Processes of laser energy absorption and electron heating in an expanding plasma in the range of irradiances $I\unicode[STIX]{x1D706}^{2}=10^{15}{-}10^{16}~\text{W}\,\cdot \,\unicode[STIX]{x03BC}\text{m}^{2}/\text{cm}^{2}$ are studied with the aid of kinetic simulations. The results show a strong reflection due to stimulated Brillouin scattering and a significant collisionless absorption related to stimulated Raman scattering near and below the quarter critical density. Also presented are parametric decay instability and resonant excitation of plasma waves near the critical density. All these processes result in the excitation of high-amplitude electron plasma waves and electron acceleration. The spectrum of scattered radiation is significantly modified by secondary parametric processes, which provide information on the spatial localization of nonlinear absorption and hot electron characteristics. The considered domain of laser and plasma parameters is relevant for the shock ignition scheme of inertial confinement fusion.

We report on the observation of subcycle interferences of electron wave packets released during strong field ionization of $\text{H}_{2}$ with cycle-shaped two-color laser fields. With a reaction microscope we measure three-dimensional momentum distributions of photoelectrons correlated with either $\text{H}_{2}^{+}$ or protons within different energy ranges generated by dissociation of $\text{H}_{2}^{+}$. We refer to these different types of photoelectrons as channels. Our results show that the subcycle interference structures of electron wave packets are very sensitive to the cycle shape of the two-color laser field. We explain this behavior by the dependence of the ionization time within an optical cycle on the shape of the laser field cycle. The subcycle interference structures can be further used to obtain insight into the subcycle dynamics of molecules during strong field interaction.

To overcome Yb lasing, a kilowatt-level 1535 nm fiber laser is utilized to in-band pump an Er:Yb co-doped fiber (EYDF) amplifier. The output power of a 301 W narrow-linewidth EYDF amplifier operating at 1585 nm, with 3 dB bandwidth of 150 pm and ${M}^2$< 1.4, is experimentally demonstrated. To the best of our knowledge, it is the highest output power achieved in L-band narrow-linewidth fiber amplifiers with good beam quality. Theoretically, a new ion transition behavior among energy levels for in-band pumping EYDF is uncovered, and a spatial-mode-resolved nonlinearity-assisted theoretical model is developed to understand its internal dynamics. Numerical simulations reveal that the reduction in slope efficiency is significantly related to excited-state absorption (ESA). ESA has a nonlinear hindering effect on power scaling. It can drastically lower the pump absorption and slope efficiency with increasing pump power for in-band pumped EYDF amplifiers. Meanwhile, optimized approaches are proposed to improve its power to the kilowatt level via in-band pumping.

Ytterbium-doped fibers have become the optimum gain media of high-power fiber lasers thanks to a simple energy structure, which strongly reduces the excited state absorption, and a low quantum defect and a high optic–optic conversion efficiency, which means the low thermal load. In this paper, we take a review of the current state of the art in terms of $\text{Yb}^{3+}$ doped fibers for high-power fiber lasers, including the development of the fabrication techniques. The research work to overcome the challenges for $\text{Yb}^{3+}$ doped fibers, which affect the stability of output power and beam quality, will be demonstrated. Direction of further research is presented and the goal is to look for a fiber design, to boost single fiber output power, stabilize the laser power and support robust single-mode operation.

In this paper, we innovatively conduct a Porro prism-based beam pointing stability promotion technique research and realize a high-performance rod-type photonic crystal fiber-based chirped pulse amplification (CPA) system, mainly including a frequency-reduced all-fiber pre-amplification stage, photonic crystal rod-based main amplification stage, and 1600 lines/mm transmission grating-pair compressor. Laser output with average power of 50 W, repetition rates of 500 kHz, pulse energy of 100 μJ, pulse duration of 830 fs, beam quality of M2<1.3, power fluctuation of 0.55% root mean square, and beam pointing drift of 19 μrad/°C over 8 h is realized. The high-performance laser system has an enormous application potential in fundamental research and precision manufacturing fields.

A 1.7 μm gain-switched thulium-doped all-fiber laser with a master oscillator power amplifier (MOPA) configuration, utilizing a bandpass fiber filter and a 1550 nm erbium/ytterbium-codoped fiber MOPA, is demonstrated. The influences of pump pulse parameters (repetition rate and pulse duration) and laser cavity structures (ring and linear) on the laser performances were experimentally investigated. To the best of our knowledge, the power quenching and drop were observed in the 1.7 μm gain-switched thulium-doped fiber lasers for the first time, resulting from the mode-locked-resembling operation and nonlinear effects. Moreover, the fiber ring-cavity laser was more stable than the linear-cavity laser in the time domain and power. Finally, a laser with a maximum average power of 1.687 W, a slope efficiency of 19.7%, a single-pulse energy of 16.87 μJ, a pulse width of 425 ns, a repetition rate of 100 kHz and a peak power of 39.69 W was obtained.

