Evolutionary optimization and long-term stabilization of a white-light seeded two-stage OPCPA seed laser.

Ultrafast laser systems, such as optical parametric chirped pulse amplifiers (OPCPA), are complex tools. Optimizing laser performance for a given application is often plagued by intricate couplings between different output parameters, making simultaneous control of multiple pulse properties difficult. Here, we experimentally demonstrate an autonomous tuning procedure of a white-light seeded two-stage OPCPA using an evolutionary strategy to reliably reach an optimized working point. We use the data collected during the tuning procedure to calibrate a performance model of the laser system, which we then apply to stabilize the intricately coupled laser output energy and spectrum simultaneously. Our approach ensures reliable day-to-day operation at optimized working points without manual tuning. We demonstrate shot-to-shot energy stability of <0.18 % rms, in combination with <25 pm rms wavelength stability and <0.2 % rms bandwidth stability during multi-day operation.

Out-of-plane multilayer-dielectric-grating compressor for ultrafast Ti:sapphire pulses.

Extreme heat loads on optics, in particular the final pulse compression gratings, are a major hurdle to overcome in the ongoing push towards high average power (kW) and high repetition rate (kHz) operation of terawatt-class Ti:sapphire lasers. Multilayer dielectric (MLD) diffraction gratings have been suggested as a potential alternative to traditionally gold-coated compressor gratings, which are plagued by high energy absorption in the top gold layer. However, to support the required bandwidth (and ultimately the desired pulse duration) with MLD gratings, the gratings have to be operated in an out-of-plane geometry near the Littrow angle. Here, we report on the design of an MLD-based out-of-plane test compressor and a matching custom stretcher. We present a full characterization of the MLD compressor, focusing on its spectral transmission and the significance of laser pulse polarization in the out-of-plane geometry. To demonstrate compression of 40 µJ pulses centered at 800 nm wavelength to 26 fs pulse duration, we use the compressor with an MLD and gold grating configuration, and fully characterize the compressed pulses. Extrapolating our results indicates that MLD-grating-based out-of-plane compressors can support near-transform-limited pulses with sub-30 fs duration and good quality, demonstrating the viability of this concept for kW-level ultrafast Ti:sapphire laser systems.

Nonlinear pulse compression of a 200 mJ and 1†kW ultrafast thin-disk amplifier.

We present a high-energy laser source consisting of an ultrafast thin-disk amplifier followed by a nonlinear compression stage. At a repetition rate of 5 kHz, the drive laser provides a pulse energy of up to 200 mJ with a pulse duration below 500 fs. Nonlinear broadening is implemented inside a Herriott-type multipass cell purged with noble gas, allowing us to operate under different seeding conditions. Firstly, the nonlinear broadening of 64 mJ pulses is demonstrated in an argon-filled cell, showing a compressibility down to 32 fs. Finally, we employ helium as a nonlinear medium to increase the energy up to 200 mJ while maintaining compressibility below 50 fs. Such high-energy pulses with sub-50 fs duration hold great promise as drivers of secondary sources.

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.

Spatio-spectral couplings in saturated collinear OPCPA.

Ultrafast laser pulses featuring both high spatio-temporal beam quality and excellent energy stability are crucial for many applications. Here, we present a seed laser with high beam quality and energy stability, based on a collinear optical parametric chirped pulse amplification (OPCPA) stage, delivering 46 µJ pulses with a 25 fs Fourier limit at 1 kHz repetition rate. While saturation of the OPCPA stage is necessary for achieving the highest possible energy stability, it also leads to a degradation of the beam quality. Using simulations, we show that spectrally dependent, rotationally symmetric aberrations dominate the collinear OPCPA in saturation. We experimentally characterize these aberrations and then remove distinct spatial frequencies to greatly improve the spectral homogeneity of the beam quality, while keeping an excellent energy stability of 0.2 % rms measured over 70 hours.

Bayesian Optimization of a Laser-Plasma Accelerator.

