Optical fiber sensor development for health monitoring and diagnosis in smart structure applications

This dissertation describes the development of several optical fiber sensor systems for four main areas of interest in fiber optic smart structure applications, including cost-effective/high bandwidth/large dynamic range sensor systems, sensor multiplexing, absolute strain measurement, and low velocity impact-induced damage detection.; The sensor systems revolve around Fabry-Perot (FP) sensor configurations, and phase modulation and demodulation techniques. Intrinsic Fabry-Perot interferometric (IFPI), extrinsic Fabry-Perot interferometric (EFPI), and in-line fiber etalon (ILFE) sensors are all investigated. Extensive use is made of path-matched differential interferometry (PMDI) based FP sensors and FP read-out interferometers with PZT stack modulation because this configuration is well suited for demodulating ILFE sensors. The demodulation schemes investigated in this dissertation include synthetic heterodyne with a differential-cross-multiplier (DCM), pseudo-heterodyne, single channel phase tracker, and sin({dollar}Delta phi{dollar})/cos({dollar}Delta phi{dollar}) phase tracker. Two ILFE/EFPI sensor multiplexing techniques are described by using a combination of coherence division multiplexing (CDM) and frequency division multiplexing (FDM). These hybrid multiplexing techniques are accomplished by merging the PMDI and serrodyne techniques. The first multiplexing technique uses multiple different lengths of ILFEs and multiple respective FP read-out interferometers, and the second multiplexing technique uses multiple different lengths of low finesse FP sensors and a single high-finesse FP read-out interferometer. Then a concept similar to the second multiplexing technique is used for absolute strain measurements. In this case, the first and second optical return paths in a moderate to high finesse EFPI sensor are used to measure the absolute phase in the EFPI sensor. The impact detection sensor system development is described using the PMDI based ILFE sensors and two phase tracker techniques. Finally, a metal coated damage encoded ILFE sensor in composites is described. The relationship between impact-induced damage and sensor response is discussed using X-ray radiograph evaluation and the strain-load curve from the sensor and load cell.