurements. A Fourier transform Bragg demodulator (FTBD) is built for simultaneously interrogating fiber gratings at 780 nm and 1550 nm wavelength ranges. The demodulator sensitivity and the grating sensor multiplexibility are investigated in terms of constraints arising from optical detectors, optical loss, and the spectral shape of the fiber gratings and mirrors. Finally, the measurement resolution of the whole sensor system is examined using simulated sensor data under two different laser tuning schemes; a single continuous sweep of the entire grating spectra and a multiple successive sweeps of the entire grating spectra. It is shown that the strain and temperature can be measured with a resolution of ±150 × 10−6 and ±25°C using first and second order fiber grating (1550 nm/780 nm) sensors bonded to a 30.48 cm x 2.54 cm x 2.3 mm aluminum beam and interrogated by a FTBD system with single laser sweep.