Abstract

The progress in high power fibre lasers is mainly due to the improvement of the fibre geometry. High power fibre lasers rely mostly on fibres with large mode area (LMA) and very low numerical aperture (NA) [1]. This allows achieving the high power output beam with excellent beam quality. However such fibres only weakly confine light, and thus they are very sensitive to the bending. Due to the large bending loss, these fibres have to be kept straight when used in lasers, which is detrimental to the compactness of the laser system and limits their potential applications. To allow bending of LMA fibres we have demonstrated the concept of a low bending loss LMA photonic crystal fibre (PCF) with asymmetric air hole structure in the cross-section of the microstructured cladding [2]. The structure consists of two regions with different filling factor d/Λ. The region with the large d/Λ factor yields low bending loss for the fundamental mode (FM) whereas the region with the small d/Λ factor enlarges the bending loss of higher order modes (HOMs). The region with high filling factor can be obtained either by employing air holes with enlarged diameter [2] or by using a decreased lattice constant [3]. Our simulations revealed that the double lattice constant fibre (DLCF) provides a superior performance over the fibre with different air hole diameters and the main advantages of this solution are the larger mode field area (MFA) in straight fibre, the larger bending loss of HOMs and the better tolerance to manufacturing errors.

© 2013 IEEE