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 Er3+ ion transition 4I13/2→4I15/2) than corresponding glass fibers (the batch composition 48SiO211Al2O3-7Na2CO3-10CaO-10PbO-12PbF2- 1.5/0.6YbF3-0.5/0.2ErF3). 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 PbF2 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 4I13/2 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 48SiO211Al2O3-7Na2CO3-10CaO-10PbO-12PbF2-1.5/0.6YbF3-0.5/0.2ErF3) 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 PbF2, erbium and ytterbium enriched cubic PbF2 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.
Lead silicate glasses co-doped with Yb3+/Er3+ have been investigated. Up-conversion luminescence spectra of Er3+ ions were registered under excitation of Yb3+ ions by 980 nm diode laser line. Green and red luminescence bands correspond to the 4S3/2 - 4I15/2 and 4F9/2 - 4I15/2 transitions of Er3+, respectively. The luminescent transitions of Er3+ ions have been also examined with temperature.
The tellurite-tungstate glasses containing small amounts of rare-earth ions have been studied experimentally at 77 and
293 K using spectroscopic methods. The photoluminescence (PL) studies reveal the emission of efficient green-yellow
light from Tb3+ ions and red light from Eu3+ ions. The Judd-Ofelt intensity parameters have been derived for Nd3+ and
Er3+ ions from the absorption spectra and they have used to calculate the radiative lifetimes and branching ratios. The
quantum efficiency η = 0.95 of the 4F3/2 level of Nd3+ ion is higher than the typical value of other tellurite-based glasses.
For Er3+ ions, PL originating from the 4S3/2,4I11/2 and
4I13/2 levels has been observed and the luminescence decay of the
first two levels has been found to be governed by radiative transitions and multiphonon relaxation involving the highest
energy of Te-O vibrations.
Heavy metal oxide and oxyhalide glasses containing Er3+ ions have been investigated. Near-infrared luminescence at
about 1550 nm and up-conversion spectra of Er3+ ions were registered under excitation by 980 nm diode laser line.
Several luminescence bands are observed, which correspond to the 4I13/2 - 4I15/2 (1550 nm), 2H11/2,4S3/2 - 4I15/2 (550 nm),
4F9/2 - 4I15/2 (670 nm) and 4I9/2 - 4I15/2 (800 nm) transitions of Er3+, respectively. The optical transitions of Er3+ ions have
been examined as a function of glass host and PbX2 (X = F, Cl, Br) content.
Lasing and fluorescence behavior of thulium doped YVO4, GdVO4, and LuVO4 single crystals were investigated
under pulsed pumping with variable duty cycle up to CW. This allowed us to study properties of these crystals
in dependence on thermal load in a broad range. Following crystals were investigated: Tm:YVO4 (5 at.% Tm/Y,
grown by the Czochralski technique), Tm:GdVO4 (2, 4, and 6 at.% Tm/Gd, grown by the floating-zone technique),
and Tm:LuVO4 (3 at.% Tm/Y, grown by the floating-zone technique). For pumping a fibre-coupled (core diameter
400 μm) laser diode operating in range from 800 up to 803nm was used (available CW power 20 W). All tested
crystals were investigated under CW and pulsed pumping (pulse length 4 ms). Under pulsed pumping (4% duty
cycle), the lasing was demonstrated with all samples. Under CW pumping only Tm:GdVO4 crystal was lasing.
For Tm:YVO4 and Tm:LuVO4 crystals, a lasing was not reached for pumping with duty cycle higher than 60 %,
and the strong blue emission was observed. Detailed measurement of visible emission for broad range of pumping
duty cycles (from 4 up to 60%) showed the exponential increase of Tm3+ integral emission intensity in bands
around 480 and 700 nm. Comparison with the results obtained for fixed duty cycle and variable crystal holder
temperature (290 - 310 K) allowed us to find a relation between the duty cycle and temperature of pumped part
of the crystal. Measurement of infrared fluorescence temporal behavior in dependence on duty cycle gives us
possibility to study a relative population of lasing level in dependence on temperature.
Thulium doped vanadates Tm:YVO4 (5 at.% Tm/Y, grown by the Czochralski technique), Tm:GdVO4 (2 and
6 at.% Tm/Gd, grown by the floating-zone technique), and Tm:LuVO4 (3 at.% Tm/Y, grown by the floating-zone
technique) were investigated as an active medium for diode pumped tunable laser operating around 1.9 μm. For
thulium laser tuning single 1.5mm thick Brewster-angled birefringent quartz plate (Lyot filter) was placed in
simple 80mm long linear quasi-hemispherical resonator. For thulium doped vanadates pumping a fibre-coupled
(core diameter 400 μm) temperature-tuned laser diode operating in range from 799 up to 810nm was used
(max available power 20 W). All tested crystals were investigated under CW and pulsed pumping. Under pulsed
pumping (4% duty-cycle, reduced heat generation) lasing and laser tuning was demonstrated with all available
samples. Lasers were tunable in following wavelength ranges: Tm:YVO4 5 at.% Tm/Y (1841 - 1927 nm),
Tm:GdVO4 2 at.% Tm/Gd (1830 - 1982 nm), 6 at.% Tm/Gd (1850 - 2010 nm), and Tm:LuVO4 3 at.% Tm/Lu
(1860 - 1940 nm). Under CW pumping only Tm:GdVO4 crystal was lasing (lasing of Tm:YVO4 and Tm:LuVO4
was not reached under elevated pumping duty factor). Using Tm:GdVO4 (2 at.% Tm/Gd) the power up to 2.6W
and slope effciency ~ 30% (with respect to absorbed power at 808nm under lasing condition) was obtained at
wavelength 1.91 μm. Tunable operation with greater that 1W output and 130nm tuning range (1842 - 1972 nm)
was demonstrated for Tm:GdVO4 (2 at.% Tm/Gd) pumped at 802 nm.