Abstract

Recently it has been demonstrated that a continuously twisted photonic crystal fibre (PCF) with a "propeller-like" core, formed from three satellite subcores, supports a pair of non-degenerate helical Bloch modes whose dominant azimuthal harmonics have OAM order ℓ = ±1 [1]. Such a twisted system can be understood by realising that the guided light takes the form of a helical Bloch wave whose azimuthally periodic field pattern must spin with the structure. The non-degeneracy of the –1 and +1 OAM modes makes twisted PCF of particular interest because both the sign and magnitude of the OAM are preserved against external perturbations even over long fibre lengths. Here we report a more complex version of this structure that has six satellite subcores, and show experimentally that it supports four non-degenerate modes with OAM order ℓ = ±1 and ±2.

© 2015 IEEE