Cladding shaping of optical fibre preforms via CO2 laser machining

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Metadata	Details
Publication Date	2015-06-01
Authors	P. C. Shardlow, R.J. Standish, J. K. Sahu, W.A. Clarkson
Citations	9

Abstract

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. The laser processing set-up (shown in Fig.1) employs a 100W 10.6µm pulsed CO2 laser (Coherent G-100). The beam is passed through a periscope system before being directed vertically down through a focusing lens (f=100mm) onto the fibre preform surface. The preform itself is mounted within a rotation stage on an X-Y translation stage. The X stage translates the preform through the laser beam along its length at speeds of up to 300mm/s, with a slower Y stage utilised for repositioning for raster scanning along the surface. The laser is modulated at up to 3kHz repetition rates with varying pulse lengths from

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