Contact Daniel Albach Email Phone +49 351 260 2831
Markus Loeser Email Phone +49 351 260 3006
Mathias Siebold Email Phone +49 351 260 3049

a High Energy Diode Pumped Laser System for Femtosecond Pulses

FWKT is currently constructing a fully diode-pumped laser system - PEnELOPE (Petawatt, Energy-Efficient Laser for Optical Plasma Experiments). A five stage amplifier system relying on Yb:CaF2 as gain medium is designed for pulse energies of 150 J and a pulse duration of <150 fs operating at a repetition rate of 1 Hz [Proc. SPIE 8780, 878 005 (2013)]. Pulses as short as 60 fs with a pulse energy of 25 nJ are delivered by a commercial Yb:KGW oscillator at a center-wavelength of 1035 nm. Stretcher and compressor of the CPA-system are designed for a stretching factor of 195 ps/nm and a hard-clip bandwidth of 50 nm. Highly efficient multilayer dielectric reflection gratings are utilized with a line density of 1760 lines per mm, allowing for a compressor efficiency of >85%.
The first 3 amplification stages (1 regenerative and 2 multipass amplifiers) are working as disk lasers with disk thicknesses of 2 and 5 mm. The 2 subsequent final amplifiers include a Helium gas cooled four-slab laser head with aperture sizes of 55 and 110 mm, allowing for up to 150 J. A He gas flow at a temperature range of 200-300 K, a pressure of up to 5 bars and a flow rate of 10 l/s will be used to obtain a longitudinal temperature gradient within the Yb:CaF2 slabs with 5 mm thickness [Opt. Lett. 37, 2175 (2012).]. A total pump peak power of 120 kW at 940 nm for the 10 J amplifier and 1.2 MW at 980 nm for the final amplifier system are under installation. With an expected gain of only 1.25 per pass, 12 extraction passes are necessary to achieve the required gain. Both last amplifiers are fully reflective and image relayed to ensure a maximum beam quality. The measured throughput is better than 90% with more than 200 surfaces hit. Figure 1 shows the optical setup of the 10 J amplifier constructed within a vacuum chamber with a size of 6.5 x 0.6 x 0.5 m3 (in comparison to the 100 J amplifier: 13 x 1.2 x 1.0 m3) to prevent breakdown near the focal spots and any air turbulences. Spherical and planar mirrors are aligned on circular structures.
The vacuum chamber of the compressor with a size of 6 x 3 x 2 m3 includes 2 gratings with a dimension of 94 x 42 cm2 and a grating distance of 2.2 m, a telescope to adapt the output beam diameter to 25 cm, and an adaptive mirror to compensate for wavefront distortions. Finally, the laser beam will be transported to the experiment area where a peak-intensity above 1021 W/cm2 is envisioned for ion acceleration experiments.


Optical setup of the 10 J laser amplifier.
Fig.1 - Optical setup of the 10 J laser amplifier.
Photographic view of the 10 J laser amplifier.
Fig.2 - Photo of the 10 J laser amplifier.