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Combining high peak and high average power has long been a key challenge of ultrafast laser technology, crucial for applications such as laser-plasma acceleration and strong-field physics. A promising solution lies in post-compressed ytterbium lasers, but scaling these to high pulse energies presents a major bottleneck. Post-compression techniques, particularly Herriott-type multi-pass cells (MPCs), have enabled large peak power boosts at high average powers but their pulse energy acceptance reaches practical limits defined by setup size and coating damage threshold. In this work, we address this challenge and demonstrate a novel type of compact, energy-scalable MPC (CMPC). By employing a novel MPC configuration and folding the beam path, the CMPC introduces a new degree of freedom for downsizing the setup length, enabling compact setups even for large pulse energies. We experimentally and numerically verify the CMPC approach, demonstrating post-compression of 8 mJ pulses from 1 ps down to 51 fs in atmospheric air using a cell roughly 45 cm in length at low fluence values. Additionally, we discuss the potential for energy scaling up to 200 mJ with a setup size reaching 2.5 m. Our work presents a new approach to high-energy post-compression, with up-scaling potential far beyond the demonstrated parameters. This opens new routes for achieving the high peak and average powers necessary for demanding applications of ultrafast lasers., Comment: 18 pages, 13 figures

Advancing ultrafast high-repetition-rate lasers to shortest pulse durations comprising only a few optical cycles while pushing their energy into the multi-millijoule regime opens a route towards terawatt-class peak powers at unprecedented average power. We explore this route via efficient post-compression of high-energy 1.2 ps pulses from an Ytterbium InnoSlab laser to 9.6 fs duration using gas-filled multi-pass cells (MPCs) at a repetition rate of 1 kHz. Employing dual-stage compression with a second MPC stage supporting a close-to-octave-spanning bandwidth enabled by dispersion-matched dielectric mirrors, a record compression factor of 125 is reached at 70% overall efficiency, delivering 6.7 mJ pulses with a peak power of about 0.3 TW. Moreover, we show that post-compression can improve the temporal contrast at picosecond delay by at least one order of magnitude. Our results demonstrate efficient conversion of multi-millijoule picosecond lasers to high-peak-power few-cycle sources, opening up new parameter regimes for laser plasma physics, high energy physics, biomedicine and attosecond science.

Control over intensity, shape, direction, and phase of coherent light is essential in numerous fields, reaching from gravitational wave astronomy over quantum metrology and ultrafast sciences to semi-conductor fabrication. Modern laser optics, however, frequently demands parameter regimes where either the wavelength or the optical power restricts control due to linear absorption, light-induced damage or optical nonlinearity. The properties of solid media, upon which most photonic control schemes rely, impose these limitations. We propose to circumvent these constraints using gaseous media tailored by high-intensity ultrasound waves. We demonstrate a first implementation of this approach by deflecting ultrashort laser pulses using ultrasound waves in ambient air, entirely omitting transmissive solid media. At optical peak powers of 20 GW exceeding previous limits of solid-based acousto-optic modulation by about three orders of magnitude, we reach a deflection efficiency greater than 50% while preserving excellent beam quality. Our approach is not limited to laser pulse deflection via acousto-optic modulation: gas-phase photonic schemes controlled by sonic waves can prospectively be translated to various optical methods, e.g., lenses or waveguides, rendering them effectively invulnerable against damage and opening up new spectral regions., Comment: 23 pages, including 9 pages of main text and 9 pages of Methods and Extended Data, 3 figures, 3 Extended Data figures, 1 Extended Data table

Few-cycle pulses present an essential tool to track ultrafast dynamics in matter and drive strong field effects. To address photon-hungry applications, high average power lasers are used which, however, cannot directly provide sub-100 fs pulse durations. Post-compression of laser pulses by spectral broadening and dispersion compensation is the most efficient method to overcome this limitation. Here, we demonstrate a notably compact setup which turns a 0.1 GW peak power, picosecond laser into a 2.9 GW peak power, 8.2 fs source. The 120-fold pulse duration shortening is accomplished in a two-stage hybrid multi-pass, multi-plate compression setup. To our knowledge, neither shorter pulses, nor higher peak powers have been reported to-date from bulk multi-pass cells alone, manifesting the power of the hybrid approach. It puts, for instance, compact, cost-efficient and high repetition rate attosecond sources within reach.

