Measurement and extraction of the Giles parameters in Ytterbium-doped fibre
Thesis (MSc (Physics))--University of Stellenbosch, 2009.
The role fulfilled by theoretical models is rapidly increasing due to lasers becoming appli- cation driven to satisfy certain criteria and demands. Construction of high precision lasers requires good theoretical models and consequently good approximations of the parameters that such models are based upon. Despite the di erent model formalisms, most share a com- mon set of input parameters, including fibre waveguiding properties, input powers, transition cross-sections and overlaps between guided modes and the dopand distribution. Experimental and numerical work which was aimed at obtaining the wide-band emission and absorption cross-sections of fibre indirectly by means of the Giles parameters was done. The Giles parameters were used rather than the well known ionic cross-sections primarily because of the convenient encapsulation of the cumbersome overlap factors and the ionic cross-sections within the Giles parameters. The wide band spectral characteristics of the Giles parameters are indispensable in the design of fibre lasers and amplifiers, as they form the key parameters for laser models. These parameters are normally obtained utilizing absorption spectroscopy to obtain the absorption cross-sections and models such as the Fuchtbauer Ladenberg relation, the Mc- Cumber relation or uorescence spectroscopy to obtain the emission cross-sections. Recent research however indicates that these methods are inaccurate in certain spectral regions. An investigation was launched to extract the Giles parameters from measurements of the ampli- fied spontaneous emission (ASE) and pump absorption in ytterbium-doped fibre for several lengths of fibre and subsequent computer simulations, utilizing an ampli fier model. The Giles parameters are extracted with a fitting algorithm that adjusts the relevant numerical values to minimize the least square difference between the numerical data obtained from the amplifier model and the measured data. Using the model devised in this project on literature data, the Giles parameters were extracted and compared to the Giles parameters extracted in literature on the same data. This comparison conforms the extraction of the Giles parameters, utilizing the model devised in this project, as successful. Subsequently the model devised in this project was applied to extract the Giles parameters from experimental data measured at Stellenbosch, using a double cladding ytterbium-doped fibre. Finally a fibre laser was built utilizing the double cladding ytterbium-doped fibre and the output was measured. The Giles parameters extracted were then used in a fibre laser model to calculate the output and compare it to the measurements taken. This served as suffcient verification that the Giles parameters extracted can be used to model a fibre laser effciently.