KEYWORDS: Hydrogen, Laser beam diagnostics, Laser systems engineering, Plasma, Cryogenics, Ion beams, Computer simulations, Data conversion, Data analysis, Mode locking
The development of high-intensity short-pulse lasers in the Petawatt regime offers the possibility to design new compact accelerator schemes by utilizing high-density targets for the generation of ion beams with multiple 10 MeV energy per nucleon. The optimization of the acceleration process demands comprehensive exploration of the plasma dynamics involved, for example via spatially and temporally resolved optical probing. Experimental results can then be compared to numerical particle-in-cell simulations, which is particularly sensible in the case of cryogenic hydrogen jet targets [1]. However, strong plasma self-emission and conversion of the plasma’s drive laser wavelength into its harmonics often masks the interaction region and interferes with the data analysis. Recently, the development of a stand-alone and synchronized probe laser system for off-harmonic probing at the DRACO laser operated at the Helmholtz-Zentrum Dresden–Rossendorf showed promising performance [2].
Here, we present an updated stand-alone probe laser system applying a compact CPA system based on a synchronized fs mode-locked oscillator operating at 1030 nm, far off the plasma’s drive laser wavelength of 800 nm. A chirped volume Bragg grating (Optigrate Corp) is used as a hybrid stretcher and compressor unit. The system delivers 160 fs pulses with a maximum energy of 0.9 mJ. By deploying the upgraded probe laser system in the laser-proton acceleration experiment with the renewable cryogenic hydrogen jet target, the plasma self-emission could be significantly suppressed while studying the temporal evolution of the expanding plasma jet. Recorded probe images resemble those of z-pinch experiments with metal wires and indicate a sausage-like instability along the jet axis, which will be discussed.
References
[1] L. Obst, et al. Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets. Sci. Rep., 7:10248, 2017.
[2] T. Ziegler, et al. Optical probing of high intensity laser interaction with micron-sized
cryogenic hydrogen jets. Plasma Phys. Control. Fusion, 2018. doi:10.1088/1361-6587/
aabf4f.
[3] C.P. João, et al. Dispersion compensation by two-stage stretching in a sub-400 fs, 1.2 mJ
Yb:CaF2 amplifier. Opt. Express, 22:10097–10104, 2014.
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.
The demand of highly efficient transparent electrodes without the use of rare earth materials such as indium requires a new generation of thin metallic films with both high transparency and electrical conductivity. For this purpose, Direct Laser interference Patterning was used to fabricate periodic hole-like surface patterns on thin metallic films in order to improve their optical transparency by selective laser ablation of the material and at the same time keeping the electrical properties at an acceptable level. Metallic films consisting of aluminum and copper with film thicknesses ranging between 5 and 40 nm were deposited on glass substrates and treated with nanosecond and picosecond pulse laser system. In order to analyze the processability of the films, the laser ablation threshold for each material as function of the layer thickness and pulse duration was firstly determined. After analyzing these initial experiments, the samples were structured with a 1.7 μm spatial period hole-like-pattern using three beam direct laser interference patterning. The structural quality of the fabricated structures was analyzed as function laser energy density (laser fluence) using scanning electron microscopy (SEM), atom force microscopy (AFM). Finally, optical and electrical properties of the films were characterized using optical spectroscopy, as well as surface impedance measurements.
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.
Highly transparent CaF2 has found many applications from the deep UV- to the IR-range. The optical quality and the
laser damage threshold are influenced by the purity and the real structure of the crystal. Both properties strongly depend
on raw material quality and growth conditions.
Production of pure CaF2 single crystals and their characterization are described. The authors´ process enables to produce
crystals up to diameters of 425mm with an internal transmittance of higher than 99.7% at 193nm (thickness 100mm) and
a homogeneity of refractive index below 1ppm for diameters >200mm.
A new approach is the growth of Yb3+ doped CaF2 crystals in such furnaces dedicated to large volumes. The advantage
of higher volume is a better homogeneity of the dopant concentration and the diffractive index in the crystal. Critical
mechanical properties especially of the doped fluoride have to be taken into account. The growth process has to be
adopted carefully to avoid stress, cracks and other crystal defects.
Data of refractive index homogeneity and stress birefringence are presented. A comparison of doped and undoped
crystals is made and an outlook for further improvement is given.
The segregation coefficient of the dopant which is important to be near to one is reported. The ratio Yb3+ /Yb2+ is
characterized spectroscopically. Differences between top and bottom of the crystal are shown.
Results of the real structure evaluation are presented. The most critical feature for high energy applications which are
strength and concentration of small angle grain boundaries are compared with that of undoped crystals.
We present temperature dependent gain measurements with different Ytterbium doped laser media, such as Yb:YAG, Yb:FP15-glass and Yb:CaF2 in a multi-pass amplifier setup. The temperature of these materials was adjusted arbitrarily between 100K and 300K, while heat removal was realized by transverse cooling. In order to obtain a good beam profile throughout the amplification process, we used an all-mirror based relay imaging setup consisting of a telescope accomplishing a 4f-imaging with a plane mirror in each image plane. The amplification beam is then coupled into the cavity and doing several round trips wandering over the surface of the spherical mirrors. Hence the laser material is placed in one of the image planes, the beam quality of the amplifier was ruled by the intensity profile of the pumping laser diodes consisting of two stacks with 2.5kW peak output power each. Due to the given damage threshold fluence, the output energy of the amplifier was limited to about 1J at a beam diameter of 4.5 mm (FWHM). The seed pulses with a duration of 6 ns were generated in a Yb:FP15-glass cavity dumped oscillator with further amplification up to the 100mJ level by a room temperature Yb:YAG multi pass amplifier. The 1 Hz repetition rate of the system was limited by the repetition rate of the front-end. With Yb:YAG for instance an output energy of 1.1 J with an record high optical to optical efficiency of more than 35% was achieved, which was further increased to 45% for 500 mJ output energy.
