Carrier-envelope offset stable, coherently combined ytterbium-doped fiber CPA delivering 1 kW of average power.

We present a carrier-envelope offset (CEO) stable ytterbium-doped fiber chirped-pulse amplification system employing the technology of coherent beam combining and delivering more than 1 kW of average power at a pulse repetition rate of 80 MHz. The CEO stability of the system is 220 mrad rms, characterized out-of-loop with an f-to-2f interferometer in a frequency offset range of 10 Hz to 20 MHz. The high-power amplification system boosts the average power of the CEO stable oscillator by five orders of magnitude while increasing the phase noise by only 100 mrad. No evidence of CEO noise deterioration due to coherent beam combining is found. Low-frequency CEO fluctuations at the chirped-pulse amplifier are suppressed by a "slow loop" feedback. To the best of our knowledge, this is the first demonstration of a coherently combined laser system delivering an outstanding average power and high CEO stability at the same time.

High average power nonlinear compression to 4 GW, sub-50 fs pulses at 2 µm wavelength.

The combination of high-repetition-rate ultrafast thulium-doped fiber laser systems and gas-based nonlinear pulse compression in waveguides offers promising opportunities for the development of high-performance few-cycle laser sources at 2 µm wavelength. In this Letter, we report on a nonlinear pulse compression stage delivering 252 µJ, sub-50 fs-pulses at 15.4 W of average power. This performance level was enabled by actively mitigating ultrashort pulse propagation effects induced by the presence of water vapor absorptions.

High speed and high resolution table-top nanoscale imaging.

We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.

Nonlinear compression of an ultrashort-pulse thulium-based fiber laser to sub-70 fs in Kagome photonic crystal fiber.

Nonlinear pulse compression of ultrashort pulses is an established method for reducing the pulse duration and increasing the pulse peak power beyond the intrinsic limits of a given laser architecture. In this proof-of-principle experiment, we demonstrate nonlinear compression of the pulses emitted by a high-repetition-rate thulium-based fiber CPA system. The initial pulse duration of about 400 fs has been shortened to <70 fs with 19.7 µJ of pulse energy, which corresponds to about 200 MW of pulse peak power.

Real-time and sub-wavelength ultrafast coherent diffraction imaging in the extreme ultraviolet.

Coherent Diffraction Imaging is a technique to study matter with nanometer-scale spatial resolution based on coherent illumination of the sample with hard X-ray, soft X-ray or extreme ultraviolet light delivered from synchrotrons or more recently X-ray Free-Electron Lasers. This robust technique simultaneously allows quantitative amplitude and phase contrast imaging. Laser-driven high harmonic generation XUV-sources allow table-top realizations. However, the low conversion efficiency of lab-based sources imposes either a large scale laser system or long exposure times, preventing many applications. Here we present a lensless imaging experiment combining a high numerical aperture (NA = 0.8) setup with a high average power fibre laser driven high harmonic source. The high flux and narrow-band harmonic line at 33.2 nm enables either sub-wavelength spatial resolution close to the Abbe limit (Δr = 0.8λ) for long exposure time, or sub-70 nm imaging in less than one second. The unprecedented high spatial resolution, compactness of the setup together with the real-time capability paves the way for a plethora of applications in fundamental and life sciences.

Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources.

We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 µW and µJ per harmonic using the respective generation mechanisms.

Improving carrier-envelope phase stability in optical parametric chirped-pulse amplifiers by control of timing jitter.

It is shown that timing jitter in optical parametric chirped-pulse amplification induces spectral drifts that transfer to carrier-envelope phase (CEP) instabilities via dispersion. Reduction of this effect requires temporal synchronization, which is realized with feedback obtained from the angularly dispersed idler. Furthermore, a novel method to measure the CEP drifts by utilizing parasitic second harmonic generation within parametric amplifiers is presented. Stabilization of the timing allows the obtainment of a CEP stability of 86 mrad over 40 min at 150 kHz repetition rate.

Generation of µW level plateau harmonics at high repetition rate.

The process of high harmonic generation allows for coherent transfer of infrared laser light to the extreme ultraviolet spectral range opening a variety of applications. The low conversion efficiency of this process calls for optimization or higher repetition rate intense ultrashort pulse lasers. Here we present state-of-the-art fiber laser systems for the generation of high harmonics up to 1 MHz repetition rate. We perform measurements of the average power with a calibrated spectrometer and achieved µW harmonics between 45 nm and 61 nm (H23-H17) at a repetition rate of 50 kHz. Additionally, we show the potential for few-cycle pulses at high average power and repetition rate that may enable water-window harmonics at unprecedented repetition rate.

