Fabrication and characterisation of ytterbium:erbium codoped phosphosilicate fibres for optical amplifiers and lasers
This thesis deals with the fabrication, characterisation, and applications of ytterbium: erbium codoped fibres. The fibres are fabricated by modified chemical vapour deposition (MCVD) combined with the solution doping technique. The properties of phosphate glasses are reviewed and we show that the fabrication process must be adapted to cope with the low viscosity of the phosphate glass. We identify the three major pitfalls which initially plagued the fabrication process and show how they can be avoided. We also describe how all glass double-clad fibres can be fabricated. The composition of the core glass is measured by energy dispersive X-ray spectrometry and inductively coupled plasma mass spectrometry. The latter technique offers sufficient accuracy to evaluate erbium and ytterbium absorption cross-sections. Raman and infrared spectroscopy are also used to gain insight into the structure of the glass. Techniques to evaluate the suitability of our fibres for efficient devices are presented and some loss mechanisms are investigated. Addition of aluminium to Er/Yb codoped phosphosilicate fibres in the region [Al]<[P] is studied for the first time to our knowledge. We show that the ternary Al/P/Si system is not a simple juxtaposition of the binary Al/Si and P/Si systems. Nevertheless, we find evidence that the rare-earths remain essentially coupled to phosphate sites for the region [Al] < [P], although addition of aluminium still appears to reduce rare-earth clustering. Finally, we present a model of Er/Yb devices based on the formalism of the rate equations, and give examples of how it can be useful to predict the effect of modifications in fibre design. To conclude we present the devices which have benefited from this work, particularly single frequency grating feedback lasers and cladding pumped lasers.