For laser performance simulations, optical properties of applied active materials have to be exactly known. Here we
report on temperature dependent emission and absorption cross section measurements for Yb:YAG, Yb:CaF2 and
Yb:FP15-glass. The temperature of the samples was aligned in steps of 20 K between 100 K and room temperature with
a liquid nitrogen driven cryostat. Absorption spectra were obtained with a fiber coupled white light source and
fluorescence spectra by excitation with a fiber coupled 10W laser diode at 970 nm. All spectral measurements were
performed with a scanning spectrum analyzer, providing a spectral resolution down to 0.05 nm. By applying the
McCumber relation in combination with the Fuchtbauer-Ladenburg method, we were able to obtain a valid emission
cross section over the whole range of interest from the measured data.
This paper will describe a comparative study on four different Yb-doped laser materials in a diode-pumped
femtosecond chirped-pulse amplifier. Broadband multi-pass amplification using single crystalline Yb:CaF2,
Yb:KGW, and Yb:YAG as well as Yb-doped fluoride phosphate glass was demonstrated. The seed pulses as
short as 85 fs were generated in a Ti:Sapphire oscillator tuned to a center-wavelength of 1030nm and then
boosted to the 100 μJ-level in a diode-pumped Yb:glass regenerative amplifier. Furthermore, the amplifier performance
at nanosecond and microsecond time-scales was analyzed. With regard to the 10-nm line-width of the
emission cross section the gain narrowing in Yb:YAG was investigated in particular. The pulse spectrum was
narrowed to 1.9nm at a total gain of 103 in an Yb:YAG multi-pass amplifier. This corresponds to a compressed
pulse duration of 0.85 ps. When seeding with nanosecond pulses of a Q-switched oscillator a maximum pulse
energy of 220mJ at a repetition rate of 10Hz has been achieved.
We report on a comparative study of the damage threshold of ytterbium-doped laser materials which are important
for diode-pumped, high-energy class short pulse lasers. Both surface and bulk damage thresholds at the lasing
wavelength of 1030 nm were investigated. A pulse duration of 6.4 ns was chosen which allows a scaling of the
damage threshold for gain media in q-switched lasers as well as chirped-pulse amplifiers. In order to achieve
comparability and repeatability of the damage measurements the surface preparation of the used samples was kept
constant. Furthermore, the correlation of the bulk damage threshold and the UV absorption spectra was analyzed.
Mathias Siebold, Christoph Wandt, Sandro Klingebiel, Zsuzsanna Major, Sergei Trushin, Izhar Ahmad, Tie-Jun Wang, Joachim Hein, Ferenc Krausz, Stefan Karsch
A diode-pumped chirped-pulse amplifier (CPA) system based on Yb:glass and Yb:YAG as a picosecond pump
source for a future ultra-high peak-power optical parametric chirped-pulse amplifier (OPCPA) is currently under
development. Pulses as short as 300 fs generated in an Yb:glass mode-locked oscillator have been stretched to the
nanosecond level. The seed pulses are then amplified up to an output pulse energy of 100mJ in a diode-pumped
Yb:glass regenerative amplifier and a subsequent Yb:YAG booster. At a small-signal gain of 103 in Yb:YAG the
initial pulse bandwidth of 4 nm (FWHM) has been gain-narrowed to 1.5 nm, which allows the re-compression to
1.1 ps. With a multi-pass Yb:YAG amplifier which has been seeded by a Q-switched sub-10-nanosecond laser
an output pulse energy of 2.9 J has been achieved. In quasi-cw-mode a peak output power of 7.6kW and a
tuning range of 5nm have been obtained. The foot-print-size of the multi-pass amplifier is 0.8m×1.1m which
illustrates the degree of system compactness.
Multi-pass amplification to the 10 joule level for a femto-second CPA laser system is aimed at diode-pumping Yb3+ doped fluoride-phosphate glass with an energy of 240 J at 940 nm. Collimated pump light of 1000 laser diode bars is focussed onto an a circular glass disk with 28mm diameter. A two-sided ring shaped assembly of diode stacks and attached optics is applied for longitudinal pumping. We developed a computer aided optimization routine for positioning single pump foci with size of 4 × 8mm2 to achieve a smooth homogeneously distributed top-hat shaped pump profile with a diameter of 18 mm. For monitoring purpose the pulse energy of each diode stack is measured with a solar panel placed behind a reflecting mirror.
The coherent adding (tiling) of gratings is a promising alternative to large single gratings for highest-power CPA lasers. However, in order to obtain both temporally and spatially undistorted beam profiles it is necessary to align the mosaic gratings accurately to less than λ/20 with respect to one another. This paper is aimed at a thorough description of the alignment procedure, which uses both spatial and temporal properties of the laser beam to detect grating misalignments. The main emphasis lies within the analysis of the k-space, which becomes accessible by focussing the compressed laser-pulse or a monochromatic alignment laser, respectively. We present the effect of grating misalignments on both focal area and near field by propagating continuous-wave, monochromatic laser light through a misaligned tiled grating compressor.
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