Results of far-field red laser beam profile measurements after propagation over spun silica 7-hollow-GeO2-doped-ring core microstructured optical fibers

This work presents the results of red laser beam profile measurements performed for earlier on designed and successfully fabricated silica spun few-mode microstructured optical fiber (MOF) with included seven GeO2-doped capillaries (or seven GeO₂-doped hollow ring cores), placed in the central part of optical fiber cross-section (7-GeO₂-doped HRC MOF). We compared two drawn from the same cane 7-GeO₂-doped HRC MOF samples with weak, medium and strong induced twisting – 130, 300 and 730 revolutions per meter. Researched 7-GeO₂-doped HRC MOF has conventional "telecommunication" outer diameter 125 μm; each GeO₂-doped HRC inner diameter is 5.0 µm with refractive index difference Δn=0.030 and pitch Λ₁=8.80 μm; air holes are placed over typical MOF hexagonal geometry, their diameter 8.80 μm and pitch Λ₂=9.80 μm. We present comparison results of red laser (λ=650 nm) beam profile measurements, performed under radially overfilled launching conditions via singlemode optical fiber (SMF) of ITU-T Rec. G.652 with free space connection to the tested weakly and strongly twisted MOF samples.