Two-color pumped OPCPA system emitting spectra spanning 1.5 octaves from VIS to NIR

Ultrabroad bandwidth of quasi-parametric amplification beyond the phase-matching limit

Quasi-parametric amplification (QPA), a variant of optical parametric amplification, can release the phase-matching requirement owing to the introduction of idler dissipation, and thus may support ultrabroad bandwidth. Here we establish the gain-dispersion equation for QPA, which reveals the interplay of signal gain, idler dissipation and phase mismatch. The idler dissipation dramatically enhances the gain bandwidth, which breaks the limit set by phase matching. We theoretically demonstrate that QPA with strong dissipation allows high-efficiency few-cycle pulse amplification in those nonlinear crystals without a magic phase-matching solution.

Serial synthesis of mid-infrared optical parametric amplifiers for enlarging a gain bandwidth

Broadband optical parametric amplifiers (OPAs) require a group-velocity matching between the signal and the idler. For mid-infrared OPAs, however, the group-velocity matching is usually difficult to meet, rendering a limited gain bandwidth. Here, we report a serial synthesis of bandwidth-limited OPAs to provide a broad gain bandwidth. In a proof-of-principle experiment, two mid-IR OPAs based on KTA crystals with different phase-matching angles are sequentially employed to amplify different spectral regions of a broad seed pulse centered at 3.1 µm. Compared to the traditional two-stage OPA, here the gain bandwidth is nearly doubled, resulting in a much shorter compressed pulse. Such a serial synthesis approach, independent of a nonlinear crystal and an interaction wavelength, particularly suits for enlarging the gain bandwidth of OPAs when broadband amplification is impossible to achieve by a single crystal.

400nm ultra-broadband gratings for near-single-cycle 100 Petawatt lasers

Compressing high-energy laser pulses to a single-cycle and realizing the "λ³ laser concept", where λ is the wavelength of the laser, will break the current limitation of super-scale projects and contribute to the future 100-petawatt and even Exawatt lasers. Here, we have realized ultra-broadband gold gratings, core optics in the chirped pulse amplification, in the 750-1150 nm spectral range with a > 90% -1 order diffraction efficiency for near single-cycle pulse stretching and compression. The grating is also compatible with azimuthal angles from -15° to 15°, making it possible to design a three-dimensional compressor. In developing and manufacturing processes, a crucial grating profile with large base width and sharp ridge is carefully optimized and controlled to dramatically broaden the high diffraction efficiency bandwidth from the current 100-200 nm to over 400 nm. This work has removed a key obstacle to achieving the near single-cycle 100-PW lasers in the future.

Light-field synthesizer based on multidimensional solitary states in hollow-core fibers

Few-cycle, long-wavelength sources for generating isolated attosecond soft x ray pulses typically rely upon complex laser architectures. Here, we demonstrate a comparatively simple setup for generating sub-two-cycle pulses in the short-wave infrared based on multidimensional solitary states in an N₂O-filled hollow-core fiber and a two-channel light-field synthesizer. Due to the temporal phase imprinted by the rotational nonlinearity of the molecular gas, the redshifted (from 1.03 to 1.36 µm central wavelength) supercontinuum pulses generated from a Yb-doped laser amplifier are compressed from 280 to 7 fs using only bulk materials for dispersion compensation.

Spatio-spectral couplings in optical parametric amplifiers

Optical parametric amplification (OPA) is a powerful tool for the generation of ultrashort light pulses. However, under certain circumstances, it develops spatio-spectral couplings, color dependent aberrations that degrade the pulse properties. In this work, we present a spatio-spectral coupling generated by a non-collimated pump beam and resulting in the change of direction of the amplified signal with respect to the input seed. We experimentally characterize the effect, introduce a theoretical model to explain it as well as reproduce it through numerical simulations. It affects high-gain non-collinear OPA configurations and becomes especially relevant in sequential optical parametric synthesizers. In collinear configuration, however, beyond the direction change, also angular and spatial chirp is produced. We obtain with a synthesizer about 40% decrease in peak intensity in the experiments and local elongation of the pulse duration by more than 25% within the spatial full width at half maximum at the focus. Finally, we present strategies to correct or mitigate the coupling and demonstrate them in two different systems. Our work is important for the development of OPA-based systems as well as few-cycle sequential synthesizers.

Simulating an ultra-broadband concept for Exawatt-class lasers

The rapid development of the optical-cycle-level ultra-fast laser technologies may break through the bottleneck of the traditional ultra-intense laser [i.e., Petawatt (PW, 10¹⁵ W) laser currently] and enable the generation of even higher peak-power/intensity lasers. Herein, we simulate an ultra-broadband concept for the realization of an Exawatt-class (EW, 10¹⁸ W) high peak-power laser, where the wide-angle non-collinear optical parametric chirped-pulse amplification (WNOPCPA) is combined with the thin-plate post-compression. A frequency-chirped carrier-envelope-phase stable super-continuum laser is amplified to high-energy in WNOPCPA by pumping with two pump-beamlets and injected into the thin-plate post-compression to generate a sub-optical-cycle high-energy laser pulse. The numerical simulation shows this hybrid concept significantly enhances the gain bandwidth in the high-energy amplifier and the spectral broadening in the post-compression. By using this concept, a study of a prototype design of a 0.5 EW system is presented, and several key challenges are also examined.