Analysis and suppression of parasitic processes in noncollinear optical parametric amplifiers.

The influence of parasitic processes on the performance of ultra-broadband noncollinear optical parametric amplifiers (NOPA's) is investigated for walk-off and non-walk-off compensating configurations. Experimental results with a white-light-seeded NOPA agree well with numerical simulations. The same model shows that 10% of the output energy of an amplified signal can be transferred into a parasitic second harmonic of the signal. These findings are supported by quantitative measurements on a few-cycle NOPA, where a few percent of the signal energy is converted to its second harmonic in the walk-off compensating case. This effect is reduced by an order of magnitude in the non-walk-off compensating configuration. A detailed study of the phase-matching conditions of the most common nonlinear crystals provides guidelines for designing NOPA systems.

Yb:YAG Innoslab amplifier: efficient high repetition rate subpicosecond pumping system for optical parametric chirped pulse amplification.

We report on a Yb:YAG Innoslab laser amplifier system for generation of subpicsecond high energy pump pulses for optical parametric chirped pulse amplification (OPCPA) at high repetition rates. Pulse energies of up to 20 mJ (at 12.5 kHz) and repetition rates of up to 100 kHz were attained with pulse durations of 830 fs and average power in excess of 200 W. We further investigate the possibility to use subpicosecond pulses to derive a stable continuum in a YAG crystal for OPCPA seeding.

Multi-gigawatt ultrashort pulses at high repetition rate and average power from two-stage nonlinear compression.

We present simple and compact (1.5m x 0.5m footprint) post-compression of a state-of-the-art fiber chirped pulse amplification system. By using two stage nonlinear compression in noble gas filled hollow core fibers we shorten 1 mJ, 480 fs, 50 kHz pulses. The first stage is a 53 cm long, 200 µm inner diameter fiber filled with xenon with subsequent compression in a chirped mirror compressor. A 20 cm, 200 µm inner diameter fiber filled with argon further broadens the spectrum in a second stage and compression is achieved with another set of chirped mirrors. The average power is 24.5 W/19 W after the first/second stage, respectively. Compression to 35 fs is achieved. Numerical simulations, agreeing well with experimental data, yield a peak power of 5.7 GW at a pulse energy of 380 µJ making this an interesting source for high harmonic generation at high repetition rate and average power.

High harmonic generation by novel fiber amplifier based sources.

Significant progress in high repetition rate ultrashort pulse sources based on fiber technology is presented. These systems enable operation at a high repetition rate of up to 500 kHz and high average power in the extreme ultraviolet wavelength range via high harmonic generation in a gas jet. High average power few-cycle pulses of a fiber amplifier pumped optical parametric chirped pulse amplifier are used to produce µW level average power for the strongest harmonic at 42.9 nm at a repetition rate of 96 kHz.

High average and peak power few-cycle laser pulses delivered by fiber pumped OPCPA system.

We report on a high power optical parametric amplifier delivering 8 fs pulses with 6 GW peak power. The system is pumped by a fiber amplifier and operated at 96 kHz repetition rate. The average output power is as high as 6.7 W, which is the highest average power few-cycle pulse laser reported so far. When stabilizing the seed oscillator, the system delivered carrier-envelop phase stable laser pulses. Furthermore, high harmonic generation up to the 33(th) order (21.8 nm) is demonstrated in a Krypton gas jet. In addition, the scalability of the presented laser system is discussed.

Fiber-amplifier pumped high average power few-cycle pulse non-collinear OPCPA.

We report on the performance of a 60 kHz repetition rate sub-10 fs, optical parametric chirped pulse amplifier system with 2 W average power and 3 GW peak power. This is to our knowledge the highest average power sub-10 fs kHz-amplifier system reported to date. The amplifier is conceived for applications at free electron laser facilities and is designed such to be scalable in energy and repetition rate.

CW seeded optical parametric amplifier providing wavelength and pulse duration tunable nearly transform limited pulses.

An optical parametric amplifier that delivers nearly transform limited pulses is presented. The center wavelength of these pulses can be tuned between 993 nm and 1070 nm and, at the same time, the pulse duration is varied between 206 fs and 650 fs. At the shortest pulse duration the pulse energy was increased up to 7.2 microJ at 50 kHz repetition rate. Variation of the wavelength is achieved by applying a tunable cw seed while the pulse duration can be varied via altering the pump pulse duration. This scheme offers superior flexibility and scaling possibilities.

