We report on hollow-core fibers displaying rms core roughness under the surface capillary waves thermodynamic limit and record loss in the short-wavelength range (50.0dB/km at 290nm, 9.7dB/km at 369nm, 5.0dB/km at 480nm, 0.9dB/km at 558nm).

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 for loss reduction regardless of the cladding confinement power. Here, we report on the reduction of the core surface roughness of hollow-core fibers by modifying their fabrication technique. The effect of the modified process has been quantified and the results showed a root-mean-square surface roughness reduction from 0.40 to 0.15 nm. The improvement in the core surface entailed fibers with ultralow loss at short wavelengths. The results reveal this approach as a promising path for the development of hollow-core fibers with loss that can potentially be orders of magnitude lower than the ones achievable with silica-core counterparts.

<p>This is the pre-review draft of the first of two papers submitted to Journal of Lightwave Technology, where we propose and apply a methodology for the analysis of the loss in tube lattice fibers (TLFs) based on the azimuthal Fourier decomposition (AFD) of the fiber’s modes along the perimeters of the tubes composing the fiber cladding. This technique combined with the coupled mode theory constitutes an effective approach for gaining insight in the waveguiding mechanism and for additional loss analysis due to fiber non-idealities. In this part I we describe the approach and apply it to loss analysis of ideal TLFs. The approach is then applied to the analysis of tube thickness variation effects in the part II.</p>

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<p>This is the pre-review draft of the second of two papers submitted to Journal of Lightwave Technology, where the effect on confinement loss of thickness variations along the perimeter of the tubes composing the cladding of inhibited-coupling guiding Tubular Lattice hollow-core fibers is investigated by using the Azimuthal Fourier Decomposition technique (developed in Part I) for the description of the cladding modal dynamics and their interaction with fundamental core mode. The results show that the thickness inhomogeneity affects the confinement loss spectrum through confinement loss increase and frequency red- and blue-shift of the high loss spectral regions. The magnitudes of the confinement loss increase and the high-loss region frequency shift strongly depend on the spatial distribution of the thickness inhomogeneity. The study provides insight into the loss mechanism of non-ideal tube lattice fibers, it allows quantifying the impact of such kind of structural deformations, identifying the route to make fibers more resilient to such fabrication imperfections, and highlighting once again the importance played in inhibited-coupling fibers by the interaction between core modes and the intricate set of cladding modes.</p>