Innovative Characterisation Techniques and Methods for Developing Ultra-Broadband Bi/Er Doped Optical Fibres

The unavailability of ultra-broadband optical fibre amplifier has restricted further development of optical communication networks. Aiming at ultra- broadband optical fibre amplifier to meet the demand for greater capacity, Bi/Er doped fibre (BEDF) is being developed as a promising optical gain medium with ultra-broadband emission generated from multiple active centres.However, these active centres generate complex emissions that they overlap in both emission and excitation bands and that consequently make the existing techniques and methods inadequate for characterising BEDFs. Thus the properties and contributions of these active centres have not been properly evaluated yet.In my PhD research, I developed novel characterisation methods and obtained significant results for developing BEDFs with desirable spectral properties. My main contributions include: 1. Fabrication of BEDFs doped with Bi, Er, Al, P and Ge by using MCVD and {\it in-situ} solution doping techniques. The fabricated fibres had ultra-broadband emission from 920 to 1700 nm, which covers the low-loss communication window of silica fibre. 2. Development of a novel convolution based fluorescence lifetime (FLT) measurement method for single and multiple FLTs. The method is able to achieve accurate microsecond FLT resolution, using the spectroscopic setup with only millisecond resolution. This method has been satisfactorily used to characterise BEDFs with multiple FLTs. 3. Development of a systematic approach to characterise spectral properties and contributions of individual active centres for cases with complex and overlapped emission bands. This approach utilises conventional experimental setups to measure and decompose FLT range, emission spectrum and absorption spectrum. This enables us to investigate key properties of individual active centre, including emission spectrum, emission contribution, FLT, absorption spectrum, emission efficiency, emission cross section and energy level diagram to acquire essential information and evaluate fibre emission performance. 4. Identification of properties and contributions of bismuth active centre (BAC) in BEDFs. Key information on both fibre compositions and spectral properties of the interested active centres including BAC-Al, BAC-Ge, BAC-P and BAC-Si and Er, was obtained. This is essential for assessment and optimisation of fibre composition and fabrication processes for developing ultra-broadband BEDFs.In summary, this research paves the way towards the development of advanced characterisation methods and obtains comprehensive spectral properties of BACs in BEDFs, as promising optical gain media for ultra-broadband applications