Improved large-mode area endlessly single-mode photonic crystal fibers
  We numerically study the possibilities for improved large-mode area endlessly single mode photonic crystal fibers for use in high-power delivery applications. By carefully choosing the optimal hole diameter we find that a triangular core formed by three missing neighboring air holes considerably improves the mode area and loss properties compared to the case with a core formed by one missing air hole. In a realized fiber we demonstrate an enhancement of the mode area by ~30 % without a corresponding increase in the attenuation. 
N.A. Mortensen, M.D. Nielsen, J.R. Folkenberg, A. Petersson, and H.R.   Simonsen (Crystal Fibre A/S)