Hollow-core endlessly single-mode (hESM) PCF

A drawback of HC-PCFs is that they are typically multimode, and because higher-order modes (HOMs) often have relatively low loss, HOM contamination is particularly acute in applications using relatively short fibre lengths. The ultralumina team, along with the TDSU fibre fabrication and the Russell division has succeeded in designing & fabricating a single-ring HC-PCF in which HOMs resonantly couple to highly-leaky modes in the cladding, an effect that holds for all wavelengths in which the fundamental mode is confined with low loss [Uebel (2016)]. The fabricated fibres show >100× higher HOM suppression compared to kagomé-style PCFs. They are very useful for applications in which M² ~ 1 beams are needed, such as the broadband light source, being developed in the ultralumina project. In a recent development, we have also reported that HOM suppression can be tuned by introducing a permanent twist by spinning the preform in the drawing tower [Günendi (2016)].

(a) SEM of a typical single-ring hollow-core PCF with dimensions marked in. (b) 3D sketch of an effectively "endlessly" single-mode PCF. A broadband, multimode beam is coupled into the core modes of the fibre. If the ratio of satellite capillary inner diameter d to core diameter D equals 0.682, optimal HOM suppression is achieved, causing HOMs to leak away rapidly into the cladding while the fundamental LP01 mode survives with low loss.