Active Fiber Systems (Germany)

Two-stage multipass-cell compression of a fiber-chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality (M2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.

Recent milestones in power-scaling of ultrafast fiber-based lasers were achieved by the simultaneous mitigation of thermal and nonlinear effects through the coherent combination of ultrafast pulses [1] . This technique is based on splitting the light into several spatially separated amplification channels that are subsequently coherently recombined into a single beam. Besides the performance scaling aspect, the laser wavelength is an important parameter for many applications, e.g. for high-field physics due to the quadratic wavelength-dependence of the ponderomotive potential [2] . Tm-doped fiber lasers have proven to be promising and relatively straightforward candidates for the realization of efficient high average- and peak-power ultrafast lasers in the 2-µm wavelength region [3] , [4] . Here we demonstrate the coherent combination [1] , [5] of four thulium-doped fiber amplifiers. The fiber-based chirped pulse amplifier (CPA [6] ) delivers pulses with <120 fs full-width at half-maximum duration with up to 228 µJ of pulse energy at a center wavelength of 1940 nm.

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.

We generate multi-mJ, 10fs pulses at average powers beyond 300W by 30-times compression of the output of an Yb-fiber CPA in a 6m long stretched capillary. Further up-scaling of the parameters is in progress.

Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

We present sub-10-fs, 3.2mJ pulses at 100kHz repetition-rate. This significant upscaling of average power was achieved by using a 6-m-long stretched flexible capillary. Such a technique has the potential to revolutionize few-cycle laser beamlines.

Today, carrier-envelope-phase (CEP) stable laser pulses have become a versatile tool for a plethora of scientific applications. Many years their generation relied on either optical parametric amplification or the use of titanium-sapphire amplifiers. Although impressive results have been achieved using these technologies [1, 2], their main drawback is the restricted average power (and therewith repetition rate for a given energy) due to thermo-optical limitations. Here we report on another approach, the nonlinear compression of ultrafast ytterbium-based high-power fiber lasers [3]. The first commercially available source employing this technology is the HR1 laser constructed for the ELI-ALPS research facility in Szeged, Hungary. The Extreme Light Infrastructure (ELI) is currently being installed in several European countries aiming to provide unique user facilities with beyond state-of-the-art laser systems. The attosecond facility ELI-ALPS in Szeged, for example, will host several laser systems that will be used for attosecond pulse generation at unprecedented pulse parameters (energy and repetition rate). One of these laser systems is the HR1 (high repetition rate) laser that targets pulse parameters of 1mJ, 6fs pulses at 100kHz repetition rate (100W average power) and with CEP stable operation in its first implementation phase.