Earlier on we introduced model of piece-wise regular fiber optic link, operating in a few-mode regime: laser-based data transmission over large core optical fiber. Presented model is based on piecewise regular representation with general approach of split-step method application. It allows to take into account laser-excited optical signal launching conditions, differential mode delay, differential mode attenuation, higher-order mode chromatic dispersion and mode mixing / power diffusion, occurring due to optical fiber irregularity and fiber optic cable bends / twisting / stress / tension. While optical fiber irregularity can be directly set by protocols of optical fiber outer diameter monitoring system of drawing tower, cable external mechanical influences are simulated via equivalent angular misalignment at the splices of regular spans. Therefore, this work is concerned with issues of selection of this equivalent angular misalignment (EAM). We performed a computational test series under various values of mentioned above EAM under following comparison with experimentally measurements of few-mode optical pulse responses at the output of multimode optical fibers with strong differential mode delay effect.
This work presents results of test series, performed for earlier on designed and successfully fabricated silica few-mode microstructured optical fibers (MOF) with six GeO2-doped cores, induced twisting 100 and 600 revolutions per meter, typical “telecommunication” outer diameter 125 µm, core diameter 8.7 µm, air hole diameter 4.6 µm, pitch 7.2 µm, and core graded refractive index profiles with height 0.0275. While Part I introduced results of differential mode delay map measurements with laser source excitation / laser-based data transmission over multimode optical fibers (MMFs) with core diameters 50 and 100 µm, combined with 6-core MOF, and Part II was concerned with researches of spectral responses, measured for fiber Bragg gratings, recorded in these MOFs, Part III reports results of far-field white light beam profile measurements, performed for weakly and strong twisted 6-core-MOFs.
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.
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