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Reconstruction of Guided Helical Bloch Modes in Twisted Coreless Photonic Crystal Fibre using Bessel Beams

Light beams carrying orbital angular momentum (OAM) are of fundamental interest in many fields of research [1]. Recently it has been reported that helical Bloch modes (HBMs) carrying multiple values of OAM are guided in chirally twisted photonic crystal fibre (PCF), even when there is no core [2]. HBMs are not supported by conventional waveguides and would be very difficult to realise using multi-beam interference. Here we report the first complete experimental characterisation of the phase and amplitude of the complex HBM fields in twisted coreless PCF. We compare the measurements with finite element simulations, based on precise scanning electron micrographs of the fibre structure, obtaining excellent agreement. We then expand the numerically calculated HBM fields as a series of Bessel beams, allowing us to calculate the strengths and azimuthal orders of the HBM harmonics with a fidelity greater than 95%.