Porous materials have many applications for laser–matter interaction experiments related to inertial confinement fusion. Obtaining new knowledge about the properties of the laser-produced plasma of porous media is a challenging task. In this work, we report, for the first time to the best of our knowledge, the time-dependent measurement of the reflected light of a terawatt laser pulse from the laser-produced plasma of low-Z foam material of overcritical density. The experiments have been performed with the ABC laser, with targets constituted by foam of overcritical density and by solid media of the same chemical composition. We implemented in the MULTI-FM code a model for the light reflection to reproduce and interpret the experimental results. Using the simulations together with the experimental results, we indicate a criterion for estimating the homogenization time of the laser-produced plasma, whose measurement is challenging with direct diagnostic techniques and still not achieved.

This study investigates the influence of seismic activities on the optical synchronization system of the European X-ray Free-Electron Laser. We analyze the controller input/output data of phase-locked loops in length-stabilized links, focusing on the response to earthquakes, ocean-generated microseism and civilization noise. By comparing the controller data with external data, we were able to identify disturbances and their effects on the control signals. Our results show that seismic events influence the stability of the phase-locked loops. Even earthquakes that are approximately 5000 km away cause remarkable fluctuations in the in-loop control signals. Ocean-generated microseism in particular has an enormous influence on the in-loop control signals due to its constant presence. The optical synchronization system is so highly sensitive that it can even identify vibrations caused by civilization, such as road traffic or major events like concerts or sport events. The phase-locked loops manage to eliminate more than 99% of the existing interference.

The Atomic Weapons Establishment (AWE) is tasked with supporting Continuous At Sea Deterrence (CASD) by certifying the performance and safety of the national deterrent in the Comprehensive Test Ban Treaty (CTBT) era. This means that recourse to further underground testing is not possible, and certification must be achieved by supplementing the historical data with the use of computer calculation. In order to facilitate this, AWE operates some of the largest supercomputers in the UK. To validate the computer codes, and indeed the designers who are using them, it is necessary to carry out further experiments in the right regimes. An excellent way to meet many of the requirements for material property data and to provide confidence in the validity of the algorithms is through experiments carried out on high power laser facilities.

The Weibel instability and the induced magnetic field are of great importance for both astrophysics and inertial confinement fusion. Because of the stochasticity of this magnetic field, its main wavelength and mean strength, which are key characteristics of the Weibel instability, are still unobtainable experimentally. In this paper, a theoretical model based on the autocorrelation tensor shows that in proton radiography of the Weibel-instability-induced magnetic field, the proton flux density on the detection plane can be related to the energy spectrum of the magnetic field. It allows us to extract the main wavelength and mean strength of the two-dimensionally isotropic and stochastic magnetic field directly from proton radiography for the first time. Numerical calculations are conducted to verify our theory and show good consistency between pre-set values and the results extracted from proton radiography.

Pulse shaping is a powerful tool for mitigating implosion instabilities in direct-drive inertial confinement fusion (ICF). However, the high-dimensional and nonlinear nature of implosions makes the pulse optimization quite challenging. In this research, we develop a machine-learning pulse shape designer to achieve high compression density and stable implosion. The facility-specific laser imprint pattern is considered in the optimization, which makes the pulse design more relevant. The designer is applied to the novel double-cone ignition scheme, and simulation shows that the optimized pulse increases the areal density expectation by 16% in one dimension, and the clean-fuel thickness by a factor of four in two dimensions. This pulse shape designer could be a useful tool for direct-drive ICF instability control.

Laser pulses of 200 ps with extremely high intensities and high energies are sufficient to satisfy the demand of shock ignition, which is an alternative path to ignition in inertial confinement fusion (ICF). This paper reports a type of Brillouin scheme to obtain high-intensity 200-ps laser pulses, where the pulse durations are a challenge for conventional pulsed laser amplification systems. In the amplification process, excited Brillouin acoustic waves fulfill the nonlinear optical effect through which the high energy of a long pump pulse is entirely transferred to a 200-ps laser pulse. This method was introduced and achieved within the SG-III prototype system in China. Compared favorably with the intensity of $2~\text{GW}/\text{cm}^{2}$ in existing ICF laser drivers, a 6.96-$\text{GW}/\text{cm}^{2}$ pulse with a width of 170 ps was obtained in our experiment. The practical scalability of the results to larger ICF laser drivers is discussed.

Numerical estimates for electrons and mesons particle–antiparticle creation from vacuum in the presence of strong electromagnetic fields are derived, using the complete probability density relation of Popov’s imaginary time method (Popov, JETP Lett. 13, 185 (1971); Sov. Phys. JETP 34, 709 (1972); Sov. Phys. JETP 35, 659 (1972); Popov and Marinov, Sov. J. Nucl. Phys. 16, 449 (1973); JETP Lett. 18, 255 (1974); Sov. J. Nucl. Phys. 19, 584 (1974)); (Popov, Phys. Let. A 298, 83 (2002)), and within the framework of an experimental setup like the E144 (Burke et al., Phys. Rev. Lett. 79, 1626 (1997)). The existence of crossing point among pair creation efficiency curves of different photon energies and the role of odd/even multiphoton orders in the production rates are discussed. Finally a kind of tunability process between the two creation processes is discussed.

