In one AR-HCF configuration, a 109-µm-diameter silica tube includes seven interior capillary tubes nearly equidistantly spaced in the core with individual core wall thickness and inner diameter averaging 132 nm and 7.3 µm, respectively. The AR-HCF capillary tubes act as resonant cavities, guiding the light within the core rather than in the solid silica cladding, significantly reducing attenuation and solarization while increasing optical-damage thresholds. For this fiber, the attenuation at a deep-UV wavelength of 218 nm is 0.1 dB/m—significantly lower than fused silica glass. Another AR-HCF optimized for 266 nm laser delivery produces nearly circular near-field pattern images even after 19.8 m transmission distances. Calculated attenuation values are 0.7 and 0.83 dB/m for 263.7 and 380 nm UV transmission wavelengths, respectively, without requiring hydrogen passivation. Reference: F. Yu et al., Opt. Express, 26, 8, 10879–10887 (2018); https://doi.org/10.1364/oe.26.010879.