Development of high function in-fibre tilted gratings and their applications

Abstract

This thesis presents a detailed description of the fabrication process, spectral characteristics and applications of in-fibre gratings of normal structures, such as fibre Bragg gratings and long period gratings, and tilted structures of small, large and 45 angle tilted fibre gratings. All these in-fibre gratings were fabricated by UV-laser inscription in standard telecom single mode fibres. The key part of this research work is the fabrication and systematic spectral and sensitivity characterisation of the fibre Bragg gratings (FBGs) and long period gratings (LPGs). Their temperature sensitivities were compared for different wavelength ranges from near-IR to mid-IR. The LPGs, which have multiple transmission loss peaks were characterised for temperature and refractive index sensitivities for each transmission loss peak, obtaining the correlation between the cladding mode order and sensitivity. The results of this investigation have enabled to select the best LPGs for two bio sensing applications: (i) investigation of Foetal Bovine Degradation due to change in temperature and (ii) sensing different haemoglobin concentrations. The other major contribution of this Ph.D research is the systematic approach used in fabricating and characterising tilted fibre gratings (TFGs) with small, large and 45°angle tilted structures. All these types of TFG have been investigated in terms of inscription methods, spectral characteristics, polarisation properties and thermal responses. The three fabrication techniques used to inscribe TFG structures, two-beam holographic, phase-mask and amplitude-mask, have been fully discussed. The TFGs were subjected to various temperature sensing experiments to evaluate their responses and how the temperature change could affect their performance in real environment. In addition, for the small and large angle TFGs, their refractive index (RI) sensing characteristics have been investigated to show their unique RI sensing capability to the surrounding medium. And due to the unique polarisation property of large angle TFG, it was employed in an all fibre twist sensor. Finally, a chemical sensing application was evaluated using a pair of large angle TFGs forming a high sensitivity interferometer. Based on their unique optical properties, a power tapper working at 800nm wavelength region has been demonstrated using 45°-TFGs. The in-fibre tapper system was characterised for its dispersion, side-tapped beam width and side-tapped power variation along the grating length. This system was then used as a temperature sensor, showing side-tapping functionality. Finally, as another major contribution, the 800nm 45°-TFG combined with CCD array were developed into an optical fibre signal interrogation system and evaluated for FBG temperature sensing, which clearly demonstrated the design concept of an in-fibre spectrometer of low cost, compact structure and high function. In collaboration with Bern University of Applied Sciences, Bangor University and Jiangnan University, the 800nm 45°-TFG was first used to develop an OCT system for bio sensing applications.