Hollow-Core PCF
The main physical principle behind propagation of light in conventional optical fibers is total internal reflection (TIR). However, engineering of optical materials with features on the scale of the wavelength of light offers many new possibilities for manipulating light. In particular, some microstructured fibres make it possible to guide light by a mechanism different from total internal reflection. In these fibres, light is trapped in the core by an out-of-plane band-gap, which appears over a range of axial wavevectors and prevents propagation of light in the microstructured cladding [Cregan (1999)], allowing guided modes to form in the central hollow core [Russell (2003); Russell (2006)].
Nonlinear optics in gas-filled hollow-core PCF
Gas-filled hollow-core photonic crystal fibre (HC-PCF), with pressure-tunable dispersion, low propagation loss, a very high optical damage threshold and a small mode area, is ideally suited to controlling the propagation of laser pulses in gases [Travers (2011); Russell (2014)]. Depending on the design, two guidance mechanisms are possible: photonic bandgap (PBG) and anti-resonant reflection (ARR). In the first the losses can be as low as 1 dB/km, but over a relatively narrow bandwidth (a few 10s of nm). In the second case, the loss is of order 1 dB/m but the guidance band can be as wide as ~1000 nm. This makes ARR-PCF particularly attractive for controlling the propagation dynamics of high-bandwidth ultrashort pulses, even down to sub-cycle durations. In its original form, ARR-PCF had a kagomé-style cladding lattice, but more recently a much simpler structure, consisting of a ring of thin-walled capillaries arranged around a central hollow core, is showing much promise. ARR-PCFs allow many interesting nonlinear phenomena to be investigated experimentally, including different types of soliton dynamics and strong field physics.
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)].
(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.
