Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to achieve a high spatial overlap with the core. This shaping is traditionally undertaken via diamond milling of the fibre preform into an octagon or hexagon, leaving a rough surface that is subsequently fire polished. We report on a new approach for shaping the inner-cladding using a CO2 laser to machine the fibre preform. This process is shown to allow fabrication of novel cladding structures, which include concave and convex surfaces, as well as a significant increase in the processing speeds and avoids the need for fire polishing prior to fibre drawing.
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