Results of laser beam profile measurements after propagation over untwisted and twisted silica hollow-GeO2-doped-ring core microstructured optical fibers, combined with large core multimode optical fiber

This work reports results of laser beam profile measurements, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fiber (MOF) with hollow-GeO₂-doped-ring core (HRC). We compared two drawn from the same preform HRC MOF samples without and with induced during the drawing process twisting of 790 revolutions per meter. Researched silica HRC MOF with outer diameter 65 µm contains hollow ring-core inner diameter of 30.5 µm with wall thickness of 1.7 µm and refractive index difference Δn = 0.03; 90 air holes, placed over typical hexagonal geometry in the periphery domain from the outside HRC at the distance 14 µm, with hole averaged diameter 2.5 µm and pitch 7.5 µm. According to simulation results (mode analysis, performed by rigorous finite element method via commercially available software COMSOL Multiphysics®), it supports two guided LP-modes (fundamental LP₀₁ and the first higher-order LP₁₁) or 4 HE/EH odd and even eigenmodes – HE₁₁/EH₁₁ and HE₂₁/EH₂₁, respectively. We present some results of laser beam profile measurements, performed under various launching conditions (different laser sources as well as excited optical fibers (both commercially available single-mode optical fiber of ITU-T Rec. G.652 and multimode optical fiber 50/125 of ISO/IEC Cat. OM2+/OM3)) at the output ends of researched HRC MOF twisted and untwisted samples as well as at the output end of large core multimode optical fiber 100/125, aligned with excited HRC MOF.