An Yb:CaF<inf>2</inf> thin-disk laser

Abstract

Summary form only given. Novel amplifier systems and laser oscillators generating short pulses and high pulse energies are required in numerous applications, not least in material processing. Currently there are significant efforts to develop such systems [1]. Among other Ytterbium-doped laser gain materials, Yb:CaF 2 is an interesting candidate for highpower cw and passively mode-locked operation for the generation of short pulses because it combines a very broad emission spectrum with a high undoped thermal conductivity (~ 9.7 W/(mK)) [2]. The proof-of principle in ultrafast laser operation was already shown with a bulk crystal by demonstrating pulses shorter than 100 fs in a SESAM passively mode-locked laser system [3]. In the present contribution we report on the first investigation of a Yb:CaF 2 thin-disk laser as this concept is ideally suited for power scaling of ultrafast lasers [4]. For the experimental investigation, two crystals from the same boule with different thicknesses were compared. The nominal doping concentration was specified by the supplier to be around 4 %. The first and non-wedged crystal has a thickness of 200 μm whereas that of the second and wedged (~0.1 °) crystal is 250μm. The thin-disk crystals were glued on copper heat sinks and water cooled from the backside with nominal operating temperatures between 15 and 18 °C. A thin-disk pumping module with a total of 24 pump light passes through the laser crystal was used for sufficient absorption of the pump radiation. The crystals were pumped either with a fiber-coupled laser diode with up to 40 W of power centered at a wavelength of 976 nm with a spectral bandwidth of about 10 nm or at high-powers or, with a 1.2 kW pump source exhibiting a spectral bandwidth of 4 nm centered at a wavelength of 976 nm. In this latter case, only one half of the pump power was used to avoid the damage of the crystal.Figure 1 shows the thermal behavior of the crystal which was examined using a thermal-imaging camera to measure the temperature at its front surface in fluorescence and laser operation. It is recognizable that the temperature at the front surface of the disk in fluorescence operation is significantly lower than in laser operation because of the smaller quantum defect of the shorter centre wavelength of the fluorescence emission. The fact that the temperature is not exceeding 100 °C at power densities of around 8 kW/cm2 represents a relevant and extremely promising information for the further development of high-power Yb:CaF2 thin-disk lasers. After this confirmation, high-power laser operation was demonstrated in a multimode resonator. As shown in Figure 2 a maximum output power of 250 W with an optical efficiency of 47 % was obtained with the 200 μm thick laser crystal. In a further experiment the wedged 250 μm thick crystal was characterized in the perspectives of modelocked operation. The pump spot was set to 1 mm. In a fundamental mode V-shaped resonator (M2 <; 1.1) a maximum of 12.9 W of CW output power with an optical efficiency of 34.2 % has been achieved. The successful demonstration of high power capability and fundamental mode operation were the first steps to set-up a passively mode-locked thin-disk laser. This work is currently under progress and will be presented during the talk.The demonstrated results confirm the potential of Yb:CaF2 for high-power thin-disk laser operation and offer the promising prospect of generating short pulse durations in a passively mode-locked thin-disk laser. Future improvements of the crystal quality, a better control of the doping concentration and a better quality of the polishing will lead to further enhancement of the laser performance.