Reduction of the 1.55 μm Er³⁺ emission band half-width in Er doped and Er/Yb co-doped oxy-fluoride glass-ceramics fibers

In earlier papers the authors have shown by XRD measurements and HRTEM imaging/SAED (selected area electron diffraction)/STEM imaging/EDS X-ray spectra that erbium or erbium/ytterbium-enriched nano-crystals are formed in erbium doped and erbium/ytterbium co-doped oxy-fluoride glass-ceramics fibers by their controlled heat-treatment. By the analysis of XRD, HRTEM and SAED patterns three crystalline compounds have been identified (Pb₅Al₃F ₁₉, Er₄F₂O₁₁Si₃, Er ₃FO₁₀Si₃). Additionally, STEM imaging combined with EDS X-ray analysis revealed higher erbium/ytterbium content in nano-crystals than in glassy host. According to several reports on homogeneous/inhomogeneous broadening of emission lines we can expect in glass-ceramics material the distinct reduction of the 1.55 μm Er³⁺ linewidth (FWHM) as a consequence of structurally ordered (crystalline) vicinity of erbium ions in glass-ceramics fibers. Additionally the Stark splitting of Er³⁺ ions sub-levels should be observed due to the crystalline electric field surrounding the erbium ion, which lifts the atomic state degeneracy, however identified crystals possess rather low symmetry (monoclinic or triclinic unit cell).