High Power Laser Science and Engineering
Co-Editors-in-Chief
Colin Danson, Jianqiang Zhu
Contents 1 Issue (s), 13 Article (s)
Vol. 10, Iss.6—Nov.1, 2022 • pp: e34- Spec. pp:
Vol. 10, Iss.6-Nov..1,2022
Research Articles
How the laser beam size conditions the temporal contrast in pulse stretchers of chirped-pulse amplification lasers
Simon Roeder, Yannik Zobus, Christian Brabetz, and Vincent Bagnoud

In this work, we propose and verify experimentally a model that describes the concomitant influence of the beam size and optical roughness on the temporal contrast of optical pulses passing through a pulse stretcher in chirped-pulse amplification laser systems. We develop an analytical model that is capable of predicting the rising edge caused by the reflection from an optical element in a pulse stretcher, based on the power spectral density of the surface and the spatial beam profile on the surface. In an experimental campaign, we characterize the temporal contrast of a laser pulse that passed through either a folded or an unfolded stretcher design and compare these results with the analytical model. By varying the beam size for both setups, we verify that optical elements in the near- and the far-field act opposed to each with respect to the temporal contrast and that the rising edge caused by a surface benefits from a larger spatial beam size on that surface.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e34 (2022)
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Research Articles
High laser damage threshold reflective optically addressed liquid crystal light valve based on gallium nitride conductive electrodes
Zhibo Xing, Wei Fan, Dajie Huang, He Cheng, and Tongyao Du

In this paper, the feasibility of a high laser damage threshold liquid crystal spatial light modulator based on gallium nitride (GaN) transparent conductive electrodes is proved. The laser-induced damage threshold (LIDT) is measured, and a high LIDT reflective optically addressed liquid crystal light valve (OALCLV) based on GaN is designed and fabricated. The proper work mode of the OALCLV is determined; the OALCLV obtained a maximum reflectivity of about 55% and an on–off ratio of 55:1, and an image response is demonstrated.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e35 (2022)
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Research Articles
Record power and efficient mid-infrared supercontinuum generation in germania fiber with high stability
Linyong Yang, Yukun Yang, Bin Zhang, Xiran Zhu, Desheng Zhao, Shuailin Liu, and Jing Hou

We report the demonstration of a mid-infrared (MIR) supercontinuum (SC) laser delivering a record-breaking average output power of more than 40 W with a long-wavelength edge up to 3.5 μm. The all-fiberized configuration was composed of a thulium-doped fiber amplifier system emitting a broadband spectrum covering 1.9–2.6 μm with pulse repetition rate of 3 MHz, and a short piece of germania fiber. A 41.9 W MIR SC with a whole spectrum of 1.9–3.5 μm was generated in a piece of 0.2-m-long germania fiber, with a power conversion efficiency of 71.4%. For an even shorter germania fiber (0.1 m), an SC with even higher output power of 44.9 W (corresponding to a conversion efficiency of 76.5%) was obtained, but the energy conversion toward the long-wavelength region was slightly limited. A continuous operation for 1 hour with output power of 32.6 W showed outstanding power stability (root mean square 0.17%) of the obtained SC laser. To the best of the authors’ knowledge, for the first time, this work demonstrates the feasibility of germania fiber on generating a 40-W level MIR SC with high efficiency and excellent power stability, paving the way to real applications requiring high power and high reliability of MIR SC lasers.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e36 (2022)
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Research Articles
Designing a toroidal crystal for monochromatic X-ray imaging of a laser-produced He-like plasma
Miao Li, Tong Yao, Zuhua Yang, Jun Shi, Feng Wang, Guohong Yang, Minxi Wei, Ao Sun, and Yang Li

