Abstract

High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic “all-fibre” architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. [1]. The gain fiber utilizes a low index polymer coating to provide the wave-guiding for the multimode pump as well as for compatibility with the NA increase (typically 0.22-0.45) which occurs in the fused taper combiners. While this all fibre approach has been shown to be viable, it is not trivial to implement at power levels in excess of several hundred watts Issues include polymer coating degradation, transverse mode-coupling induced instability at splices or FBG’s, grating walk-off, and modal instability, [2]. The latter issue arises because these fiber laser designs are focussed on single-transverse-mode operation, [3,4], even though the fibres themselves are multimode to avoid nonlinear impairments. This is despite the fact that most cutting and welding applications actually utilize a multimode fibre for delivery to the cutting head. The BBP of such systems is typically 2.5mm.mRad at a wavelength around 1080nm. However, single mode operation allows power scaling by incoherently combining several lower power fiber lasers into a single beam with that BPP.

© 2011 Optical Society of America