  While optical fibers display excellent performances in the infrared, visible
and ultraviolet ranges remain poorly addressed by them. Obtaining better fibers
for the short-wavelength range has been restricted, in all fiber optics, by
scattering processes. In hollow-core fibers, the scattering loss arises from
the core roughness and represents the limiting factor in reducing their loss
regardless of the fiber cladding confinement power. To attain fibers performing
at short wavelengths, it is paramount developing means to minimize the height
variations on the fiber microstructure boundaries. Here, we report on the
reduction of the core surface roughness of hollow-core fibers by modifying
their fabrication technique. In the novel process proposed herein, counter
directional gas fluxes are applied within the fiber holes during fabrication to
attain an increased shear rate on its microstructure. The effect of the process
on the surface roughness has been quantified by optical profilometry and the
results showed that the root-mean-square surface roughness has been reduced
from 0.40 nm to 0.15 nm. The improvement in the fiber core surface quality
entailed fibers with ultralow loss in the short-wavelength range. We report on
fibers with record loss values as low as 50 dB/km at 290 nm, 9.7 dB/km at 369
nm, 5.0 dB/km at 480 nm, and 1.8 dB/km at 719 nm. The results reveal this new
approach as a promising path for the development of hollow-core fibers guiding
at short wavelengths with loss that can potentially be orders of magnitude
lower than the ones achievable with their silica-core counterparts.
