Already the very first hollow-core PCFs demonstrated in 1999 had a complex microstructure consisting of several (~10) layers of air-holes in the cladding. In the case of the photonic bandgap fibres it is known that the confinement loss decreases for each layer added to the cladding. In recent years, however, it has become increasingly clear that in many scientifically and technically interesting cases, it is sufficient to have a single ring of hollow glass capillaries surrounding the air-core, as long as the cladding capillaries are thin enough compared to the wavelength. In close collaboration with the Russell division, we have explored several aspects of these new types of single-ring hollow-core fibres. First, we used a tube with a hexagonal inner bore to precisely position six capillaries 60° apart at the tube vertices (advantageous compared to a circular tube) [Edavalath (2015), Edavalath (2017)]. Second, we demonstrated theoretically and experimentally that this fibre design can provide endlessly single-mode guidance even though the core is hollow, provided the capillary-to-core diameter ratio is close to 0.682 [Uebel (2016)]. This value ensures phase-matching between LP11, which is the most critical higher-order mode (HOM) in the core, and the fundamental mode in the cladding capillaries, thereby eliminating it from the core, as proven by prism-assisted side-coupling [Trabold (2014)]. Third, we have demonstrated the twisting of these types of fibre directly during fibre drawing, which provides another route to optimizing the suppression of unwanted HOMs [Günendi (2016)]. Finally, we have derived a simple analytical expression to estimate the bend-radius at which single-ring hollow-core PCF acquires high bend-loss, and demonstrated its validity experimentally [Frosz (2017)].
A typical single-ring hollow-core fibre with six capillaries in the cladding, each ~19 µm in diameter and ~400 nm thick [Uebel (2016)].