Near-field to far-field transition of photonic crystal fibers:   symmetries and interference phenomena
  The transition from the near to the far field of the fundamental mode radiating out of a photonic crystal fiber is investigated experimentally and theoretically. It is observed that the hexagonal shape of the near field rotates two times by pi/6 when moving into the far field, and eventually six satellites form around a nearly gaussian far-field pattern. A semi-empirical model is proposed, based on describing the near field as a sum of seven gaussian distributions, which qualitatively explains all the observed phenomena and quantitatively predicts the relative intensity of the six satellites in the far field. 
Niels Asger Mortensen and Jacob Riis Folkenberg (Crystal Fibre A/S,   Denmark)