Structured illumination ptychography and at-wavelength characterization with an EUV diffuser at 13.5†nm wavelength.

Structured illumination is essential for high-performance ptychography. Especially in the extreme ultraviolet (EUV) range, where reflective optics are prevalent, the generation of structured beams is challenging and, so far, mostly amplitude-only masks have been used. In this study, we generate a highly structured beam using a phase-shifting diffuser optimized for 13.5 nm wavelength and apply this beam to EUV ptychography. This tailored illumination significantly enhances the quality and resolution of the ptychography reconstructions. In particular, when utilizing the full dynamics range of the detector, the resolution has been improved from 125 nm, when using an unstructured beam, to 34 nm. Further, ptychography enables the quantitative measurement of both the amplitude and phase of the EUV diffuser at 13.5 nm wavelength. This capability allows us to evaluate the influence of imperfections and contaminations on its "at wavelength" performance, paving the way for advanced EUV metrology applications and highlighting its importance for future developments in nanolithography and related fields.

In-band pumped, Q-switched thulium-doped fiber laser system delivering 140 W and 7†mJ pulse energy.

We report on a highly efficient, in-band pumped, Q-switched, Tm-doped, rod-type master oscillator power amplifier (MOPA) system delivering up to 140 W average output power and 7 mJ pulse energy with a slope efficiency of 77% at 20 kHz repetition rate. The amplifier is pumped with Raman-shifted fiber lasers centered at 1692 nm. This in-band pump scheme for Tm-doped fiber lasers can significantly mitigate their quantum defect-related heat load limitations. At the same time, this pump wavelength yields a similar amount of storable and extractable energy to the state-of-the-art pumping at 793 nm. This approach has allowed for the development of highly efficient Tm-doped fiber laser systems combining a high average power and a high output pulse energy.

Mitigation of transverse mode instability by heat-load modulation.

We present the first experimental realization of a new mitigation strategy for TMI based on controlling the phase shift between the modal intensity pattern and the thermally induced refractive index grating. If specific modulation parameters are applied while pulsing the seed and/or pump radiation, the direction of energy transfer is forced from the higher-order modes into the fundamental mode. In this way, the fiber amplifier can operate at an average output power significantly higher than the TMI threshold with a diffraction-limited beam profile. A stable beam profile is observed at an average output power that is 83% higher than the TMI threshold of the free-running system, with an intra-burst average power that is 4.15 times higher than the TMI threshold.

Characterization of transverse mode instability with a 4-quadrant photodiode.

Transverse mode instability (TMI) represents the main limitation for the power scaling of fiber laser systems with a diffraction-limited beam quality. In this context, it has become increasingly important to find a cheap and reliable way to monitor and characterize TMI and distinguish this effect from other dynamic perturbations. In this work, with the help of a position-sensitive detector, a novel method is developed to characterize the TMI dynamics even in the presence of power fluctuations. The position information of the fluctuating beam is recorded in the X- and Y-axis of the detector, which are used to track the temporal evolution of the center of gravity of the beam. The trajectories described by the beam within a specific time window contain rich information about TMI, which can be used to gain further insight into this phenomenon.

Broadband ptychography using curved wavefront illumination.

We examine the interplay between spectral bandwidth and illumination curvature in ptychography. By tailoring the divergence of the illumination, broader spectral bandwidths can be tolerated without requiring algorithmic modifications to the forward model. In particular, a strong wavefront curvature transitions a far-field diffraction geometry to an effectively near-field one, which is less affected by temporal coherence effects. The relaxed temporal coherence requirements allow for leveraging wider spectral bandwidths and larger illumination spots. Our findings open up new avenues towards utilizing pink and broadband beams for increased flux and throughput at both synchrotron facilities and lab-scale beamlines.

Analysis of optical core-to-core coupling: challenges and opportunities in multicore fiber amplifiers.

In this work we study in detail core-to-core coupling effects in multicore fibers (MCFs) using a simulation tool based on supermodal interference. We pay particular attention to the impact of core area scaling, which plays an important role in prospective amplifier systems. We consider geometrical and optical properties of the MCF structure, including the ability for dense packaging of the cores but also the influence on the core guidance (V-parameter). In general, this study is important to unlock the power and energy scaling potential of the next-generation MCF amplifiers.

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.

High-speed and wide-field nanoscale table-top ptychographic EUV imaging and beam characterization with a sCMOS detector.

We present high-speed and wide-field EUV ptychography at 13.5 nm wavelength using a table-top high-order harmonic source. Compared to previous measurements, the total measurement time is significantly reduced by up to a factor of five by employing a scientific complementary metal oxide semiconductor (sCMOS) detector that is combined with an optimized multilayer mirror configuration. The fast frame rate of the sCMOS detector enables wide-field imaging with a field of view of 100 µm × 100 µm with an imaging speed of 4.6 Mpix/h. Furthermore, fast EUV wavefront characterization is employed using a combination of the sCMOS detector with orthogonal probe relaxation.

Mitigation of transverse mode instability by modal birefringence in polarization-maintaining fibers.

