The radiated fields of the fundamental mode of photonic crystal fibers
  The six-fold rotational symmetry of photonic crystal fibers has important manifestations in the radiated fields in terms of i) a focusing phenomena at a finite distance from the end-facet and ii) the formation of low-intensity satellite peaks in the asymptotic far field. For our study, we employ a surface equivalence principle which allows us to rigorously calculate radiated fields starting from fully-vectorial simulations of the near field. Our simulations show that the focusing is maximal at a characteristic distance from the end-facet. For large-mode area fibers the typical distance is of the order 10 Lambda with Lambda being the pitch of the triangular air-hole lattice of the photonic crystal fiber. 
Ali Dabirian, Mahmood Akbari, and Niels Asger Mortensen