Susanne T. Fredrich-Thornton

Thermal and laser properties of Yb:LuAG for kW thin disk lasers

Thin disk laser experiments with Yb:LuAG (Yb:Lu(3)Al(5)O(12)) were performed leading to 5 kW of output power and an optical-to-optical efficiency exceeding 60%. Comparative analyses of the laser relevant parameters of Yb:LuAG and Yb:YAG were carried out. While the spectroscopic properties were found to be nearly identical, investigations of the thermal conductivities revealed a 20% higher value for Yb:LuAG at Yb(3+)-doping concentrations of about 10%. Due to the superior thermal conductivity with respect to Yb:YAG, Yb:LuAG offers thus the potential of improved performance in high power thin disk laser applications.

Model for the calculation of radiation trapping and description of the pinhole method

Radiation trapping is a well-known process that results in the lengthening of observed fluorescence lifetimes in laser materials with significant overlap in their emission and absorption spectra. The pinhole method is a measurement technique that allows the intrinsic fluorescence lifetime of an excited state to be determined in a nondestructive manner. A theoretical description of this method is proposed. A model is developed that identifies the lifetime extrapolated to a zero radius pinhole as the intrinsic fluorescence lifetime. The application of this method to bulk materials and thin discs is discussed.

Switching of emissivity and photoconductivity in highly doped Yb3+:Y2O3 and Lu2O3 ceramics

Reversible jump of thermal emission accompanied with jump of photocurrent is observed in highly doped Yb3+:Y2O3 and Lu2O3 bulk ceramics pumped at 940nm wavelength. In contrast, when these materials are heated with a CO2 laser at 10.6μm wavelength, only a gradual increase of thermal emission and photoconductivity are observed up to the melting point. These results are interpreted as a ytterbium mediated, photoassisted avalanche of thermal emission.

Ultrashort pulse generation from diode pumped mode-locked Yb 3+ :sesquioxide single crystal lasers

We present diode pumped SESAM supported Kerr-lens mode locked laser operation based on Yb3+:Sc2O3 and Yb3+:Lu2O3 single crystals. Pulses as short as 71 fs with an average power of 1.09 W were obtained from an Yb3+:Lu2O3 single crystal. Yb3+:Sc2O3 delivered pulses as short as 81 fs with an average power of 840 mW. The mode locked laser operation was stable for longer than 2 hours.

Yb-Doped Thin-Disk Laser Materials: A Comparison between Yb:LuAG and Yb:YAG

The impact of Yb-doping concentration on thermal conductivity is much lower in Yb:LuAG compared to Yb:YAG. As the materials have very similar spectroscopic properties, Yb:LuAG is supposed to outperform Yb:YAG in high power laser applications.

Up-conversion to the Conduction Band in highly doped Yb:YAG and Yb:Y 2 O 3 and its effect on Thin-Disk Lasers

The photoconductivity of high Yb-concentration YAG and Y2O3 samples has been measured, confirming the occurrence of up-conversion in these materials. High intensity pumping reveals a very broad emission spectrum that exhibits avalanche behaviour.

Photoconductivity in Yb-doped materials at high excitation densities and its effect on highly Yb-doped thin-disk lasers

Due to the simple energy structure of Yb3+ with only two 4f manifolds, energy transfer processes such as cross-relaxation and up-conversion are not expected in Yb-doped materials. Yet, photoconductivity measurements have confirmed the creation of free charge carriers in Yb-doped single crystals when irradiated with pump light of 940nm [1,2]. These experiments focussed particularly on Yb:YAG samples of various doping concentrations and the measurements displayed a nonlinear increase of photocurrent with the excitation density. Further investigations of Yb:YAG ceramics as well as Yb:Lu2O3 show a similar behaviour, indicating that this phenomenon is neither limited to crystalline samples nor to the YAG host. Yet, a different behaviour is observed in Yb-doped fluorides which may be due to the hosts different band structure.

Quenching processes in Yb lasers; Correlation to the valence stability of the Yb ion

We present an overview of quenching processes in Yb-doped lasers. Experiments made on Yb:YAG crystals and Yb-doped fiber lasers show that induced losses appear upon UV-irradiation through a charge-transfer process. The valence stability of the Yb ion is believed to be the key issue for the quenching processes in Yb-doped high power laser systems.

Ultra-Short Pulses from Diode-Pumped Yb 3+ -Doped Crystal and Ceramic Lasers with High Average Power

From Yb3+:Sc2O3, Yb3+:Lu2O3crystals and an Yb3+:Y2O3ceramic, sub-70 fs pulses with above 500 mW average power were obtained, respectively. Additionally, 128 fs pulses with 1.35 W average power were achieved with an Yb:YAG ceramic.

Highly Doped Yb:YAG Thin-Disk Lasers: A Comparison between Single Crystal and Ceramic Active Media

Despite the very similar spectroscopic properties, efficient lasing in the thin-disk laser set-up has not been possible for highly doped Yb:YAG single crystals so far, whereas 20%Yb:YAG ceramics display a 60.6% slope efficiency.