Supercontinuum generation and optical damage of sapphire and YAG at high repetition rates

We have experimentally investigated supercontinuum (SC) generation and the evolution of optical damage in sapphire and YAG crystals with 180 fs, 1035 nm pulses from an amplified Yb:KGW laser with variable repetition rates, both in tight and loose focusing conditions. In this Letter, we demonstrate that the extinction of the SC spectrum always correlates with an occurrence of conical third harmonic generation, which readily serves as an indication of the onset of in-bulk optical damage. Damage-related structural changes of the nonlinear material are also justified by an increased intensity and large red shift of crystal luminescence spectrum corresponding to the F center emission. The SC spectrum in sapphire starts shrinking on the time scale between seconds and minutes by varying the focusing condition from tight to loose at the laser repetition rate of 200 kHz, whereas the YAG crystal produces stable performance for several hours at least.

2.5 TW, two-cycle IR laser pulses via frequency domain optical parametric amplification

Broadband optical parametric amplification in the IR region has reached a new milestone through the use of a non-collinear Frequency domain Optical Parametric Amplification system. We report a laser source delivering 11.6 fs pulses with 30 mJ of energy at a central wavelength of 1.8 μm at 10 Hz repetition rate corresponding to a peak power of 2.5 TW. The peak power scaling is accompanied by a pulse shortening of about 20% upon amplification due to the spectral reshaping with higher gain in the spectral wings. This source paves the way for high flux soft X-ray pulses and IR-driven laser wakefield acceleration.

Next Generation Driver for Attosecond and Laser-plasma Physics

The observation and manipulation of electron dynamics in matter call for attosecond light pulses, routinely available from high-order harmonic generation driven by few-femtosecond lasers. However, the energy limitation of these lasers supports only weak sources and correspondingly linear attosecond studies. Here we report on an optical parametric synthesizer designed for nonlinear attosecond optics and relativistic laser-plasma physics. This synthesizer uniquely combines ultra-relativistic focused intensities of about 10²⁰ W/cm² with a pulse duration of sub-two carrier-wave cycles. The coherent combination of two sequentially amplified and complementary spectral ranges yields sub-5-fs pulses with multi-TW peak power. The application of this source allows the generation of a broad spectral continuum at 100-eV photon energy in gases as well as high-order harmonics in relativistic plasmas. Unprecedented spatio-temporal confinement of light now permits the investigation of electric-field-driven electron phenomena in the relativistic regime and ultimately the rise of next-generation intense isolated attosecond sources.

40-µJ passively CEP-stable seed source for ytterbium-based high-energy optical waveform synthesizers

We demonstrate experimentally for the first time a ~40-µJ two-octave-wide passively carrier-envelope phase (CEP)-stable parametric front-end for seeding an ytterbium (Yb)-pump-based, few-optical-cycle, high-energy optical parametric waveform synthesizer. The system includes a CEP-stable white-light continuum and two-channel optical parametric chirped pulse amplifiers (OPCPAs) in the near- and mid-infrared spectral regions spanning altogether a two-octave-wide spectrum driven by a regenerative amplifier. The output pulses are compressed and fully characterized to demonstrate the well-behaved spectral phase of this seed source.

Tunable broadband intense IR pulse generation at non-degenerate wavelengths using group delay compensation in a dual-crystal OPA scheme

A robust group delay compensated dual-crystal optical parametric amplification (DOPA) scheme is proposed that will be used to prove the positive effect of group delay compensation on a DOPA as predicted by the simulations in the previously published literature. Through simple adjustments, it is also capable of providing 20 fs pulses (theoretically compressible to 12 fs, corresponding to sub-four-cycle for 1300 nm components), broadband IR pulses at non-degenerate wavelengths using short pulse (broadband) pump laser. In our table-top DOPA system, group delay compensation has been realized using a simple optical crystal. Our design provides output power in order of 100 mW. We managed to achieve minimum 20 nm improvement on the bandwidth, compared to single-crystal OPA (SOPA) structure whilst keeping total conversion efficiency above 30%. Adjusting our configuration by optimizing the phase-matching angles of the two BBO crystals, we also have realized a practical scheme that benefitting from group delay compensation can obtain 75 nm bandwidth improvement while keeping the conversion efficiency constant. This achievement will open the doors to the realm of multiple crystals OPA systems and provide a solution to the imposed limitation on the effective lengths of applicable non-linear crystals and hence limited power gain of such broadband OPA systems.

