Spectral dips in t-PCF with single core

This topological effect makes it possible for example to phase-match light guided in a central solid glass core (modal index nc) to the fundamental space-filling mode in the cladding (phase index nSM in the untwisted fibre) with the result that light can leak out into cladding modes at certain wavelengths. This results in a series of dips in the transmission spectrum, caused by anti-crossings between the core mode and leaky ring-shaped cladding modes carrying orbital angular momentum (OAM), each dip corresponding to a different OAM order [Wong (2012)]. Since the cladding light is diverted by the hollow channels into a spiral path, the azimuthal component of its wavevector must take values that yield a round-trip phase advance that is an integer multiple  of 2π, where  is the OAM order. This leads to a resonance condition that yields remarkably good agreement with experimental measurements, showing in particular that the dip wavelengths scale linearly with the twist rate. We have used the twist and strain sensitivity of these dips to construct an all-optical twist-strain transducer [Xi (2013)].

(a) Transmission spectrum in a solid-core PCF with a twist rate of 10.8 rad/mm. Experiment and numerical modelling agree quite well. (b) Calculated field intensity profiles of the coupled core-cladding modes at two of the spectral dips, showing the ring-shaped helical Bloch modes in the cladding.