High-energy, diode-pumped Yb:CaF2 disk-laser

Previous works on Ytterbium-doped alcaline-earth fluoride have impressively shown that these materials are suitable for high-peak power operation [1,2] due to their broad emission spectrum comparable with rare-earth doped laser glasses. From the scaling point of view Yb:CaF2 is the most promising material since crystal growth at large sizes with diameters up to 100 mm and more with high optical quality have been demonstrated so far. Also the thermo-mechanical and thermo-optical properties such as the negative thermal lens of Yb:CaF2 are superior for high-average power lasers [3]. Moreover, the high saturation fluence (i.e. 80 J/cm²) pushes the potential energy limit of thin disk lasers by about one order of magnitude compared to oxide laser materials such as Yb:YAG. This means that at a given stored energy parasitic lasing is reduced by a low transverse gain [4]. However, the low single-pass gain of Yb:CaF2 lasers must compensated by a large number of amplifier passes. We built a multi-pass Yb:CaF2 disk amplifier pumped with a diode laser module having a peak power of 16.8 kW (provided by Lastronics GmbH, Germany). A maximum pump energy of 40.3 J at a wavelength of 940 nm was obtained at a pump duration of 2.4 ms while the flat-top shaped pump profile (6 × 6 mm²) was homogenized by a doublet of micro-lens arrays inside the module and imaged into the gain medium. The seed pulses with a duration of 6 ns (FWHM) were generated in a Q-switched Yb:YAG oscillator and then pre-amplified from 400 μJ to 100 mJ at a repetition rate of 1 Hz in a booster Yb:YAG amplifier. For comparison a rod type and a disk Yb:CaF2 amplifier were build and analyzed. While the laser rod with a diameter of 28 mm and a length of 20 mm was 2.2 mol% Yb-doped the doping concentration of the disk with a thickness of 2.7 mm and diameter of 20 mm was 4.5 mol% Yb. In case of the rod a two-side anti-reflection (AR) coating and a highly reflective (HR) mirror was used, whereas the disk was HR-coated from the rear side and glued on a plane copper-mirror for thermal contact. A maximum output pulse energy of 1.2 J at a repetition rate of 1 Hz of the multi-pass Yb:CaF2 amplifier with the rod-type material was measured. Above 1.2 J laser induced damage occurred in the rod volume while at 270 mJ surface damage on the AR-side was observed in the case of the disk. This very low damage resistance of ~1 J/cm² is owed to the surface quality in our specific case and therefore the optical-to-optical conversion efficiency was limited to 1.5%. In the case of the rod type amplifier an efficiency of 4% was obtained. Fig. 1 illustrates the setup and the dynamic of both Yb:CaF2 amplifier configurations. Although the efficiency achieved was limited it becomes obvious that the performance of the disk is improved compared to the rod with regard to the total gain and minimum pump energy in order to bleach out the reabsorption at the laser wavelength of 1030 nm. Future work is now in progress in order to boost the pulse energy and hence the extraction efficiency of the disk Yb:CaF2 amplifier.