In this paper, we have experimentally demonstrated a high power and high brightness narrow-linewidth fiber amplifier seeded by an optimized fiber oscillator. In order to improve the temporal stability, the fiber oscillator consists of a composite FBG-based cavity with an external feedback structure. By optimizing the forward and backward pumping ratio, nonlinear effects and SRS-induced mode distortion of the fiber amplifier are suppressed comprehensively, accompanying with the simultaneous improvement of beam quality and output power. Furthermore, the TMI threshold is also improved by ~1.0 kW by coiling the gain fiber with a novel curvature shape. Finally, a 6 kW narrow linewidth laser is achieved with beam quality (M2) of ~1.4. The laser brightness has doubled comparing to the results before optimization. At the maximum output power,. To the best of our knowledge, it is the highest brightness narrow linewidth fiber laser based on one-stage MOPA structure.

Double cone ignition (DCI) [Zhang et al., Phil. Trans. R. Soc. A 378: 20200015 (2020)] was proposed recently as a novel path for direct-drive inertial confinement fusion (ICF) using high power lasers. In this scheme, plasma jets with both high density and high velocity are required for collisions. Here we report preliminary experimental results obtained at the Shenguang-II upgrade laser facility, employing a CHCl shell in a gold cone irradiated with a two-ramp laser pulse. The CHCl shell was pre-compressed by the first laser ramp to a density of 3.75 g/cm3 along the isentropic path. Subsequently, the target was further compressed and accelerated by the second laser ramp in the cone. According to the simulations, the plasma jet reached a density of up to 15 g/cm3, while measurements indicated a velocity of 126.8 ± 17.1 km/s. The good agreements between experimental data and simulations are documented.

Power scaling in conventional broad-area (BA) lasers often leads to the operation of higher-order lateral modes, resulting in a multiple-lobe far-field profile with large divergence. Here, we report an advanced sawtooth waveguide (ASW) structure integrated onto a wide ridge waveguide. It strategically enhances the loss difference between higher-order modes and the fundamental mode, thereby facilitating high-power narrow-beam emission. Both optical simulations and experimental results illustrate the significant increase in additional scattering loss of the higher-order modes. The optimized ASW lasers achieve an impressive output power of 1.1 W at 4.6 A at room temperature, accompanied by a minimal full width at half maximum (FWHM) lateral divergence angle of 4.91°. Notably, the far-field divergence is reduced from 19.61° to 11.39° at the saturation current, showcasing a remarkable 42% improvement compared to conventional BA lasers. Moreover, the current dependence of divergence has been effectively improved by 38%, further confirming the consistent and effective lateral mode control capability offered by our design.

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 (TDFL) 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 the SCF lasers in the 2-μm spectral range.

In this study, we investigated the influence of fiber parameters on stimulated Raman scattering (SRS) and identified a unique pattern of SRS evolution in the counter tandem pumping configuration. Our findings revealed that the SRS threshold in counter-pumping is predominantly determined by the length of the output delivery fiber rather than the gain fiber. By employing counter tandem pumping scheme and optimizing fiber parameters, a 10 kW fiber laser was achieved with beam quality M2 of 1.92. No mode instability (MI) or severe SRS limitation were observed. To our knowledge, this study achieved the highest beam quality in over 10 kW fiber lasers based on conventional double-clad Yb-doped fiber.

The problem of optimizing the parameters of a laser pulse compressor consisting of four identical diffraction gratings is solved analytically. The goal of optimization is to obtain maximum pulse power, completely excluding both beam clipping on gratings and the appearance of spurious diffraction orders. The analysis is carried out in a general form for an out-of-plane compressor. Two particular “plane” cases attractive from a practical point of view are analyzed in more detail: a standard Treacy compressor (TC) and a compressor with an angle of incidence equal to the Littrow angle (LC). It is shown that in both cases the LC is superior to the TC. Specifically, for 160-cm diffraction gratings, optimal LC design enables 107 PW for XCELS and 111 PW for SEL-100 PW, while optimal TC design enables 86 PW for both projects.

To overcome Yb laser, kilowatt-level 1535 nm fiber laser is utilized to in-band pump an Er:Yb co-doped fiber (EYDF) amplifier. An output power of 301 W narrow-linewidth EYDF amplifier operating at 1585 nm, with 3 dB bandwidth of 150 pm and M²<1.4, is experimentally demonstrated. To the best of our knowledge, it is the highest output power 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 optical 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, to improve power to kilowatt level via in-band pumping, optimized approaches are proposed.

A colliding microjet liquid sheet target system was developed, and tested for pairs of round nozzles of 10, 11, and 18µm in diameter. The sheet’s position stability was found to be better than a few micrometers. Upon interaction with 50mJ laser pulses, the 18µm jet has a resonance amplitude of 16µm at a repetition rate of 33Hz, while towards 100Hz it converges to 10µm for all nozzles. A white-light interferometric system was developed to measure the liquid sheet thickness in the target chamber both in air and in vacuum, with a measurement range of 185nm – 1µm and an accuracy of ±3%. The overall shape and 3D shape of the sheet follow the Hasson-Peck model in air. In vacuum versus air, the sheet gradually loses 10% of its thickness, so the thinnest sheet achieved was below 200nm at a vacuum level of 10-4mbar, and remained stable for several hours of operation.

The high-energy/high-intensity laser facility PHELIX of the GSI Helmholtzzentrum f ̈ur Schwerionenforschung in Darmstadt, Germany, has been in operation since 2008. Here, we review the current system performance, which is the result of continuous development and further improvement. Through its versatile frontend architecture, PHELIX can be operated in both long and short pulse modes, corresponding to ns pulses with up to 1 kJ pulse energy and sub-ps, 200 J pulses, respectively. In the short-pulse mode, the excellent temporal contrast and the control over the wavefront make PHELIX an ideal driver for secondary sources of high-energy ions, neutrons, electrons, and X-rays. The long-pulse mode is mainly used for plasma heating, which can then be probed by the heavy-ion beam of the linear accelerator of GSI. In addition, PHELIX can now be used to generate X-rays for studying exotic states of matter created by heavy-ion heating using the ion beam of the heavy-ion synchrotron of GSI.

Dr. Jie Zhang, a distinguished physicist, has made significant contributions in the fields of high-energy density physics and inertial confinement fusion. Because of these, he was elected academician of the Chinese Academy of Sciences in 2003, academician of the German National Academy of Sciences in 2007, Fellow of the Academy of Sciences for the Developing World (TWAS) in 2008, foreign member of the Royal Academy of Engineering in the United Kingdom in 2011, and foreign associate of the National Academy of Sciences in the United States in 2012. In 2015, he was awarded the prestigious Edward Teller Medal, the most important international award in inertial confinement fusion and high-energy density physics. In 2021, he was awarded the Future Science Prize in Physical Sciences.