A Macro-bend Fiber Based Wavelength Demodulation System for Optical Fiber Sensing Applications

This thesis considers the design and development of a wide range all fiber interrogation system with a rapid measurement capability for extracting reliable measure and information from fiber Bragg grating sensors. Existing passive and active interrogation systems have certain limitations and do not satisfy all requirements. In this thesis a wavelength demodulation system is proposed, based on a macro-bend fiber filter employed in a ratiometric detection scheme. A theoretical model and analysis of an edge filter based ratiometric wavelength measurement (RWM) system is presented. A fiber edge filter is realized by carefully selecting an appropriate bend radius and number of bend turns of a standard single-mode fiber with an applied absorption layer. The calibration and characterization of the system are carried out experimentally and the effect of the signal-to-noise ratio (SNR) of the source is studied. It is shown that the SNR limits the measurable wavelength range of the system. A complete noise model for an edge filter based ratiometric system is developed and the noise analysis shows that increasing the slope of the edge filter is not necessarily a guaranteed approach to increasing the resolution of the system. The effect of polarization on the system is also investigated. The origins of the polarization dependency of the components, the macro-bend fiber filter and the 3 dB coupler, are identified and the effect of polarization dependent loss (PDL) on the RWM system with concatenated PDL components is investigated. To eliminate the polarization dependency, a novel twisted macro-bend fiber edge filter based system is proposed and demonstrated. As the bend loss in single-mode fiber is a function of temperature, an investigation to estimate the temperature induced instabilities in the system is carried out. The macro-bend fiber based interrogation system is used to extract information from an array of FBGs with different peak wavelengths using a best fit slope. A performance evaluation of the interrogation system is carried out and static strain, dynamic strain and temperature are measured. The investigation of the temperature dependence of the bend loss edge filter also led to the development of a new all-fiber temperature sensor, which is described, demonstrating a linear relation between the bend loss of a single-mode fiber and temperature in a sensing application for the first time