2005 Conference on Lasers and Electro-Optics Europe

Yb:NaGd(WO4)2: Spectroscopic Characterisation

and Laser Demonstration

R. Peters, J. Johannsen, M. Mond, K. Petermann, and G. HuberInstitutfiir Laser-Physik, Universitdt Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany

Fax: +49-40-8998-5160, e-mail: mond@physnet.uni-hamburg.de

SummaryFor high-power laser applications such as laser cutting, welding, and drilling ytterbium doped crystals are the preferredactive laser media. The absence of upconversion processes and the low quantum defect between pump and laserradiation are common for all Yb-doped crystals. But among these crystals, Yb:YAG is of outstanding importance due toits thermo-optical and mechanical properties. For some applications, such as the thin-disk lasers, high Yb-concentrations are necessary to exploit the specific advantages of the thin-disk laser design at the best.

However, a strong decrease in quantum efficiency towards higher Yb-concentrations is observed in many laser crystals,that is for example in Yb:YAG. This is based on an energy migration combined with an energy transfer to certainimpurities. In case of efficient and fast migration, the excitation energy is dispersed over a large volume in the crystal.The transfer to impurities followed by nonradiative decay vanishes the excitation energy leading to the decrease inquantum efficiency. For this effect the distances between the ytterbium ions is a key parameter, because the energymigration rate depends on the inverse sixth order of the ion separation.

In recent years, Yb-doped tungstates, like Yb:KY(WO4)2 (Yb:KYW), have gained strong importance. because thedistance between the Yb-ions is larger than in YAG, thus leading to a reduced energy migration rate. Efficient laseroperation has already been demonstrated in a standard laser design [1], and also in the thin-disk setup. The crystals canbe doped up to Yb-concentrations of 100 % with nearly no concentration quenching. However, these tungstates canonly be grown from the flux, which - as major drawback - leads to low crystal growth rates and a cost-intensiveproduction.

Here, Yb:NaGd(WO4)2 (Yb:NGW) is presented, which - in contrast to the potassium tungstates - can be grown by theCzochralski-technique and exhibits even larger Yb-Yb-distances. The crystal has a tetragonal Scheelite-structure withspace group C64h. The Yb-ions replace the gadolinium- ions in this structure, which are statistically distributed over twodifferent sites in the crystal lattice. However, the absorption and emission cross section spectra exhibit a concentrationdependence [2], which is supposed to be caused by an increased ordering of the structure. Apparently, the decreasingaverage ionic radius of the Gd-site by doping of the smaller Yb-ions makes the Gd-ions and thus the sustituting Yb-ionspreferentially occupying just one of the two sites. This effect is going to be confirmed by low temperature absorption,emission, and excitation measurements. The results will be presented at the conference.

The increasing ordering towards higher doping levels can already be seen in lifetime measurements: In Figure 1 thelifetime of the 2F5/2 multiplet is shown for ditTerent Yb-concentrations. The increase of lifetime is obvious for higherconcentrations. In many cases this increase is based on the well-known radiation trapping effect, which increases the

600 measured lifetime by reabsorption and delayedemission of the initial radiation. But here, the effect of

550- radiation trapping is largely excluded due to the usedexperimental technique, i.e. the so-called pinhole-

soo - method. This change in ion-site occupation may cover500 a possible concentration quenching; therefore no

information concerning the quantum efficiency can beE deduced from this figure.aD U

400- Furthermore, Yb:NGW has been tested in first laser. experiments. Under titanium sapphire laser pumping, a

350- " slope efficiency of 36 % and a maximum output power350

,____ -_________ of nearly 200 mW were achieved. Using a diode laser0 5 10 15 20 25 at a wavelength of 975 nm a slope etficiency of 20 %

Yb concentration (at. % in melt) and a maximum output power of 230 mW were

Fig. 1: Lifetimes of Yb:NGW for the different ytterbium- obtained.concentrations. [I] N.V. Kuleshov et al., Appl. Phys. B 64. 409 (1997)

[2] D. Lis et al., ASSP 2003, paper TuB9-1, 183 (2003)

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