The serrodyne principle enables shifting the frequency of an electromagnetic signal by applying a linear phase ramp in the time domain [1]. This phenomenon has been exploited to frequency-shift signals in the radiofrequency (RF), microwave and optical region of the electromagnetic spectrum over ranges of up to a few GHz e.g. to analyse the Doppler shift of RF signals, for noise suppression and frequency stabilization [2-9]. Here, we employ this principle to shift the center frequency of high power femtosecond laser pulses over a range of several THz with the help of a nonlinear multi-pass cell. We demonstrate our method experimentally by shifting the central wavelength of a state-of-the-art 75 W frequency comb laser from 1030\,nm to 1060\,nm and to 1000\,nm. Furthermore, we experimentally show that this wavelength shifting technique supports coherence characteristics at the few Hz-level while improving the temporal pulse quality. The technique is generally applicable to wide parameter ranges and different laser systems, enabling efficient wavelength conversion of high-power lasers to spectral regions beyond the gain bandwidth of available laser platforms.

We report the broadband absorption spectrum of the $3\nu_{2}$ band of $^{14}$NH$_{3}$ near 4 $\mu m$. The data were recorded using a mid-infrared frequency comb coupled to a homebuilt Fourier-transform spectrometer with a resolution of 0.00501 $cm^{-1}$. Line positions, line intensities, self-broadening, and self-shift parameters for six rovibrational lines were determined at room temperature ($T=296$ K). Comparison with HITRAN 2016 shows good agreement at improved precision. The high precision and the rapid tunability of our experiment enables advanced fast spectroscopy of molecular gases.

Post-compression of ultra-short laser pulses via self-phase modulation is routinely employed for the generation of laser pulses with optical bandwidths reaching far beyond the laser gain limitations. While high compression factors can be routinely achieved, the compressed pulses typically suffer from temporal quality degradation. We numerically and experimentally analyze the deterioration of different measures of temporal quality with increasing compression factor and show how appropriate dispersion management and cascading of the post-compression process can be employed to limit the impact of this effect. The demonstrated saturation of pulse quality degradation at large compression factors puts novel femtosecond laser architectures based on post-compressed pico- or even nanosecond laser systems in sight.

As Ultrafast laser technology advances towards ever higher peak and average powers, generating sub-50 fs pulses from laser architectures that exhibit best power-scaling capabilities remains a major challenge. Here, we present a very compact and highly robust method to compress 1.24 ps pulses to 39 fs by means of only a single spectral broadening stage which neither requires vacuum parts nor custom-made optics. Our approach is based on the hybridization of the multi-plate continuum and the multi-pass cell spectral broadening techniques. Their combination leads to significantly higher spectral broadening factors in bulk material than what has been reported from either method alone. Moreover, our approach efficiently suppresses adverse features of single-pass bulk spectral broadening. We use a burst mode Yb:YAG laser emitting pulses with 80 MW peak power that are enhanced to more than 1 GW after post-compression. With only 0.19 % rms pulse-to-pulse energy fluctuations, the technique exhibits excellent stability. Furthermore, we have measured state-of-the-art spectral-spatial homogeneity and good beam quality of M$^2 = 1.2$ up to a spectral broadening factor of 30. Due to the method's simplicity, compactness and scalability, it is highly attractive for turning a high-power picosecond laser into an ultrafast light source that generates pulses of only a few tens of femtoseconds duration.

Dual-frequency comb spectroscopy permits broadband precision spectroscopic measurements with short acquisition time. A dramatic improvement of the maximal spectral bandwidth and the minimal measurement time can be expected when the lasers' pulse repetition rate is increased, owing to a quadratic dependence (Nyquist criterion). Here, we demonstrate a dual-comb system operating at a high repetition rate of 1 GHz based on mature, digitally-controlled, low-noise mode-locked lasers. Compared to conventional lower repetition rate ($\sim$100 MHz) oscillators, this represents a 100-fold improvement in terms of the Nyquist criterion, while still providing adequate spectral sampling even for trace gas absorption fingerprints. Two spectroscopy experiments are performed with acquisition parameters not attainable in a 100 MHz system: detection of water vapor absorption around 1375 nm, demonstrating the potential for fast and ambiguity-free broadband operation, and real-time acquisition of narrow gas absorption features across a spectral span of 0.6 THz (600 comb lines) in only 5 $\mu$s. Additionally, we show high mutual coherence of the lasers below the Hz-level, generating opportunities for broadband spectroscopy even with low-bandwidth detectors such as mid-infrared, imaging or photo-acoustic detectors.

