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180 FS YB:YAG thin disk regenerative amplifier
Abstract
Yb:YAG amplifiers are frequently used in the pulse regime of some few ps. To reach shorter pulse duration other materials with broader amplification spectras like Ti:Sa or tungstates are preferred, so far. In this paper it is shown, that even 180 fs can be reached with a Yb:YAG regenerative amplifier at a repetition rate of 100 kHz and a pulse energy of 1mJ.
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A review of theoretical and experimental studies of thermal effects in solid-state lasers is presented, with a special focus on diode-pumped ytterbium-doped materials. A large part of this review provides however general information applicable to any kind of solid-state laser. Our aim here is not to make a list of the techniques that have been used to minimize thermal effects, but instead to give an overview of the theoretical aspects underneath, and give a state-of-the-art of the tools at the disposal of the laser scientist to measure thermal effects.After a presentation of some general properties of Yb-doped materials (Section 1), we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally (Section 2). This is the first step before studying the complex problem of thermal lensing (Section 3). We will focus on some newly discussed aspects, like the definition of the thermo-optic coefficient: we will highlight some misleading interpretations of thermal lensing experiments due to the use of the dn/dT parameter in a context where it is not relevant. Section 4 will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in Section 5, we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.
A thin disk regenerative amplifier based on Yb:KLW (potassium lutetium tungstate) is operated in different set-ups, changing the net gain and total amount of nonlinearity during amplification. Spectrum broadening during amplification produces Lorentz-shaped pulses with an autocorrelation width of 190 fs after dispersion compenstaion. Damping of bifurcations is demonstrated by nonlinear spectrum broadening, allowing stable operation with an output energy of of nearly 400 muJ at 50 kHz.
The laser system is based on an Yb:YAG thin-disk regenerative amplifier, which is operated in different operation modes in order to address broad spectrum of pulse durations. It is especially interesting for application development tasks, when different pulse durations can be tested to find the application optimum. For sub-picosecond pulse duration the dispersion of the regenerative amplifier output is compensated with a pair of diffraction gratings. Pulses with a full width at half maximum of 334 fs at an output power level of 30 W can be produced using a nonlinear spectrum broadening during amplification. Tuning of the distance between gratings or abandoning of the compressor allows for output pulse durations of several picoseconds with an output power of 60 W. A second seed source allows for pulse durations up to several nanoseconds. Further, the amplifier was operated in cavity-dumped or in Q-switched mode just by changing of the electrical control of the Pockels cell in the amplifier. The pulse durations range is extended, correspondingly, to 100 ns and even to microseconds.
A new, scalable concept for diode-pumped high-power solid-state lasers is presented. The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink. This allows very high pump power densities without high temperature rises within the crystal. Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface. This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities. Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 4.4 W and a maximum slope efficiency of 68%. From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm3. Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs.