Design and realisation of leakage channel fibres by the powder-in-tube method

The applications of fibre lasers demand for increasing power. Limits are set by various nonlinear effects. Leakage channel fibres (LCF) are one approach to this problem. With this type of fibre, most nonlinear effects can, in principle, be mitigated simultaneously by increasing the mode field area and by maintaining the single mode regime. For its implementation, we propose to use the powder-in-tube preform technique. While the microstructure consists of commercial pure silica rods, the surrounding is filled with index-raised aluminum-doped silica oxide granulate. For the fabrication of the latter, we tested two different methods. For the first one, the oxide precursors were mixed in pure powder form. In the other method, the material was produced with the helps of the sol-gel process, where the mixing takes place in liquid phase, thus resulting in an expected improved homogeneity. Prior to the fabrication of a prototype, their feasibility has been tested with the help of a finite-difference method simulation tool (Lumerical MODE Solutions). Two such fibres have been fabricated according to this results. The influence of the granulate mixing method and of the grain size on the homogeneity in refractive index has been tested. Although the produced fibres do not yet show the desired performance, the produced prototypes prove that LCFs can indeed be realised with this approach.