The Free-Electron Laser (FEL) FLASH offers the worldwide still unique capability to study ultrafast processes with high-flux, high-repetition rate XUV and soft X-ray pulses. The vast majority of experiments at FLASH are of pump-probe type. Many of them rely on optical ultrafast lasers. Here, a novel FEL facility laser is reported which combines high average power output from Yb:YAG amplifiers with spectral broadening in a Herriott-type multi-pass cell and subsequent pulse compression to sub-100 fs durations. Compared to other facility lasers employing optical parametric amplification, the new system comes with significantly improved noise figures, compactness, simplicity and power efficiency. Like FLASH, the optical laser operates with 10 Hz burst repetition rate. The bursts consist of 800 $\mu$s long trains of up to 800 ultrashort pulses being synchronized to the FEL with femtosecond precision. In the experimental chamber, pulses with up to 50 $\mu$J energy, 60 fs FWHM duration and 1 MHz rate at 1.03 $\mu$m wavelength are available and can be adjusted by computer-control. Moreover, nonlinear polarization rotation is implemented to improve laser pulse contrast. First cross-correlation measurements with the FEL at the plane-grating monochromator photon beamline are demonstrated, exhibiting the suitability of the laser for user experiments at FLASH., Comment: 28 pages, 13 figures

We report a simple and compact design of a dispersion compensated mode-locked Yb:fiber oscillator based on a nonlinear amplifying loop mirror (NALM). The fully polarization maintaining (PM) fiber integrated laser features a chirped fiber Bragg grating (CFBG) for dispersion compensation and a fiber integrated compact non-reciprocal phase bias device, which is alignment-free. The main design parameters were determined by numerically simulating the pulse evolution in the oscillator and by analyzing their impact on the laser performance. Experimentally, we achieved an 88 fs compressed pulse duration with sub-fs timing jitter at 54 MHz repetition rate and 51 mW of output power with 5.5 * 10-5 [20 Hz, 1 MHz] integrated relative intensity noise (RIN). Furthermore, we demonstrate tight phase-locking of the laser's carrier-envelope offset frequency (fceo) to a stable radio frequency (RF) reference and of one frequency comb tooth to a stable optical reference at 291 THz., Comment: 8 pages, 11 figures, journal manuscript

We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.035 ps$^2$) to normal (+0.015 ps$^2$). We put a particular focus on the characterization of the intensity noise as well as the free-running linewidth of the carrier-envelope-offset (CEO) frequency as a function of the different operation regimes. We observe that operation points far from the spontaneous emission peak of Yb (~1030 nm) and close to zero intracavity dispersion can be found, where the influence of pump noise is strongly suppressed. For such an operation point, we show that a CEO linewidth of less than 10-kHz at 1 s integration can be obtained without any active stabilization.

In this work, we demonstrate post-compression of 1.2 picosecond laser pulses to 13 fs via gas-based multi-pass spectral broadening. Our results yield a single-stage compression factor of about 40 at 200 W in-burst average power and a total compression factor >90 at reduced power. The employed scheme represents a route towards compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

We demonstrate dual-comb generation from an all-polarization-maintaining dual-color ytterbium (Yb) fiber laser. Two pulse trains with center wavelengths at 1030 nm and 1060 nm respectively are generated within the same laser cavity with a repetition rate around 77 MHz. Dual-color operation is induced using a tunable mechanical spectral filter, which cuts the gain spectrum into two spectral regions that can be independently mode-locked. Spectral overlap of the two pulse trains is achieved outside the laser cavity by amplifying the 1030-nm pulses and broadening them in a nonlinear fiber. Spatially overlapping the two arms on a simple photodiode then generates a down-converted radio frequency comb. The difference in repetition rates between the two pulse trains and hence the line spacing of the down-converted comb can easily be tuned in this setup. This feature allows for a flexible adjustment of the tradeoff between non-aliasing bandwidth vs. measurement time in spectroscopy applications. Furthermore, we show that by fine-tuning the center-wavelengths of the two pulse trains, we are able to shift the down-converted frequency comb along the radio-frequency axis. The usability of this dual-comb setup is demonstrated by measuring the transmission of two different etalons while the laser is completely free-running.

