Development of fibre interferometer sensors based on double cladding fibre for multi-parameter sensing

Abstract

The role of sensors in the future industrial environment is becoming more crucial and complex. One of the important sensor characteristics is the multi-parameter sensing capability. Fibre interferometer is a type of optical sensor that has been proven for its excellent sensing performance, high design flexibility and high capability for multi-parameter sensing. This research work assesses the potential use of double cladding fibre (DCF) as an interferometric multi-parameter sensor. In light of the importance of multi-parameter sensing capability required in future industry environment, three novel sensor designs were proposed and experimentally demonstrated. These proposed designs incorporated DCF as the main sensor structure to maintain high commonality, as well as to fully utilize the unique sensing properties of DCF. The first design proposed in this study is the fibre Michelson interferometer based on DCF, which is used for refractive index (RI) and temperature sensing. This sensor operates based on two sensing mechanisms to detect RI and temperature. RI sensing relies on Fresnel reflection at the tip of DCF, whereby RI change is quantified from power change in the sensor spectrum. Meanwhile, temperature sensing depends on the interference between the core mode and the first cladding modes of DCF. Thermo-optic effect causes a change of wavelength in the sensor spectrum. The experimental results retrieved from the proposed sensor revealed that temperature and RI spectra responses were indeed distinguishable. The second design proposed in this study is the Mach-Zehnder interferometer with dual sensing points used for RI and discrete liquid level sensing. These two sections are separated by an RI insensitive region formed by the DCF section. The sensor can be utilised to measure RI in single- or dual-point configuration. The third design proposed in this study is the DCF-based Mach-Zehnder used for small curvature (or displacement) and large curvature (or circumference) sensing. In this proposed design, two optical paths are paved in the core and in the inner cladding of the DCF. The outer cladding of DCF provides confinement of light in the inner cladding, hence enabling higher curvature to be imposed without any significant optical loss. This research work covers conceptual sensor designs, sensor fabrication and experimentation work. At conceptual level, mathematical models of particular sensor structures were studied and further developed in order to understand the sensor behaviour. The particular sensor structures were analysed numerically using BeamProp software to understand its function from the field distribution. Systematic fabrication procedures were developed for the sensor to ensure high process efficiency and repeatability. Additionally, this thesis contributes to the development of experimentation setup and data acquisition process. The proposed multi-parameter sensors have great potential to be deployed in various industrial applications.