Twist-tuned suppression of higher-order modes in single-ring hollow-core photonic crystal fibers

TL;DR

This paper demonstrates that spinning the preform during fiber drawing introduces a twist that enhances higher-order mode suppression in single-ring hollow-core photonic crystal fibers, enabling more robust single-mode operation.

Contribution

The study shows that helical twisting during fabrication can recover and improve mode suppression in SR-PCFs with sub-optimal geometries, combining modeling and experiments.

Findings

Over 12 dB/m improvement in mode suppression achieved experimentally.

Twist rate affects higher-order mode profiles as predicted by simulations.

Helical twisting enables effectively endlessly single-mode operation.

Abstract

Optimum suppression of higher order modes in single-ring hollow-core photonic crystal fibers (SR-PCFs) occurs when the capillary-to-core diameter ratio d/D = 0.68. Here we report that, in SR-PCFs with sub-optimal values of d/D, higher-order mode suppression can be recovered by spinning the preform during fiber drawing, thus introducing a continuous helical twist. This geometrically increases the effective axial propagation constant (initially too low) of the LP01-like modes of the capillaries surrounding the core, enabling robust single-mode operation. The effect is explored by means of extensive numerical modeling, an analytical model and a series of experiments. Prism-assisted side-coupling is used to investigate the losses and near-field patterns of individual fiber modes in both the straight and twisted cases. More than 12 dB/m improvement in higher order mode suppression is achieved experimentally in a twisted PCF. The measurements also show that the higher order mode profiles change with twist rate, as predicted by numerical simulations. Helical twisting offers an additional tool for achieving effectively endlessly single-mode operation in hollow-core SR-PCFs.