Non-collinear spectral coherent combination of ultrashort laser pulses

Non-collinear spectral coherent combining (NCSCC) of ultrashort pulses is analyzed. 2D modeling of the electromagnetic field is performed in case of NCSCC using two or three pulses with different wavelengths. In the case of two pulses, a potentially unwanted spatio-temporal structure of the field appears, corresponding to spatial and temporal modulation of the pulse. By using NCSCC of three 62 fs long pulses with different spectral composition, such spatial-temporal coupling is eliminated and the combined pulse duration in the focal region drops to less than half. The method is scalable to a large number of ultrashort pulses.

High-average-power 2 μm few-cycle optical parametric chirped pulse amplifier at 100 kHz repetition rate

Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 μm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 μJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.

53 W average power few-cycle fiber laser system generating soft x rays up to the water window

We report on a few-cycle laser system delivering sub-8-fs pulses with 353 μJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.

Sub-10-fs pulses tunable from 480 to 980 nm from a NOPA pumped by an Yb:KGW source

We describe two noncollinear optical parametric amplifier (NOPA) systems pumped by either the second (515 nm) or the third (343 nm) harmonic from an Yb:KGW source. Pulse durations as short as 6.8 fs are readily obtained by compression with chirped mirrors. The availability of both the second and third harmonics for NOPA pumping allows for gap-free tuning from 520 to 980 nm. The use of an intermediate NOPA to generate seed light at 780 nm extends the tuning range of the third harmonic pumped NOPA toward 450 nm.

Two MHz tunable non collinear optical parametric amplifiers with pulse durations down to 6 fs

We report on the development of a 2 MHz non collinear optical parametric amplifier (NOPA) for high repetition rate time resolved X-ray or optical spectroscopy. Our modular and very flexible device is pumped by the second and third harmonics of a commercial femtosecond Ytterbium-doped fiber laser. The amplified pulses are tunable from 520 nm to 1000 nm with pulse durations between 15 and 30 fs over the full tuning range. The same setup is also suitable for broadband amplification and we demonstrate the generation of 6 fs pulses at a central wavelength of 850 nm as well as the generation of a broadband spectrum supporting 4.2 fs transform limited pulse duration at a central wavelength of 570 nm. Very high stability and compactness is achieved thanks to an optimized mechanical design.

Frequency domain optical parametric amplification

Today's ultrafast lasers operate at the physical limits of optical materials to reach extreme performances. Amplification of single-cycle laser pulses with their corresponding octave-spanning spectra still remains a formidable challenge since the universal dilemma of gain narrowing sets limits for both real level pumped amplifiers as well as parametric amplifiers. We demonstrate that employing parametric amplification in the frequency domain rather than in time domain opens up new design opportunities for ultrafast laser science, with the potential to generate single-cycle multi-terawatt pulses. Fundamental restrictions arising from phase mismatch and damage threshold of nonlinear laser crystals are not only circumvented but also exploited to produce a synergy between increased seed spectrum and increased pump energy. This concept was successfully demonstrated by generating carrier envelope phase stable, 1.43 mJ two-cycle pulses at 1.8 μm wavelength.

Sub-20 fs μJ-energy pulses tunable down to the near-UV from a 1 MHz Yb-fiber laser system

Optical parametric amplifiers render widely tunable ultrashort pulses, but for full spectral coverage, complex mixing schemes are needed. In particular, the blue and near-UV part of the spectrum is not directly reached with the 800 nm pump from Ti:sapphire systems or the 1030 nm pump of Yb-based lasers. We combine third harmonic pumping at 343 nm with seeding by a second harmonic (SH) pumped continuum to tune a noncollinear optical parametric amplifier down to 395 nm. Together with a SH pumped branch, the full range from 395 to 970 nm is covered with 20 fs pulse length or less. Pulse energies up to the μJ-level with an average power of up to 200 mW at 200 kHz and 480 mW at 1 MHz are achieved. With additional frequency doubling, the full range down to 210 nm is reached without gap. Two-photon absorption in the amplifier crystal is discussed as the critical issue in UV-pumped systems.

Power scaling of supercontinuum seeded megahertz-repetition rate optical parametric chirped pulse amplifiers

Optical parametric chirped-pulse amplifiers with high average power are possible with novel high-power Yb:YAG amplifiers with kW-level output powers. We demonstrate a compact wavelength-tunable sub-30-fs amplifier with 11.4 W average power with 20.7% pump-to-signal conversion efficiency. For parametric amplification, a beta-barium borate crystal is pumped by a 140 W, 1 ps Yb:YAG InnoSlab amplifier at 3.25 MHz repetition rate. The broadband seed is generated via supercontinuum generation in a YAG crystal.

