We report on the development of a carrier-envelope phase (CEP)-stable 1030 nm fiber-based laser system producing 6.2 fs pulses achieved via the multi-pass cell (MPC) post-compression technique with 402 W average power at 100 kHz repetition rate. This system employs an upgraded three-stage MPC compression scheme exhibiting excellent beam quality properties for this intensity region. Active stabilization locks the CEP noise below 430 mrad root mean square. This work represents the first demonstration of a coherently combined fiber laser system simultaneously achieving such exceptional average power, CEP stability and sub-two-cycle pulse durations. Similar to all other light sources of the Extreme Light Infrastructure Attosecond Light Pulse Source, this newly developed system is accessible to the international research community in peer-reviewed open user calls of the Extreme Light Infrastructure European Research Infrastructure Consortium.

We demonstrate enhanced spectral broadening of femtosecond pulses (330 fs, 1030 nm, 5-200 kHz) in a hollow-core fiber (HCF) using CO2/helium gas mixtures. While increasing pure CO2 pressure broadens the spectrum and induces a Raman redshift, it causes substantial optical loss. Introducing a helium buffer enables strong broadening at reduced CO2 pressures, significantly decreasing loss. Propagating pulses through a 1-m-long, 300-μm-core HCF, we show that helium enhances spectral width, extends the Raman shift, and improves transmitted power by facilitating spatial cooling. This binary molecular/noble gas approach provides a robust, low-loss route for spectral engineering in HCFs.

The ALPS Facility of ELI ERIC provides few-cycle light pulse sources to the international user community. A key group of beamlines utilizes the 1 mJ, sub-6 fs, 100 kHz fiber-based laser system, HR1, which supports attosecond high harmonic generation applications and time-resolved spectroscopy experiments. While the high repetition rate facilitates statistical analysis, it also puts a high thermal demand on beam delivery and targetry due to the high average power. In this paper, we present a laser system as a solution for the high demand of discrete repetition rates from 10 Hz to 10 kHz, spanning several orders of magnitude below the 100 kHz of the HR1 system. This apparatus, referred to as the HR Alignment system, is integrated into the same HHG beamline in the ELI ALPS facility where the 100 kHz HR1 system can also be operated. It offers tunable repetition rate (10 Hz to 10 kHz), carrier-envelope phase (CEP) stabilization, and delivers sub-6 fs pulses with 1 mJ energy. The system comprises an Yb:KGW frontend, two nonlinear compression stages based on multipass cell technology, and precisely designed chirped mirrors. Detailed characterization reveals excellent energy stability, beam quality, temporal contrast, and CEP stability below 300 mrad, all designed to match the pulse parameters of the HR1 laser. This compact, robust, and turnkey system provides high flux for reaction microscopy experiments based on high harmonic generation (HHG) and serves as a valuable asset for ultrafast research for external users of ELI ALPS.

The ELI-ALPS High Repetition Rate laser laboratory provides high-power, ultrashort laser pulses for statistical investigation of ultrafast phenomena [1]. Our laser systems deliver few-millijoule pulses in the near-infrared (1030 nm) at repetition rates up to 100 kHz. This high repetition rate enables experiments that require large number of observations, such as those resolving quantum-mechanical phenomena with single-photon sensitivity or studying events with rare occurrence.

The ELI ERIC facility offers international users ultrashort laser sources via the ALPS facility focusing on few-cycle laser drivers for attosecond pulses, particle beams and ultrahigh intensity interactions. The HR1 system supports attosecond high harmonic generation (HHG) and time-resolved spectroscopy at 100 kHz. However, its high repetition rate, while aiding statistical analysis, poses thermal challenges and limits certain applications requiring lower repetition rates. To address this, the HR Alignment laser system was developed for the HHG beamline at the ELI-ALPS facility. This new system delivers sub-6 fs, 1 mJ pulses with a tunable repetition rate (from 10 Hz to 10 kHz) and carrier-envelope phase (CEP) stabilization. It utilizes an ytterbium-doped potassium gadolinium tungstate front-end, multi-pass cell compression, and chirped mirrors. Characterization confirms excellent energy and CEP stability (below 300 mrad), beam quality and temporal contrast, matching the HR1 laser's performance. This compact, stable system provides high-flux attosecond generation for reaction microscopy enhancing ultrafast research in the ELI-ALPS facility.

This study systematically investigates the effects of beam diameter on perforation speed and hole quality in 1.5 mm thick acrylonitrile butadiene styrene (ABS) using a 1.08 mu m fiber laser. Experiments were conducted with laser powers up to 560 W and beam diameters ranging from 0.5 to 30 mm. Perforation time was defined as the duration required to achieve 80% light transmission through the sample. Statistical analysis reveals a strong correlation between laser power, beam diameter, and perforation speed. Notably, our findings highlight the significant impact of shielding effects - including smoke generation, plasma formation, and particularly the obstruction by molten material - on the penetration dynamics of 1.08 mu m laser radiation in ABS. These results provide insights for optimizing laser parameters to achieve perforation on ABS samples with large beam sizes and from large stand-off distances, which is relevant for remote laser processing.

We report on the development of a carrier-envelope phase (CEP) stable 1030 nm fiber-based laser system producing 6.2 fs pulses achieved via the multi-pass cell (MPC) post-compression technique with 402 W average power at 100 kHz repetition rate. This system employs an upgraded three-stage MPC compression scheme exhibiting excellent beam quality properties for this intensity region. Active stabilization locks the CEP noise below 430 mrad RMS. This work represents the first demonstration of a coherently combined fiber laser system simultaneously achieving such exceptional average power, CEP stability, and sub-two-cycle pulse durations. Similarly to all other light sources of the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI ALPS), this newly developed system is accessible to the international research community in peer-reviewed open user calls of Extreme Light Infrastructure European Research Infrastructure Consortium (ELI ERIC).