Optical fibers offer convenient access to a variety of nonlinear phenomena. However, due to their inversion symmetry, second-order nonlinear effects, such as second-harmonic generation (SHG), are challenging to achieve. Here, all-fiber in-core SHG with high beam quality is achieved in a random fiber laser (RFL). The fundamental wave (FW) is generated in the same RFL. The phase-matching condition is mainly achieved through an induced periodic electric field and the gain is enhanced through the passive spatiotemporal gain modulation and the extended fiber. The conversion needs no pretreatment and the average second-harmonic (SH) power reaches up to 10.06 mW, with a corresponding conversion efficiency greater than 0.04%. Moreover, a theoretical model is constructed to explain the mechanism and simulate the evolution of the SH and FW. Our work offers a simple method to generate higher brightness for in-fiber SHs, and may further provide new directions for research on all-fiber χ(2)-based nonlinear fiber optics and RFLs.

We report on a high-efficiency, high-power tandem Ho:YAG single-crystal fiber (SCF) laser in-band pumped by a Tm-doped fiber laser at 1907 nm. In addition to the uniform heat distribution resulting from the large surface-to-volume ratio of this fiber-like thin-crystal rod, the long gain region provided by the tandem layout of two SCFs enables high lasing efficiency and power handling capability. More than 100 W output power is achieved at 2.1 μm, corresponding to a slope efficiency of 70.5% and an optical-to-optical efficiency of 67.6%. To the best of our knowledge, this is the highest output power and efficiency ever reported from SCF lasers in the 2-μm spectral range.

The nonlinear absorption (NLA) properties of potassium dideuterium phosphate crystals at 515 nm under different excitation laser intensities are investigated with the Z-scan technique. Two critical intensities are highlighted: the critical intensity for exciting the NLA and the critical intensity of the multiphoton absorption mechanism transition. Experimental results indicate the existence of defect states located in the band gap, which can be manipulated by varying laser intensity. A model based on the change of multiphoton absorption mechanism induced by the transformation of defect species is proposed to interpret the experiments. Modeling results are in good agreement with the experiment data.

Experiments have shown that the ion energy obtained by laser–ion acceleration can be optimized by choosing either the appropriate pulse duration or the appropriate target thickness. We demonstrate that this behavior can be described either by the target normal sheath acceleration model of Schreiber et al. or by the radiation pressure acceleration model of Bulanov and coworkers. The starting point of our considerations is that the essential property of a laser system for ion acceleration is its pulse energy and not its intensity. Maybe surprisingly we show that higher ion energies can be reached with reduced intensities.

The fiber laser based on an oscillating-amplifying integrated structure has the potential to benefit from the advantages of a fiber laser oscillator and amplifier with the characteristics of strong anti-back-reflected light ability and high efficiency. Here, we achieved a 3.5-kW near-single-mode (M2 ∼ 1.24) oscillating–amplifying integrated fiber laser with an active fiber length of 8 m in the oscillating section and 17.6 m in the amplifying section. While operating at the maximum power, the optical-to-optical conversion efficiency is 87.0%, and the intensity of stimulated Raman scattering is about 23.61 dB lower than that of the signal light. To the best of the authors’ knowledge, this is the highest output power of an oscillating–amplifying integrated fiber laser, accompanied with the best beam quality and the highest efficiency.

Single-shot laser-induced damage threshold (LIDT) measurements of multi-type free-standing ultrathin foils were performed in a vacuum environment for 800 nm laser pulses with durations τ ranging from 50 fs to 200 ps. The results show that the laser damage threshold fluences (DTFs) of the ultrathin foils are significantly lower than those of corresponding bulk materials. Wide band gap dielectric targets such as SiN and formvar have larger DTFs than semiconductive and conductive targets by 1–3 orders of magnitude depending on the pulse duration. The damage mechanisms for different types of targets are studied. Based on the measurement, the constrain of the LIDTs on the laser contrast is discussed.

We report on a grating-free fiber chirped pulse amplifier (CPA) at 2.8 μm for the first time. The CPA system adopted Er:ZBLAN fiber with large anomalous dispersion as the stretcher and germanium (Ge) rods as the compressor with a compact structure. High-energy picosecond pulses of 2.07 μJ were generated at the repetition rate of 100 kHz. Using highly dispersive Ge rods, the amplified pulses were compressed to 408 fs with a pulse energy of 0.57 μJ, resulting in a peak power of approximately 1.4 MW. A spectral broadening phenomenon in the main amplifier was observed, which was caused by the special gain shape of the Er:ZBLAN fiber amplifier in operation and confirmed by our numerical simulation. This compact fiber CPA system at 2.8 μm will be practical and meaningful for application fields.