We experimentally demonstrate in a difference-frequency generation mid-infrared frequency comb source the effect of temporal overlap between pump- and signal- pulse to the relative intensity noise (RIN) of the idler pulse. When scanning the temporal delay between our 130 fs long signal- and pump pulses, we observe a RIN minimum with a 3 dB width of 20 fs delay and an RIN increase of 20 dB in 40 fs delay at the edges of this minimum. We also demonstrate active long-term stabilization of the mid-infrared frequency comb source to the temporal overlap setting corresponding to the lowest RIN operation point by an on-line RIN-detector and active feedback control of the pump-signal- pulse delay. This active stabilization set-up allowed us to dramatically increase the signal-to-noise ratio of mid-infrared absorption spectra.

We demonstrate an experimental technique for both transverse and longitudinal characterization of bunched femtosecond free electron beams. The operation principle is based on monitoring of the current of electrons that obtained an energy gain during the interaction with the synchronized optical near-field wave excited by femtosecond laser pulses. The synchronous accelerating/decelerating fields confined to the surface of a silicon nanostructure are characterized using a highly focused sub-relativistic electron beam. Here the transverse spatial resolution of 450 nm and femtosecond temporal resolution achievable by this technique are demonstrated.

We present a comprehensive study of laser pulse amplification with respect to operation regimes, gain dynamics, and highly stable operation points of Ho:YLF regenerative amplifiers (RAs). The findings are expected to be more generic than for this specific case. Operation regimes are distinguished with respect to pulse energy and the appearance of pulse instability, and are studied as a function of the repetition rate, seed energy, and pump intensity. The corresponding gain dynamics are presented, identifying highly stable operation points related to high gain build -up during pumping and high gain depletion during pulse amplification. These operation points are studied numerically and experimentally as a function of several parameters, thereby achieving, for our Ho:YLF RA, highly stable output pulses with measured fluctuations of only 0.19% (standard deviation).

The widespread use of high energy particle beams in basic research, medicine and coherent X-ray generation coupled with the large size of modern radio frequency (RF) accelerator devices and facilities has motivated a strong need for alternative accelerators operating in regimes outside of RF. Working at optical frequencies, dielectric laser accelerators (DLAs) - transparent laser-driven nanoscale dielectric structures whose near fields can synchronously accelerate charged particles - have demonstrated high-gradient acceleration with a variety of laser wavelengths, materials, and electron beam parameters, potentially enabling miniaturized accelerators and table-top coherent x-ray sources. To realize a useful (i.e. scalable) DLA, crucial developments have remained: concatenation of components including sustained phase synchronicity to reach arbitrary final energies as well as deflection and focusing elements to keep the beam well collimated along the design axis. Here, all of these elements are demonstrated with a subrelativistic electron beam. In particular, by creating two interaction regions via illumination of a nanograting with two spatio-temporally separated pulsed laser beams, we demonstrate a phase-controlled doubling of electron energy gain from 0.7 to 1.4 keV (2.5 percent to 5 percent of the initial beam energy) and through use of a chirped grating geometry, we overcome the dephasing limit of 25 keV electrons, increasing their energy gains to a laser power limited 10 percent of their initial energy. Further, optically-driven transverse focusing of the electron beam with focal lengths below 200 microns is achieved via a parabolic grating geometry. These results lay the cornerstone for future miniaturized phase synchronous vacuum-structure-based accelerators.

In this paper we present proof of principle experiments of an optical gating concept for free electrons. We demonstrate a temporal resolution of 1.2+-0.3 fs via energy and transverse momentum modulation as a function of time. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams. We expect that 10 as temporal resolution will be achieved in the near future using such a scheme., Comment: 5 figures

We demonstrate a Ho:YLF regenerative amplifier (RA) overcoming bifurcation instability and consequently achieving high extraction energies of 6.9 mJ at a repetition rate of 1 kHz with pulse-to-pulse fluctuations of 1.1%. Measurements of the output pulse energy, corroborated by numerical simulations, identify an operation point that allows high-energy pulse extraction at a minimum noise level. Complete suppression of the onset of bifurcation was achieved by gain saturation after each pumping cycle in the Ho:YLF crystal via lowering the repetition rate and cooling the crystal. Even for moderate cooling, a significant temperature dependence of the Ho:YLF RA performance was observed.