In this study, a toroidal quartz ( $20\overline{2}3$ ) crystal is designed for monochromatic X-ray imaging at 72.3°. The designed crystal produces excellent images of a laser-produced plasma emitting He-like Ti X-rays at 4.75 keV. Based on the simulations, the imaging resolutions of the spherical and toroidal crystals in the sagittal direction are found to be 15 and 5 μm, respectively. Moreover, the simulation results show that a higher resolution image of the source can be obtained by using a toroidal crystal. An X-ray backlight imaging experiment is conducted using 4.75 keV He-like Ti X-rays, a 3 × 3 metal grid, an imaging plate and a toroidal quartz crystal with a lattice constant of 2d = 0.2749 nm. The meridional and sagittal radii of the toroidal α-quartz crystal are 295.6 and 268.5 mm, respectively. A highly resolved image of the microgrid, with a spatial resolution of 10 μm, is obtained in the experiment. By using similar toroidal crystal designs, the application of a spatially resolved spectrometer with high-resolution X-ray imaging ability is capable of providing imaging data with the same magnification ratio in the sagittal and meridional planes.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e37 (2022)
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Research Articles
Dispersion management for a 100 PW level laser using a mismatched-grating compressor
Fenxiang Wu, Jiabing Hu, Xingyan Liu, Zongxin Zhang, Peile Bai, Xinliang Wang, Yang Zhao, Xiaojun Yang, Yi Xu, Cheng Wang, Yuxin Leng, and Ruxin Li

We report dispersion management based on a mismatched-grating compressor for a 100 PW level laser, which utilizes optical parametric chirped pulse amplification and also features large chirped pulse duration and an ultra-broadband spectrum. The numerical calculation indicates that amplified pulses with 4 ns chirped pulse duration and 210 nm spectral bandwidth can be directly compressed to sub-13 fs, which is close to the Fourier-transform limit (FTL). More importantly, the tolerances of the mismatched-grating compressor to the misalignment of the stretcher, the error of the desired grating groove density and the variation of material dispersion are comprehensively analyzed, which is crucially important for its practical application. The results demonstrate that good tolerances and near-FTL compressed pulses can be achieved simultaneously, just by keeping a balance between the residual second-, third- and fourth-order dispersions in the laser system. This work can offer a meaningful guideline for the design and construction of 100 PW level lasers.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e38 (2022)
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Research Articles
Effects of second-order dispersion of ultrashort laser pulse on stimulated Raman scattering
Yanqing Deng, Dongning Yue, Mufei Luo, Xu Zhao, Yaojun Li, Xulei Ge, Feng Liu, Suming Weng, Min Chen, Xiaohui Yuan, and Jie Zhang

The influence of second-order dispersion (SOD) on stimulated Raman scattering (SRS) in the interaction of an ultrashort intense laser with plasma was investigated. More significant backward SRS was observed with the increase of the absolute value of SOD ($\mid \kern-1pt\!{\psi}_2\!\kern-1pt\mid$). The integrated intensity of the scattered light is positively correlated to the driver laser pulse duration. Accompanied by the side SRS, filaments with different angles along the laser propagation direction were observed in the transverse shadowgraph. A model incorporating Landau damping and above-threshold ionization was developed to explain the SOD-dependent angular distribution of the filaments.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e39 (2022)
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Research Articles
Gain-switched watt-level thulium-doped fiber laser and amplifier operating at 1.7 μm
Yang Xiao, Xusheng Xiao, Lutao Liu, and Haitao Guo

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.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e40 (2022)
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Research Articles
Grating-free 2.8 μm Er:ZBLAN fiber chirped pulse amplifier
Yicheng Zhou, Zhipeng Qin, Xiabing Zhou, and Guoqiang Xie

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.

High Power Laser Science and Engineering
Dec. 22, 2022, Vol. 10 Issue 6 06000e41 (2022)
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Research Articles
Multilayer dielectric grating pillar-removal damage induced by a picosecond laser
Kun Shuai, Xiaofeng Liu, Yuanan Zhao, Keqiang Qiu, Dawei Li, He Gong, Jian Sun, Li Zhou, Youen Jiang, Yaping Dai, Jianda Shao, and Zhilin Xia

Multilayer dielectric gratings typically remove multiple-grating pillars after picosecond laser irradiation; however, the dynamic formation process of the removal is still unclear. In this study, the damage morphologies of multilayer dielectric gratings induced by an 8.6-ps laser pulse were closely examined. The damage included the removal of a single grating pillar and consecutive adjacent grating pillars and did not involve the destruction of the internal high-reflection mirror structure. Comparative analysis of the two damage morphological characteristics indicated the removal of adjacent pillars was related to an impact process caused by the eruption of localized materials from the left-hand pillar, exerting impact pressure on its adjacent pillars and eventually resulting in multiple pillar removal. A finite-element strain model was used to calculate the stress distribution of the grating after impact. According to the electric field distribution, the eruptive pressure of the dielectric materials after ionization was also simulated. The results suggest that the eruptive pressure resulted in a stress concentration at the root of the adjacent pillar that was sufficient to cause damage, corresponding to the experimental removal of the adjacent pillar from the root. This study provides further understanding of the laser-induced damage behavior of grating pillars and some insights into reducing the undesirable damage process for practical applications.