Generating high-quality laser-plasma accelerated electron beams requires carefully balancing a plethora of physical effects and is therefore challenging-both conceptually and in experiments. Here, we use Bayesian optimization of key laser and plasma parameters to flatten the longitudinal phase space of an ionization-injected electron bunch via optimal beam loading. We first study the concept with particle-in-cell simulations and then demonstrate it in experiments. Starting from an arbitrary set point, the plasma accelerator autonomously tunes the beam energy spread to the subpercent level at 254 MeV and 4.7 pC/MeV spectral density. Finally, we study a robust regime, which improves the stability of the laser-plasma accelerator and delivers sub-five-percent rms energy spread beams for 90% of all shots.

Optimal Beam Loading in a Laser-Plasma Accelerator.

Applications of laser-plasma accelerators demand low energy spread beams and high-efficiency operation. Achieving both requires flattening the accelerating fields by controlled beam loading of the plasma wave. Here, we optimize the generation of an electron bunch via localized ionization injection, such that the combination of injected current profile and averaged acceleration dynamics results in optimal beam loading conditions. This enables the reproducible production of 1.2% rms energy spread bunches with 282 MeV and 44 pC at an estimated energy-transfer efficiency of ∼19%. We correlate shot-to-shot variations to reveal the phase space dynamics and train a neural network that predicts the beam quality as a function of the drive laser.

Description of spatio-temporal couplings from heat-induced compressor grating deformation.

High average power high-intensity laser systems can suffer from a heat-induced deformation of the final compressor gratings, which introduces wavefront aberrations and spatio-temporal couplings to the pulse. Here, we use a simple numerical description, that was first introduced by Li et al. (Appl. Phys. Express, 10, 102702, 2017 and Optics Express, 26, 8453, 2018), to calculate the resulting degradation of the peak intensity and the 3-dimensional deformation of the laser pulse as a function of average power, and verify the results using experimental data. For a typical 100 TW-class laser we find that non-negligible pulse distortions can occur at an average power as low as 2.7 Watts. An open source implementation of our numerical description is available for researchers to estimate the effects of spatio-temporal couplings for their specific laser configuration.

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses.

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios.

Wavefront degradation of a 200 TW laser from heat-induced deformation of in-vacuum compressor gratings.

High-repetition-rate high-power laser systems induce a high average power heat deposition into the gold-coated diffraction gratings. To study the effects of the thermal expansion of in-vacuum Pyrex gratings on the laser properties, we scan the pulse energy and repetition rate of a 200 TW laser system while monitoring the laser wavefront. Through the measured changes in laser divergence and focusability, we define an average power limit below which the in-vacuum compressor can be used with no degradation of the laser focus quality.

Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate.

We generate narrowband terahertz (THz) radiation in periodically poled lithium niobate (PPLN) crystals using two chirped-and-delayed driver pulses from a high-energy Ti:sapphire laser. The generated frequency is determined by the phase-matching condition in the PPLN and influences the temporal delay of the two pulses for efficient terahertz generation. We achieve internal conversion efficiencies up to 0.13% as well as a record multicycle THz energy of 40 µJ at 0.544 THz in a cryogenically cooled PPLN.

Scalable spectral solver in Galilean coordinates for eliminating the numerical Cherenkov instability in particle-in-cell simulations of streaming plasmas.

Discretizing Maxwell's equations in Galilean (comoving) coordinates allows the derivation of a pseudospectral solver that eliminates the numerical Cherenkov instability for electromagnetic particle-in-cell simulations of relativistic plasmas flowing at a uniform velocity. Here we generalize this solver by incorporating spatial derivatives of arbitrary order, thereby enabling efficient parallelization by domain decomposition. This allows scaling of the algorithm to many distributed compute units. We derive the numerical dispersion relation of the algorithm and present a comprehensive theoretical stability analysis. The method is applied to simulations of plasma acceleration in a Lorentz-boosted frame of reference.

Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation.

Highly-efficient optical generation of narrowband terahertz radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband difference-frequency generation using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.

Elimination of numerical Cherenkov instability in flowing-plasma particle-in-cell simulations by using Galilean coordinates.

Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.