Abstract
A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the -like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the -like core mode phase-matches to the -like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio .) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed -like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.
© 2017 Optical Society of America
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