We present a passively mode-locked, tunable soliton Ho:fiber ring oscillator, optimized for seeding of Ho:YLF amplifiers. The oscillator is independently tunable in central wavelength and spectral width from 2040 nm to 2070 nm and from 5 nm to 10 nm, respectively. At all settings the pulse energy within the soliton is around 800 pJ. The soliton oscillator was optimized to fully meets the spectral requirements for seeding Ho:YLF amplifiers. Its Kelly sidebands are located outside the amplifier gain spectrum, resulting in a train of about 1 ps long pedestal-free pulses with relative intensity noise (RIN) of only 0.13 % RMS when integrated from 1 Hz to Nyquist frequency.

We report on a mid-infrared frequency comb source of unprecedented tunability covering the entire 3-10 {\mu}m molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at 4.7 {\mu}m wavelength., Comment: 3 pages, 3 figures

Development of the optical frequency comb has revolutionised metrology and precision spectroscopy due to its ability to provide a precise and direct link between microwave and optical frequencies. A novel application of frequency comb technology that leverages both the ultrashort duration of each laser pulse and the exquisite phase coherence of a train of pulses is the generation of frequency combs in the extreme ultraviolet (XUV) via high harmonic generation (HHG) in a femtosecond enhancement cavity. Until now, this method has lacked sufficient average power for applications, which has also hampered efforts to observe phase coherence of the high-repetition rate pulse train produced in the extremely nonlinear HHG process. Hence, the existence of a frequency comb in the XUV has not been confirmed. We have overcome both challenges. Here, we present generation of >200 {\mu}W per harmonic reaching 50 nm, and the observation of single-photon spectroscopy signals for both an argon transition at 82 nm and a neon transition at 63 nm. The absolute frequency of the argon transition has been determined via direct frequency comb spectroscopy. The resolved 10-MHz linewidth of the transition, limited by the transverse temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Due to the lack of cw lasers, these frequency combs are currently the only promising avenue towards extending ultrahigh precision spectroscopy to below the 100-nm spectral region with a wide range of applications that include spectroscopy of electronic transitions in molecules, experimental tests of bound state and many body quantum electrodynamics in He+ and He, development of next-generation "nuclear" clocks, and searches for spatial and temporal variation of fundamental constants using the enhanced sensitivity of highly charged ions.

We present detailed studies of the coherence properties of an ultra-broadband super-continuum, enabled by a new approach involving continuous wave laser sources to independently probe both the amplitude and phase noise quadratures across the entire spectrum. The continuum coherently spans more than 1.5 octaves, supporting Hz-level comparison of ultrastable lasers at 698 nm and 1.54 {\mu}m. We present the first numerical simulation of the accumulated comb coherence in the limit of many pulses, in contrast to the single-pulse level, with systematic experimental verification. The experiment and numerical simulations reveal the presence of quantum-seeded broadband amplitude noise without phase coherence degradation, including the discovery of a dependence of the super-continuum coherence on the fiber fractional Raman gain., Comment: 5 pages, 3 figures

Optical frequency-comb-based-high-resolution spectrometers offer enormous potential for spectroscopic applications. Although various implementations have been demonstrated, the lack of suitable mid-infrared comb sources has impeded explorations of molecular fingerprinting. Here we present for the first time a frequency-comb Fourier transform spectrometer operating in the 2100-to-3700-cm-1 spectral region that allows fast and simultaneous acquisitions of broadband absorption spectra with up to 0.0056 cm-1 resolution. We demonstrate part-per-billion detection limits in 30 seconds of integration time for various important molecules including methane, ethane, isoprene, and nitrous oxide. Our system enables precise concentration measurements even in gas mixtures that exhibit continuous absorption bands, and it allows detection of molecules at levels below the noise floor via simultaneous analysis of multiple spectral features. This system represents a near real-time, high-resolution, high-bandwidth mid-infrared spectrometer which is ready to replace traditional Fourier transform spectrometers for many applications in trace gas detection, atmospheric science, and medical diagnostics., Comment: 23 pages (double spaced), 5 figures, 1 table

We present a high-power optical parametric oscillator-based frequency comb in the mid-infrared wavelength region using periodically poled lithium niobate. The system is synchronously pumped by a 10-W femtosecond Yb:fiber laser centered at 1.07 um and is singly resonant for the signal. The idler (signal) wavelength can be continuously tuned from 2.8 to 4.8 um (1.76 to 1.37 um) with a simultaneous bandwidth as high as 0.3 um and a maximum average idler output power of 1.50 W. We also demonstrate the performance of the stabilized comb by recording the heterodyne beat with a narrow-linewidth diode laser. This OPO is an ideal source for frequency comb spectroscopy in the mid-IR., Comment: 4 figures