Experimental investigation of bending properties of large mode area photonic crystal fibre with double lattice constant structure

Summary form only given. 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.In this paper we demonstrate by the first time the results of experimental investigation of bending properties carried out for the DLCF fabricated according to our design. The fibre was designed to provide low bending loss and single mode operation when bent around 10 cm. Due to the structural asymmetry our fibre has to be bent in a proper plane. This is however not an issue in a case of a short fibre that is meant to be used in a high-power laser where the fibre can be fixed after proper installation. Experiments confirm the exceptional bending features of our fibre. Although it is multimode when kept straight, the beam quality significantly improve when the fibre is bent. For the bent fibre length of only 16 cm the M2 reduces from over 3.3 to merely 1.2 (see Fig. 1), which confirms the extremely large loss of HOMs. At the same time, the measured value of the FM bending loss is as low as 0.8 dB/m.We have also investigated the influence of the bending plane deviation on the optical power transmitted by our fibre. The range of the bending orientation in which the power level does not decrease by more than 1 dB/m was measured to be approximately ±3.5°. Therefore the tolerance to the bending orientation deviations is large compared to the precision of commercially available fibre rotators which is close to 2°. The above together with the very large mode field area in straight (4800 μm2) and bent fibre (-670 μm2) shows that our fibre can be the candidate of choice for fabricating small footprint high-power lasers.