Generation of flattop pump pulses for OPCPA by coherent pulse stacking with fiber Bragg gratings.

We present a simple and robust pulse shaping device based on coherent pulse stacking. The device is embedded in a polarisation maintaining step index fiber. An input pulse is sent through a fiber optical circulator. Up to four pulse replicas are reflected by fiber Bragg gratings and interfere at the output. Temperature control allows tuning of the relative pulse phases of the sub-pulses. Additionally fine tuning of the sub-pulse amplitudes is demonstrated. We experimentally generated 235 ps and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse durations up to several nanoseconds.

80 kHz repetition rate high power fiber amplifier flat-top pulse pumped OPCPA based on BIB3O6.

We present a high peak power optical parametric chirped pulse amplifier (OPCPA) seeded by a cavity dumped Ti:Sapphire oscillator. A frequency doubled high power Ytterbium-doped fiber amplifier is pumping the device. Temporal synchronization of the pump pulses is done via soliton generation in a highly nonlinear photonic crystal fiber. This soliton is fiber amplified and spectrally filtered in several fiber amplifiers. A simple birefringent pulse shaper generates a flat-top temporal pump pulse profile. Direct amplification of these pulses in large mode area fibers without using a stretcher and compressor provides significantly reduced complexity. For the first time to our knowledge broadband amplification around 800 nm central wavelength is demonstrated in BIB(3)O(6) (BIBO) crystals. The stretched Ti:Sapphire oscillator pulses are amplified up to a pulse energy of 25 microJ. Recompression with a grating compressor yields 50.7 fs pulses with 16.2 microJ pulse energy.

Compact fiber amplifier pumped OPCPA system delivering Gigawatt peak power 35 fs pulses.

We report on a compact Gigawatt peak power OPCPA system which is pumped by the second harmonic of an Yb-doped fiber amplifier and seeded by a cavity dumped Ti:Sapphire oscillator. Picosecond pump pulses for the OPCPA are generated by spectral filtering and directly amplified to 1 mJ pulse energy in several fiber amplifiers, without the need of chirped pulse amplification. Since no stretcher and compressor is required, the pump laser is very compact and easy to operate. The two stage optical parametric amplifier delivers 35 fs pulses with 53 microJ pulse energy and 1.1 GW peak power at 40 kHz repetition rate. Additionally, the scaling potential of this approach is discussed.

Degenerate optical parametric amplifier delivering sub 30 fs pulses with 2GW peak power.

Degenerated optical parametric amplification (OPA) is a well known technique to achieve broadband amplification necessary to generate ultrashort pulses. Here we present a parametric amplifier pumped by the frequency doubled output of a state-of-the-art fiber chirped pulse amplification system (FCPA) delivering mJ pulse energy at 30 kHz repetition rate and 650 fs pulse duration. The parametric amplifier and the FCPA system are both seeded by the same Yb:KGW oscillator. Additional spectral broadening of the OPA seed provides enough bandwidth for the generation of ultrashort pulses. After amplification in two 1mm BBO crystals a pulse energy of 90 microJ is yielded at 30 kHz. Subsequent compression with a sequence of chirped mirrors shortens the pulses to 29 fs while the pulse energy is as high as 81 microJ resulting in 2GW of peak power.

500 MW peak power degenerated optical parametric amplifier delivering 52 fs pulses at 97 kHz repetition rate.

We present a high peak power degenerated parametric amplifier operating at 1030 nm and 97 kHz repetition rate. Pulses of a state-of-the art fiber chirped-pulse amplification (FCPA) system with 840 fs pulse duration and 410 microJ pulse energy are used as pump and seed source for a two stage optical parametric amplifier. Additional spectral broadening of the seed signal in a photonic crystal fiber creates enough bandwidth for ultrashort pulse generation. Subsequent amplification of the broadband seed signal in two 1 mm BBO crystals results in 41 microJ output pulse energy. Compression in a SF 11 prism compressor yields 37 microJ pulses as short as 52 fs. Thus, pulse shortening of more than one order of magnitude is achieved. Further scaling in terms of average power and pulse energy seems possible and will be discussed, since both concepts involved, the fiber laser and the parametric amplifier have the reputation to be immune against thermo-optical effects.