  The remarkable recent demonstrations in ultralow loss Inhibited-Coupling (IC)
hollow-core photonic crystal fibers (HCPCFs) place them as serious candidates
for the next-generation of long-haul fiber optics systems. A hindrance to this
prospect, but also to short-haul applications such as micromachining, where
stable and high-quality beam delivery is needed, is the challenge to design and
fabricate an IC-guiding fiber that combines ultra-low loss, truly and robust
single-modeness, and polarization-maintaining operation. Design solutions
proposed up to now require a trade-off between low loss and truly single
modeness. Here, we propose a novel concept of IC HCPCF for obtaining low-loss
and effective single-mode operation. The fiber is endowed with a hybrid
cladding composed of a Kagome-tubular lattice (HKT). This new concept of
microstructured cladding allows to significantly reduce confinement loss and,
at the same time, preserving a truly and robust single-mode operation.
Experimental results show a HKT-IC-HCPCF with a minimum loss figure of 1.6
dB/km at 1050 nm and a higher-order modes extinction ratio as high as 47.0 dB
for a 10 m long fiber. The robustness of the fiber single-modeness was tested
by moving the fiber and varying the coupling conditions. The design proposed
herein opens a new route for the accomplishment of HCPCFs that combine robust
ultralow loss transmission and single-mode beam delivery and provides new
insight into the understanding of IC guidance.
