The crystallization kinetics of Er/Yb co-doped oxyfluoride glasses

摘要

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_2llAl_2O_3-7Na_2CO_3-10CaO-10PbO-12PbF_2-1.5/0.6YbF_3-0.5/0.2ErF_3) were examined in. the form of powders of diameter 45-100 um (diameter comparable to that of standard multimode fiber core 62 um). 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 (Johnson-Mehl-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_2, erbium and ytterbium enriched cubic PbF_2 isomorphic to the fluorite structure). Luminescence intensity at 1550 nm under 488/980 nm excitation has shown comparable values for glass-ceramics 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.