Novel technique for fabrication of large core, high rare earth concentration, photodarkening resistant fibres for high power fibre laser applications

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Research into high power fibre lasers is being driven by material processing and military applications and over recent years there has been much interest in scaling output powers from cladding pumped fibre lasers and amplifiers. While kW power levels with diffraction limited output have been demonstrated from a single core fibre the drive is now towards multi kW powers with high beam quality. To achieve this it is important to maximize both the core area and the rare earth doping concentration whilst maintaining robust single mode operation and good optical to optical conversion efficiency. These features are also needed in order to avoid the need for excessively long fibres for efficient pump absorption, and to raise the threshold for unwanted nonlinear processes and optical damage. The proposed research programme will develop a novel process for incorporating rare earth and aluminium dopants into fibre performs in a simple and cost effective way. The technique will allow higher concentrations of dopants with increased uniformity along the length of the preform, increased core size, reduced background loss, and no phodarkening in fibres. The use of lanthanide chelates to incorporate rare earths in preforms has been demonstrated previously. However, the previously reported techniques require complicated, bulky and expensive heated bubblers and gas lines. The proposed technique would simplify things greatly by placing the chelates in independently heated crucibles within the glassware on the lathe, in close proximity to the deposition zone thereby increasing control and versatility.It is also important that these dopants can be incorporated into large volumes of glass to create large core structures and to obtain better fibre yields, whilst also reducing the number of steps required to make multiple core fibre for high power fibre lasers and to facilitate the production of the doped capillaries required for various classes of microstructures fibre including: holey fibres, hollow core fibre, band gap fibres and ribbon fibres incorporating multiple doped core arrays.