Ultralow transmission loss in inhibited-coupling guiding hollow fibers

Attenuation in photonic bandgap guiding hollow-core photonic crystal fiber (HC-PCF) has not beaten the fundamental silica Rayleigh scattering limit (SRSL) of conventional step-index fibers due to strong core-cladding optical overlap, surface roughness at the silica cladding struts, and the presence of interface modes. Hope has been revived recently by the introduction of hypocycloid core contour (i.e., negative curvature) in inhibited-coupling guiding HCPCF. We report on several fibers with a hypocycloid core contour and a cladding structure made of a single ring from a tubular amorphous lattice, including one with a record transmission loss of 7.7 dB/km at ~750 nm (only a factor ~2 above the SRSL) and a second with an ultrabroad fundamental band with loss in the range of 10-20 dB/km, spanning from 600 to 1200 nm. The reduction in confinement loss makes these fibers serious contenders for light transmission below the SRSL in the UV-VIS-NIR spectral range and could find application in high-energy pulse laser beam delivery or gas-based coherent and nonlinear optics.