Surface scattering loss in anti-resonant fibers is analyzed using simplified and rigorous models for design optimization. To accurately match results across various fiber designs, a slight increase in the simplified model’s exponential parameter is needed.

A method using a tunable Long Period Grating is proposed to assess the microbend sensitivity of single-mode optical fibers. This method aids to understand the precise manner of how leaky modes contribute to microbending loss.

The ELI-ALPS High Repetition rate (HR-1 & HR-2) laser systems [1], [2] provide millijoule level, few-cycle laser pulses at 100 kHz repetition rate. The laser systems work at around 1030 nm in the near infrared wavelength region. We report the current status and state-of-the art parameter lists of these lasers which will become available to the users in the next commissioning user call of ELI-ALPS.

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.

Different deformation functions to find the best fit to the experimental data of micro-bending loss measurements of optical fibers are investigated. Best outcome, fitting the parameters is in the form of a Gaussian power spectrum. © 2022 The Author(s)

We present a sub-2-cycle laser system combining high average power, pulse energy and repetition rate with CEP-stable operation. The laser system creates 300 fs pulses with 1.8 mJ pulse energy that are nonlinearly post-compressed down to few optical cycles in two subsequent multipass cells (MPC). A pulse duration of 5.8fs (sub-2-cycle) at a pulse energy of 1.1mJ in combination with 110W average power (100 kHz) is achieved. This corresponds to the shortest pulses and highest compressed average power for few-cycle MPCs. Furthermore, the carrier-to-envelope-phase stability amounts to 300 mrad for frequencies above 2 kHz as measured by stereo—above-threshold-ionization (ATI).

Litz wires consist of many individual, isolated conductor strands. Certain litz wires are fabricated by simply twisting the bundle of strands, hence a wire pitch is realized. In this paper, the ac resistance of coils made of such twisted litz wire are calculated by combining two-dimensional finite element simulation and circuit laws. In so doing, the inherently three-dimensional problem can be solved at much lower computational cost. Results are presented for the current distribution, and for the alternating current resistance of coils, validated against measured data.

Attosecond pulses were widely utilized to investigate ultra-fast phenomena in the natural time scale of electron motion - attosecond (10 −18 s) - since its first demonstrations 20 years ago [1] , [2] . However, the repetition rate of most attosecond sources were limited to several kHz, which hindered many applications such as the photoelectron emission from solids where the space charge effect is needed to be alleviated, and the statistic measurements where less events every laser shot are preferred. Hence, to achieve high signal-to-noise ratio, the only method is to increase the repetition rate. In addition, the substantially shortened time for data acquisition is beneficial for most experiments. Thanks to the high-repetition-rate (HR) laser, HR attosecond pulses are possible now.

Thin diameter fibers with 80μm cladding diameters were designed and fabricated. The limitation of total fiber diameters was investigated for step- and 2-types trench-index fibers taking ITU-T standard, micro-bending loss, and mechanical reliabilities, into account.

We present a CEO-stable 1.1 kW CPA system that is designed to drive a few-cycle-generation stage ( 300W of average power at 100kHz repetition-rate providing <10fs pulses. The chirped-pulse-amplification system (CPA) demonstrates excellent noise properties with <220mrad of the integrated carrier-envelope-offset (CEO) noise (10Hz to 20MHz) at a pulse repetition rate of 80MHz while the relative-intensity-noise (RIN) stayed <0.3%. This is the first CEO-stable laser system at 1kW level average power.

Simulation model to calculate the micro-bending loss, based on the coupled mode theory with additional empirical parameters is fitted to our measurement data. Relation between micro-bending loss, effective core area and design parameters are investigated.

We present a sub-2 cycle laser system combining high average power, pulse energy and repetition rate with CEP-stable operation making this system an ideal driver for next-generation attosecond sources. The system is based on a fiber-chirped pulse amplification system incorporating coherent combination of 8 amplifier channels. The latter emits 300fs pulses, which are post-compressed down to the few-cycle regime in a two-stage multi-pass cell (MPC) nonlinear pulse compression. The first multi-pass cell is based on standard dielectric mirrors and achieves 1.7mJ, <35 fs pulses. The second MPC utilizes metal-based mirrors allowing to achieve a pulse duration of 5.8fs at a pulse energy of 1.1mJ, which corresponds to the shortest pulses and highest compressed average power for few-cycle MPCs with 110W at the used 100kHz pulse repetition rate. The stabilization of the carrier-to-envelope phase is presented.

Few-cycle laser systems providing both high repetition rates and high pulse energies are a major focus of next-generation light sources. Consequently, the research facility of the Extreme Light Infrastructure (ELI) that is devoted to the generation of isolated attoseconds pulses, (ELI-ALPS in Szeged, Hungary), has set the demand for a laser system delivering carrier-envelope phase (CEP) stable pulses with 5 mJ of pulse energy, 6 fs pulse duration at 100 kHz repetition rate, which corresponds to an average power of 500 W [1] . This laser system, that is named HR2 (the high-repetition-rate beam line), is currently under development at Active Fiber System GmbH. Achieving these ambitious laser parameters is done by merging the robust techniques of coherent combination as an average-power scaling concept and the use of stretched hollow-core fiber technology for nonlinear pulse compression [2] .