Multiphoton and material absorption, Rayleigh scattering, high attenuation, and solarization (solar degradation) are characteristics of silica fibers when guiding ultraviolet (UV) radiation. And even though passivation with hydrogen can reduce solarization, attenuation loss still approaches 1 dB/m for UV light from 400 to 200 nm. But a new single-mode antiresonant hollow-core fiber (AR-HCF) design from researchers at the University of Bath (Bath, England), the Shanghai Institute of Optics and Fine Mechanics (SIOM) at the Chinese Academy of Sciences (Shanghai, China), and M-Solv (Oxford, England) offers <1 dB/m transmission for the same UV wavelengths, but with no observed solar degradation after 1 hour when delivering 0.46 μJ pulses. The fibers also deliver broadband UV radiation, compared to Kagome photonic-crystal fiber (PCF) designs that only operate over very narrow wavelength ranges.

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.

(Source: LaserFocusworld, By Gail Overton, Senior Editor 05/01/2018)