Fabian Roeser

PEnELOPE: a high peak-power diode-pumped laser system for laser-plasma experiments

We introduce the directly diode-pumped PEnELOPE laser-system which is designed for a pulse energy of 150 J, a repetition rate of 1Hz and a pulse duration of 120 fs. The principle setup of amplifier and stretcher-compressor system as well as the pumping, energy extraction and cooling scheme of the power amplifiers will be reported. In this paper we focus on numerical modeling as well as design studies.

High-energy, ceramic-disk Yb:LuAG laser amplifier

We report the first short-pulse amplification results to several hundred millijoule energies in ceramic Yb:LuAG. We have demonstrated ns-pulse output from a diode-pumped Yb:LuAG amplifier at a maximum energy of 580 mJ and a peak optical-to-optical efficiency of 28% at 550 mJ. In cavity dumped operation of a nanosecond oscillator we obtained 1 mJ at up to 100 Hz repetition rate. A gain bandwidth of 5.4 nm was achieved at room temperature by measuring the small-signal single-pass gain. Furthermore, we compared our results with Yb:YAG within the same amplifier system.

High-energy diode-pumped D2O-cooled multislab Yb:YAG and Yb:QX-glass lasers.

We investigated the lasing performance of a multislab Yb:QX and Yb:YAG laser amplifiers using a facet-cooled design. Di-deuterium oxide (D2O) was used as the coolant flowing between the active slabs with the pump and laser light passing through the very low absorbing heavy-water films. A square pump profile at a maximum intensity of 40  kW/cm2 drove the amplifier with a peak fluence of 5.5  J/cm2 and a pulse duration of 6 ns. We demonstrated a maximum pulse energy of 1 J for each gain medium as well as a repetition rate of 10 Hz for Yb:YAG and 1 Hz for Yb:QX glass, thus showing the feasibility and scalability of directly water-cooled, diode-pumped, high-energy short-pulse lasers.

High energy CPA-free picosecond Yb:YAG amplifier

We report on a CPA free picosecond MOPA system with a fs Yb:KGW oscillator and 2 subsequent amplifiers using Yb:YAG active mirrors. A maximum pulse energy of 35mJ at 10Hz repetition rate was achieved.

Scaling EUV and X-ray Thomson sources to optical free-electron laser operation with traveling-wave Thomson scattering (Conference Presentation)

Traveling-Wave Thomson-Scattering (TWTS) allows for the realization of optical free-electron lasers (OFELs) from the interaction of short, high-power laser pulses with brilliant relativistic electron bunches. The laser field provides the optical undulator which is traversed by the electrons. In order to achieve coherent amplification of radiation through electron microbunching the interaction between electrons and laser must be maintained over hundreds to thousands of undulator periods. Traveling-Wave Thomson-Scattering is the only scattering geometry so far allowing for the realization of optical undulators of this length which is at the same time scalable from extreme ultraviolet to X-ray photon energies. TWTS is also applicable for the realization of incoherent high peak brightness hard X-ray to gamma-ray sources which can provide orders of magnitude higher photon output than classic head-on Thomson sources. In contrast to head-on Thomson sources TWTS employs a side-scattering geometry where laser and electron propagation direction of motion enclose an angle. Tilting the laser pulse front with respect to the wave front by half of this interaction angle optimizes electron and laser pulse overlap. In the side-scattering geometry the tilt of the pulse-front compensates the spatial offset between electrons and laser pulse-front which would be present otherwise for an electron bunch far from the interaction point where it overlaps with the laser pulse center. Thus the laser pulse-front tilt ensures continuous overlap between laser pulse and electrons while these traverse the laser pulse cross-sectional area. This allows to control the interaction distance in TWTS by the laser pulse width rather than laser pulse duration as is the case for head-on Thomson scattering. Utilizing petawatt class laser pulses with millimeter to centimeter scale width allows for the realization of compact optical undulators with thousands of periods. When laser pulses for TWTS are prepared, care has to be taken of laser dispersion. Especially for scenarios featuring interaction angles of several ten to over one hundred degree the angular dispersion originating from laser pulse-front tilt can significantly prolong the pulse duration during the interaction which leads to a decrease in optical undulator amplitude and eventually terminates the interaction long before the target interaction distance is reached. In the talk it is shown how a pair of two gratings can be used to first generate the pulse-front tilt and second control and compensate dispersion during the interaction by utilizing the plane of optimum compression. Furthermore an experimental setup strategy is presented allowing for an interaction outside the laser pulse focus. This is a necessity for TWTS OFELs requiring focusing to reach optical undulator strengths on the order of unity since the centimeter scale laser pulse width at the interaction point result in turn in Rayleigh lengths on the order of one hundred meter and thus in laser focusing distances of several hundred meter. The talk shows how an out-of-focus interaction geometry utilizing strong focusing of the incident laser pulse needs to be designed in order to regain compactness by reducing the focusing distance by one to two orders of magnitude.

Broadband, diode-pumped Yb-doped fused bulk silica laser

We successfully demonstrated cw lasing of ytterbium-doped fused bulk silica glass. We achieved a highly polarized output with a slope efficiency of 52% and a wavelength tuning range from 1005–1110 nm.

High-energy diode-pumped D2O-cooled multislab Yb:YAG and Yb:QX-glass lasers

We investigated the performance of multislab Yb:QX and Yb:YAG laser amplifiers using low absorption heavy-water (D2O) as coolant. We demonstrated a pulse energy of 1 J at a repetition rate of up to 10 Hz.

Diode-pumped Yb:LuAG and Yb:YAG disk laser amplifiers with high pulse energies

Ytterbium-doped garnets are attractive for use in high-energy diode-pumped solid-state lasers due to their spectral and thermal properties. Yb:YAG is one of the most well developed thin-disk laser materials. Recently, Yb:LuAG [1-4] was investigated as a promising candidate for kW thin-disk lasers because of its superior thermal conductivity especially at high doping concentrations compared to Yb:YAG. In this paper we demonstrate high-energy ns-pulse multipass amplification in Yb:YAG and Yb:LuAG. Pulses with a duration of 6 ns at a center wavelength of 1030 nm were generated in a cavity dumped oscillator and amplified to 20 mJ in an Yb:YAG based booster amplifier. The gain medium of a further (main) amplifier was pumped by a stack of micro-lens homogenized laser diodes operating at 940 nm a peak output power of 4kW. The pump light is focussed to a spot size of 4 mm into the disk while the reflected (not absorbed) light was recovered by an imaging system leading to two double-passes through the gain medium. The output of the booster is sent into a semi-stable resonator cavity (mirror based 4-f imaging system around the laser disk) forming a 10-pass amplifier.

High-energy Laser Amplifiers using Ultra-broad Emitting Yb-doped Materials

High-energy laser amplifiers using Yb-doped gain media often show reduced efficiencies especially when operating at ultra-short pulses. We summarize techniques to improve storage and extraction efficiencies of Yb-based laser amplifiers.

Pulse Shortening by spectral gain modulation in a regenerative Yb:CaF2 laser amplifier

We successfully demonstrate bandwidth enhancement via gain modulation in a regenerative Yb:CaF2 amplifier implementing a birefringent quartz crystal. 260 fs pulses of a Yb:KGW oscillator can be shortened down to 220 fs after amplification.