Fibre Optic Sensors Interrogation by Interferometric Illumination

With the conventional interrogation methods, multiple fibre Bragg gratings (FBG) with similar wavelengths cannot be interrogated simultaneously and, at the most one tapered sensor (TS) can be interrogated in a single scan. This drawback is overcome by interferometric illumination of optical sensors and by interrogating the sensor modulated interferograms. The capability and the efficacy of the different optical fibre sensor classes’ interrogation by interferometric illumination and coherence filtering is investigated in this thesis using an all-fibre interferometer. The use of dedicated photodiodes for the interferometer’s measurement arms ensures independent processing of the interferograms from the different sensors and sensor classes without spectral overlap. When interrogating multiple arrays of multiple FBGs by interferometric illumination, the sensors subjected to a temperature test demonstrated a ~9pm/0C temperature response, typical of an FBG. The experiment also demonstrated the interrogation of FBGs with similar wavelengths across multiple arrays without spectral overlap. Interferometric interrogation of the TS provides for measurements using both the temporal and spectral fringes. Measurements from processing of the sensor’s temporal fringes were found to have a higher accuracy than the ones obtained by processing of the spectral fringes as in the case of an optical spectrum analyser (OSA). Multiple tapered sensors used for sensing glucose solution concentration changes yielded sensitivity values of ~(3.0±1.4)e-7/ppm and ~(5.2±2.3)e-7/ppm, demonstrating the capability and efficacy of the system to interrogate multiple tapered sensors simultaneously even under conditions of reduced spectral fringe visibility unlike an OSA, which uses only the spectral fringes for measurements. Simultaneous interrogation of multiple FBGs and a TS without spectral overlap demonstrated the multi-class sensor interrogation capability of the interferometric illumination scheme. The TS’s response to ethanol evaporation and an FBG’s temperature response were found to be in concurrence with the respective sensor’s typical behaviour.