Small-core photonic crystal fibres with weakly disordered air-hole cladding

Motivated by recent experimental work by Folkenberg et al (2003 Opt. Lett. 28 1882–4) we consider the effect of weak disorder in the air-hole lattice of small-core photonic crystal fibres. We find that the broken symmetry leads to higher-order modes which have generic intensity distributions resembling those found in standard fibres with elliptical cores. This explains why recently reported experimental higher-order mode profiles appear very different from those calculated numerically for ideal photonic crystal fibres with inversion and six-fold rotational symmetry. The splitting of the four higher-order modes into two groups fully correlates with the observation that these modes have different cut-offs.