Abstract

Over the past few years, impressive power scaling of rare-earth-doped fibre lasers well above the kW-level has been demonstrated in the continuous-wave regime with high beam quality [1]. Excellent natural heat dissipation of fibres and small quantum defect of rare earths such as Yb lead to low thermal loading. Furthermore, innovative fibre designs are developed with the aim of enlarging mode field areas (2000 μm² in an Yb-doped fibre were reported [2]) to reduce nonlinear effects and increase the number of rare earth ions contributing to amplification. IN the so-called low- NA Large Mode Area Fibres, light always propagates thanks to Total Internal Reflection (TIR) or modified TIR. Consequently, these fibres exhibit some major drawbacks limiting their applications apart from the laboratory. Indeed, the very low refractive index difference between core and cladding (often ~10"⁴ or less) is responsible for high bending loss which can limit laser efficiency or considerably reduce the compactness of packaging. In addition, the refractive index profile (RIP) of the core region must be controlled with good accuracy as its value must remain close to that of silica. An efficient Yb doping implying an increase of the refractive index, the last condition limits the total amount of ions in the core and complicates fabrication processes. Nonlinear interaction length could be reduced to the length of the laser cavity length if a high refractive index rare-earth-doped large mode area core would be combined with singlemode emission. In this paper, we explore some new concepts of optical fibre designs exhibiting large active core surrounded by resonant cladding for high power delivery.

© 2007 IEEE