Dispersion engineering in nonlinear multipass cells for high-quality pulse compression.

A dispersion-engineered multipass cell operating in the enhanced frequency regime is presented. Through the use of dispersive cavity mirrors, the nonlinear interaction is reshaped resulting in a smoother broadened spectrum, which yields a significant improvement in compressed pulse quality. The 70 W average power output of an Yb:fiber laser at 50 kHz repetition rate is compressed from 205 fs to 32 fs with more than 96% of the energy contained in the temporal main feature of the pulse. This first, to the best of our knowledge, experimental demonstration of a pulse quality improvement through enhanced frequency chirping in a multipass cell displays the opportunities for dispersion-tailored pulse compression.

Pulses of 32 mJ and 158 fs at 20-kHz repetition rate from a spatiotemporally combined fiber laser system.

A high-energy, high-power ultrafast fiber laser system based on spatiotemporal coherent combination is presented. Bursts of eight subsequent chirped-pulse amplification (CPA)-stretched pulses are amplified simultaneously in 16 parallel ytterbium-doped rod-type amplifiers. After spatial and temporal coherent combination of the total 128 amplified pulse replicas into a single pulse, it is compressed in a partially protective-gas-filled CPA compressor. Finally, nearly Fourier-transform-limited pulses with an energy of 32 mJ and a duration of 158 fs are emitted with a repetition rate of 20 kHz and a close to diffraction-limited beam quality.

Continuously tunable high photon flux high harmonic source.

In this work, a continuously tunable extreme ultraviolet source delivering a state-of-the-art photon flux of >1011 ph/s/eV spanning from 50 eV to 70 eV is presented. The setup consists of a high-power fiber laser with a subsequent multipass cell followed by a waveguide-based high harmonic generation setup. Spectral tuning over the full line spacing is achieved by slightly adjusting the lasers driving pulse energy, utilizing nonlinear propagation effects and pulse chirping. The presented method enables a high tuning speed while delivering reproducible and reliable results due to a simple experimental realization. For possible future experiments, a method for continuous, on-demand pulse-to-pulse switching of the generated XUV radiation with full spectral coverage is conceived.

Gas-plasma-based generation of broadband terahertz radiation with 640 mW average power.

We present a high-power source of broadband terahertz (THz) radiation covering the whole THz spectral region (0.1-30 THz). The two-color gas plasma generation process is driven by a state-of-the-art ytterbium fiber chirped pulse amplification system based on coherent combination of 16 rod-type amplifiers. Prior to the THz generation, the pulses are spectrally broadened in a multipass cell and compressed to 37 fs with a pulse energy of 1.3 mJ at a repetition rate of 500 kHz. A gas-jet scheme has been employed for the THz generation, increasing the efficiency of the process to 0.1%. The air-biased coherent detection scheme is implemented to characterize the full bandwidth of the generated radiation. A THz average power of 640 mW is generated, which is the highest THz average power achieved to date. This makes this source suitable for a variety of applications, e.g., spectroscopy of strongly absorbing samples or driving nonlinear effects for the studies of material properties.

Scaling potential of beam-splitter-based coherent beam combination.

The impact of nonlinear refraction and residual absorption on the achievable peak- and average power in beam-splitter-based coherent beam combination is analyzed theoretically. While the peak power remains limited only by the aperture size, a fundamental average power limit is given by the thermo-optical and thermo-mechanical properties of the beam splitter material and its coatings. Based on our analysis, 100 kW average power can be obtained with state-of-the-art optics at maintained high beam quality (M2 ≤ 1.1) and at only 2% loss of combining efficiency. This result indicates that the power-scaling potential of today's beam-splitter-based coherent beam combination is far from being depleted. A potential scaling route to megawatt-level average power is discussed for optimized beam splitter geometry.

Multipass cell for high-power few-cycle compression.

A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 µJ pulse energy is obtained at 82% cell transmission. A high output beam quality (${{\rm{M}}^2} \lt {1.2}$) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.

Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries.

Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics.

1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system.

An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air.

Kilowatt-average-power compression of millijoule pulses in a gas-filled multi-pass cell.

We demonstrate the reliable generation of 1-mJ, 31-fs pulses with an average power of 1 kW by post-compression of 200-fs pulses from a coherently combined Yb:fiber laser system in an argon-filled Herriott-type multi-pass cell with an overall compression efficiency of 96%. We also analyze the output beam, revealing essentially no spatiospectral couplings or beam quality loss.

Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power.

We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

23 mJ high-power fiber CPA system using electro-optically controlled divided-pulse amplification.

The pulse-energy scaling technique electro-optically controlled divided-pulse amplification is implemented in a high-power ultrafast fiber laser system based on coherent beam combination. A fiber-integrated front end and a multipass-cell-based back end allow for a small footprint and a modular implementation. Bursts of eight pulses are amplified parallel in up to 12 ytterbium-doped large-pitch fiber amplifiers. Subsequent spatiotemporal coherent combination of the 96 total amplified pulse replicas to a single pulse results in a pulse energy of 23 mJ at an average power of 674 W, compressible to a pulse duration of 235 fs. To the best of our knowledge, this is the highest pulse energy ever accomplished with a fiber chirped-pulse amplification (CPA) system.

3.5 kW coherently combined ultrafast fiber laser.

An ultrafast laser based on the coherent beam combination of four ytterbium-doped step-index fiber amplifiers is presented. The system delivers an average power of 3.5 kW and a pulse duration of 430 fs at an 80 MHz repetition rate. The beam quality is excellent (M2<1.24·1.10), and the relative intensity noise is as low as 1% in the frequency span from 1 Hz to 1 MHz. The system is turn-key operable, as it features an automated spatial and temporal alignment of the interferometric amplification channels.

Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays.

We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.

Electro-optically controlled divided-pulse amplification.

A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.