Broadband continuum generation in mode-locked lasers with phase-matched output couplers

The concept of intracavity phase matching is proposed and demonstrated both theoretically and experimentally with a broadband phase-matched dielectric output coupler for linear-cavity few-cycle Ti:sapphire oscillators. The spectrum in the matched wavelength range is enhanced by >10  dB while maintaining good beam quality via resonantly enhanced continuum generation. The enhanced spectral components can be continuously tuned by varying the intracavity dispersion. Because dielectric coatings offer flexible design capabilities, this approach is applicable to various lasers with different gain media to obtain custom-tailored spectra, which have the potential to benefit several applications, such as shorter pulse generation, seeding of ytterbium lasers for pumping optical parametric amplifiers, and direct f-2f detection of the carrier-envelope phase.

Pulse characterization by THG d-scan in absorbing nonlinear media

We report on few-cycle pulse characterization based on third harmonic generation dispersion scan (THG d-scan) measurements using thin films of different TiO(2)-SiO(2) compositions as nonlinear media. By changing the TiO(2) concentration in the thin film the band gap and therefore the position of the absorption edge were varied. The retrieved pulse durations from different nonlinear media agree within 5%, and the reconstructed pulse shapes prove to be immune against the absorption edges as well. The reason is the robust retrieval algorithm which takes the influence of wavelength dependent nonlinearity into account by a spectral weight function.

Generation of few-cycle infrared pulses from a degenerate dual-pump OPCPA

A degenerate dual-pump optical parametric chirped-pulse amplifier (OPCPA) for generation of few-cycle intense pulses centered at 1.6 μm is theoretically investigated. By adding the optimized linear chirp to the two pump pulses from Ti:sapphire source and carefully adjusting the delays between the two pumps and seed, the long- and short-wavelength components of the seed pulse are efficiently amplified during the parametric process. Our simulations show that a broadband spectrum spanning from 1.3 μm to 2.1 μm is attained with a conversion efficiency of 22.6%. Signal pulse with a near transform-limited (TL) duration of 10.1 fs can be achieved by simply removing the linear chirp from the output signal. Besides, the compressed signal beam manifests good quality both spectrally and temporally, which allows tightly focusing the signal beam for further use.

Temporal and spatial effects inside a compact and CEP stabilized, few-cycle OPCPA system at high repetition rates

We present a compact and ultra-stable few-cycle OPCPA system. In two non-collinear parametric amplification stages pulse energies up to 17 µJ at 200 kHz repetition rate are obtained. Recompression of the broadband pulses down to 6.3 fs is performed with chirped mirrors leading to peak powers above 800 MW. The parametric amplification processes were studied in detail employing (2 + 1) dimensional numerical simulations and compared to experimental observations in terms of spectral shapes, pulse energy, spatial effects as well as delay dependent nonlinear mixing products. This gives new insights into the parametric process and design guidelines for high repetition rate OPCPA systems.

Thermal effects in high average power optical parametric amplifiers

Optical parametric amplifiers (OPAs) have the reputation of being average power scalable due to the instantaneous nature of the parametric process (zero quantum defect). This Letter reveals serious challenges originating from thermal load in the nonlinear crystal caused by absorption. We investigate these thermal effects in high average power OPAs based on beta barium borate. Absorption of both pump and idler waves is identified to contribute significantly to heating of the nonlinear crystal. A temperature increase of up to 148 K with respect to the environment is observed and mechanical tensile stress up to 40 MPa is found, indicating a high risk of crystal fracture under such conditions. By restricting the idler to a wavelength range far from absorption bands and removing the crystal coating we reduce the peak temperature and the resulting temperature gradient significantly. Guidelines for further power scaling of OPAs and other nonlinear devices are given.

Impact of temporal, spatial and cascaded effects on the pulse formation in ultra-broadband parametric amplifiers

A 2 + 1 dimensional nonlinear pulse propagation model is presented, illustrating the weighting of different effects for the parametric amplification of ultra-broadband spectra in different regimes of energy scaling. Typical features in the distribution of intensity and phase of state-of-the-art OPA-systems can be understood by cascaded spatial and temporal effects.

Control of nonlinear spectral phase induced by ultra-broadband optical parametric amplification

Optical parametric amplifiers (OPAs) impose an optical parametric phase (OPP) onto the amplified signal. It manifests itself as a spectral phase in the case of broadband signals and, therefore, hampers pulse compression. Here we present, for the first time, a complete experimental characterization of this OPP for different ultra-broadband noncollinear OPA configurations. This measurement allows us to compensate the OPP and to achieve Fourier-limited pulses as short as 1.9 optical cycles. A numerical model is in excellent agreement with our measurements and reveals the importance of high order phase compensation in the case of noncollinear phase matching. In contrast, operation at degeneracy enables almost complete compensation of the OPP by second-order dispersion only.