According to the earlier author’s papers, the erbium/ytterbium co-doped oxyfluoride glass-ceramics fibers should demonstrate better 1550 nm emission under 488/515/980 nm excitation (the erbium Er<sup>3+</sup> ion transition <sup>4</sup>I<sub>13/2</sub>→<sup>4</sup>I<sub>15/2</sub>) than corresponding glass fibers (the batch composition 48SiO<sub>2</sub>11Al<sub>2</sub>O<sub>3</sub>-7Na<sub>2</sub>CO<sub>3</sub>-10CaO-10PbO-12PbF<sub>2</sub>- 1.5/0.6YbF<sub>3</sub>-0.5/0.2ErF<sub>3</sub>). Glass fibers provided as a core of standard multimode waveguide (the diameter of 62 μm) have been drawn with the mini-tower to the diameter between 50 μm and 80 μm, then annealed in the two-step regime (580°C/1h – nucleation of nano-crystals; 760oC/15/30 min – nano-crystals growth). This kind of heat treatment ensures the transparent glass-ceramics fibers with the microstructure of homogeneously distributed nano-crystals (lead, erbium and ytterbium enriched cubic fluorite-like crystals and hexagonal PbF<sub>2</sub> crystals) embedded in a glassy host. Their transmission covers the range of 80-90% and seems to be sufficient with respect to their provided limited length (~2m). The luminescence intensity for glass-ceramics fibers at 1530 nm is higher than that of respective glass fibers and the lifetimes of the erbium ion excited state <sup>4</sup>I<sub>13/2</sub> are of the same order (~5 ms). In that context the glass-ceramics fibers discussed above seem to be promising candidates for cores of fiber lasers at the 1550 nm band.
The intention of the authors is to show a possibility of controlled crystallization of Er/Yb co-doped oxyfluoride glass fibers provided for glass-ceramics core fiber lasers or optical amplifiers at 1550 nm. Selected glasses (the batch composition 48SiO<sub>2</sub>11Al<sub>2</sub>O<sub>3</sub>-7Na<sub>2</sub>CO<sub>3</sub>-10CaO-10PbO-12PbF<sub>2</sub>-1.5/0.6YbF<sub>3</sub>-0.5/0.2ErF<sub>3</sub>) were examined in the form of powders of diameter 45-100 μm (diameter comparable to that of standard multimode fiber core 62 μm). Powders were annealed at various temperatures and time periods in order to obtain glass-ceramics with different crystalline fraction. DTA measurements and calculations (isothermal and non-isothermal annealing) enabled to determine JMAK (JohnsonMehl-Avrami-Kissinger) equation parameters and to estimate crystal growth rate and certain features of nucleation stage. XRD/TEM/SAED/EDS/HRTEM/SEM/EDS techniques (X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy) enabled to determine crystal structure of growing crystals (erbium and ytterbium enriched hexagonal PbF<sub>2</sub>, erbium and ytterbium enriched cubic PbF<sub>2</sub> isomorphic to the fluorite structure). Luminescence intensity at 1550 nm under 488/980 nm excitation has shown comparable values for glassceramics powder and for bulk glass what suggests a considerably higher emission yield for glass-ceramics fiber. The lifetime of erbium excited state for glass-ceramics powder (~4 ms) is also comparable to that of bulk glass and is promising from the point of view of a stimulated emission. Computed parameters of JMAK equation enable to establish heat treatment conditions for glass fibers and hence to control the level (fraction) of glass crystallization.
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 (Pb<sub>5</sub>Al<sub>3</sub>F<sub>19</sub>, Er<sub>4</sub>F2O<sub>11</sub>Si<sub>3</sub>, Er<sub>3</sub>FO<sub>10</sub>Si<sub>3</sub>). 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 Er<sup>3+</sup> linewidth (FWHM) as a consequence of structurally ordered (crystalline) vicinity of erbium ions in glass-ceramics fibers. Additionally the Stark splitting of Er<sup>3+</sup> 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).