Full-field characterization of helical Bloch modes guided in twisted coreless photonic crystal fiber

It was recently reported that a photonic crystal fiber (PCF) with no structural core guides light if a permanent chiral twist is introduced by spinning the fiber preform during the draw. The intriguing guidance mechanism behind this novel effect has many remarkable features; for example, it intrinsically supports circularly polarized helical Bloch modes (HBMs) that carry multiple optical vortices, making twisted PCFs of interest in fields such as optical micro-manipulation, imaging, quantum optics, and optical communications. Here we report for the first time, to the best of our knowledge, that a twisted coreless PCF supports not just one but a family of guided HBMs, each member of which has a unique transverse field distribution and harmonic spectrum. By making detailed interferometric measurements of the near-field phase and amplitude distributions of HBMs, and expanding them as a series of Bessel beams, we are able to extract the amplitude of each azimuthal and radial HBM harmonic. Good agreement is found with the numerical solutions of Maxwell's equations. The results shed light on the properties of this curious new optical phenomenon.