The effect of transverse mode instability (TMI) poses a fundamental obstacle for a further scaling of diffraction-limited, high-power fiber laser systems. In this work we present a theoretical and experimental study on the mitigation of TMI by modal birefringence in a polarization maintaining (PM) fiber. With the help of comprehensive simulations, we show that the thermally-induced refractive index grating responsible for TMI can be modified and washed out when light is coupled with a polarization input angle detuned from the main axes of the fiber. To confirm the theoretical predictions, we have designed and manufactured an Yb-doped large-mode-area PM fiber. Using this fiber, we have systematically investigated the dependence of the TMI threshold on the polarization input angle of the seed laser. We experimentally demonstrate that when the polarization input angle of the seed is aligned at 50° with respect to the slow-axis, the TMI threshold increases by a factor of 2, verifying the theory and the numerical simulations. A high speed polarization mode-resolved analysis of the output beam is presented, which reveals that at the onset of TMI both polarization axes fluctuates simultaneously.

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.

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.

22-W average power high pulse energy multipass-cell-based post-compression in the green spectral range.

A gas-filled multipass-cell-based post-compression of 515 nm wavelength second-harmonic pulses of an Yb:fiber laser from 240 fs to 15.7 fs is presented. The system delivers 0.44 mJ of pulse energy, 22.4 W of average power at 50.8 kHz with an overall efficiency of more than 40%. These results display the capabilities of multipass-cell-based post-compression schemes to move from the well-established near infrared spectral region to the undeveloped visible regime, allowing for high efficiencies in conjunction with energetic ultrashort pulses at high repetition rates. The unique combination of parameters in the green spectral range offers an immense potential for future developments of high photon flux higher-order harmonic sources.

High-power two-color plasma-based THz generation driven by a Tm-doped fiber laser.

We report on the efficient generation of broadband THz radiation based on a two-color gas-plasma scheme. Broadband THz pulses covering the whole THz spectral region, from 0.1-35 THz, are generated. This is enabled by a high-power, ultra-fast, thulium-doped, fiber chirped pulse amplification (Tm:FCPA) system and a subsequent nonlinear pulse compression stage that uses a gas-filled capillary. The driving source delivers 40 fs pulses at a central wavelength of 1.9 µm with 1.2 mJ pulse energy and 101 kHz repetition rate. Owing to the long driving wavelength and the use of a gas-jet in the THz generation focus, the highest reported conversion efficiency for high-power THz sources (>20 mW) of 0.32% has been achieved. The high efficiency and average power of 380 mW of the broadband THz radiation make this an ideal source for nonlinear, tabletop THz science.

Nonlinear pulse compression to sub-two-cycle, 1.3†mJ pulses at 1.9†µm wavelength with 132†W average power.

We report the nonlinear pulse compression of a high-power, thulium-doped fiber laser system using a gas-filled hollow-core fiber. The sub-two cycle source delivers 1.3 mJ pulse energy with 80 GW peak power at a central wavelength of 1.87 µm and an average power of 132 W. This is, so far, to the best of our knowledge, the highest average power of a few-cycle laser source reported in the short-wave infrared region. Given its unique combination of high pulse energy and high average power, this laser source is an excellent driver for nonlinear frequency conversion, toward terahertz, mid-infrared, and soft X-ray spectral regions.

Frequency-doubled Q-switched 4 × 4 multicore fiber laser system.

Frequency doubling of a Q-switched Yb-doped rod-type 4 × 4 multicore fiber (MCF) laser system is reported. A second harmonic generation (SHG) efficiency of up to 52% was achieved with type I non-critically phase-matched lithium triborate (LBO), with a total SHG pulse energy of up to 17 mJ obtained at 1 kHz repetition rate. The dense parallel arrangement of amplifying cores into a shared pump cladding enables a significant increase in the energy capacity of active fibers. The frequency-doubled MCF architecture is compatible with high-repetition-rate and high-average-power operation and may provide an efficient alternative to bulk solid-state systems as pump sources for high-energy titanium-doped sapphire lasers.

Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers.

Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

Inband-pumped, high-power thulium-doped fiber amplifiers for an ultrafast pulsed operation.

We investigate the influence of the pump wavelength on the high-power amplification of large-mode area, thulium-doped fibers which are suitable for an ultrashort pulsed operation in the 2 µm wavelength region. By pumping a standard, commercially available photonic crystal fiber in an amplifier configuration at 1692 nm, a slope efficiency of 80 % at an average output power of 60 W could be shown. With the help of simulations we investigate the effect of cross-relaxations on the efficiency and the thermal behavior. We extend our investigations to a rod-type, large-pitch fiber with very large mode area, which is exceptionally suited for high-energy ultrafast operation. Pumping at 1692 nm leads to a slope efficiency of 74 % with a average output power of 67 W, instead of the 38 % slope efficiency obtained when pumping at 793 nm. These results pave the way to highly efficient 2 µm fiber-based CPA systems.

Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate.

High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.

High-energy Q-switched 16-core tapered rod-type fiber laser system.

High-energy Q-switched master oscillator power amplifier systems based on rod-type 4 × 4 multicore fibers are demonstrated, achieving energy up to 49 mJ in ns-class pulses. A tapered fiber geometry is tested that maintains low mode order in large multimode output cores, improving beam quality in comparison to a similar fiber with no taper. The tapered fiber design can be scaled both in the number of amplifying cores and in the dimensions of the cores themselves, providing a potential route toward joule-class fiber lasers systems.

500†W rod-type 4 × 4 multicore ultrafast fiber laser.

We present a coherently combined femtosecond fiber chirped-pulse-amplification system based on a rod-type, ytterbium-doped, multicore fiber with 4 × 4 cores. A high average power of up to 500 W (after combination and compression) could be achieved at 10 MHz repetition rate with excellent beam quality. Additionally, < 500 fs pulses with up to 600 µJ of pulse energy were also realized with this setup. This architecture is intrinsically power scalable by increasing the number of cores in the fiber.