Performance analysis and design of fibre gratings for sensing applications

Abstract

The role of optical technologies for non-contact type of measurements in the field of instrumentation and sensing is revolutionary. The exceptional sensitivity of fiber towards external perturbation has made research world extremely benefited. This property is explored to construct a large variety of instruments and sensors. This thesis also deals with the performance analysis and design of fiber gratings for sensing applications. Long Period Gratings (LPGs) are proposed as simple yet versatile optical fiber sensors. In case of LPG, periodic modulation of refractive index within core region is written in such a way that light propagating in the core can interact with cladding and outside thereby yielding an output spectrum that includes changes due to all external parameters to which the fiber may be subjected to. The phase matching condition causes light from the fundamental mode to couple with discrete forward-propagating cladding modes. These cladding modes attenuate rapidly on propagation and result in loss bands at discrete wavelengths in the grating transmission spectrum. One of the aims of this work has been to develop an analytical model to predict the location of resonance bands as function of grating periods as well as core and cladding refractive indices. The transmission characteristics of uniform LPGs are simulated theoretically by using coupled-mode theory. Studies undertaken in the thesis include review of number of LPG fabrication methods and understanding the mechanism of photosensitivity in germanosilicate fibers using different reported models. The fabrication of these gratings has been done using TeraXian grating writing station. In this work, uniform LPGs are fabricated by using a line-narrowed KrF excimer laser adopting point by point method. The fibers used in experiments are photosensitive optical fibers (Boron co-doped) and gratings can be written directly into these fibers without hydrogenation.