High Power Laser Science and Engineering
Dec. 23, 2022, Vol. 10 Issue 6 06000e42 (2022)
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Research Articles
Large temporal window and high-resolution single-shot cross-correlator with two separate measurement channels
Jingui Ma, Xiaoping Ouyang, Liangze Pan, Peng Yuan, Dongfang Zhang, Jing Wang, Guoqiang Xie, Jianqiang Zhu, and Liejia Qian

In strong-field physics experiments with ultraintense lasers, a single-shot cross-correlator (SSCC) is essential for fast optimization of the pulse contrast and meaningful comparison with theory for each pulse shot. To simultaneously characterize an ultrashort pulse and its long pedestal, the SSCC device must have both a high resolution and a large temporal window. However, the resolution and window in all kinds of single-shot measurement contradict each other in principle. Here we propose and demonstrate a novel SSCC device with two separate measurement channels: channel-1 for the large-window pedestal measurement has a moderate resolution but a large window, while channel-2 for the ultrashort pulse measurement has a small window but a high resolution; this allows the accurate characterization of the pulse contrast in a single shot. A two-channel SSCC device with a 200-fs resolution and 114-ps window has been developed and tested for its application in ultraintense lasers at 800 nm.

High Power Laser Science and Engineering
Dec. 27, 2022, Vol. 10 Issue 6 06000e43 (2022)
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Research Articles
Transverse mode instability mitigation in a high-power confined-doped fiber amplifier with good beam quality through seed laser control
Hanshuo Wu, Haobo Li, Yi An, Ruixian Li, Xiao Chen, Hu Xiao, Liangjin Huang, Huan Yang, Zhiping Yan, Jinyong Leng, Zhiyong Pan, and Pu Zhou

In this work, a confined-doped fiber with the core/inner-cladding diameter of 40/250 μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique, and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability (TMI) mitigation. Notably, the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments. It is found that the TMI threshold could be significantly affected by the seed laser mode purity. The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis. Finally, a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83, which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier. This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.

High Power Laser Science and Engineering
Nov. 15, 2022, Vol. 10 Issue 6 06000e44 (2022)
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Research Articles
Electron pulse train accelerated by a linearly polarized Laguerre–Gaussian laser beam
Yin Shi, David R. Blackman, Ping Zhu, and Alexey Arefiev

A linearly polarized Laguerre–Gaussian (LP-LG) laser beam with a twist index $l = -1$ has field structure that fundamentally differs from the field structure of a conventional linearly polarized Gaussian beam. Close to the axis of the LP-LG beam, the longitudinal electric and magnetic fields dominate over the transverse components. This structure offers an attractive opportunity to accelerate electrons in vacuum. It is shown, using three-dimensional particle-in-cell simulations, that this scenario can be realized by reflecting an LP-LG laser off a plasma with a sharp density gradient. The simulations indicate that a 600 TW LP-LG laser beam effectively injects electrons into the beam during the reflection. The electrons that are injected close to the laser axis experience a prolonged longitudinal acceleration by the longitudinal laser electric field. The electrons form distinct monoenergetic bunches with a small divergence angle. The energy in the most energetic bunch is 0.29 GeV. The bunch charge is 6 pC and its duration is approximately $270$ as. The divergence angle is just ${0.57}^{\circ }$ (10 mrad). By using a linearly polarized rather than a circularly polarized Laguerre–Gaussian beam, our scheme makes it easier to demonstrate the electron acceleration experimentally at a high-power laser facility.

High Power Laser Science and Engineering
Dec. 27, 2022, Vol. 10 Issue 6 06000e45 (2022)
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Research Articles
Intense harmonic generation driven by a relativistic spatiotemporal vortex beam
Lingang Zhang, Liangliang Ji, and Baifei Shen

Spatiotemporal optical vortex (STOV) pulses carrying purely transverse intrinsic orbital angular momentum (TOAM) are attracting increasing attention because the TOAM provides a new degree of freedom to characterize light–matter interactions. In this paper, using particle-in-cell simulations, we present spatiotemporal high-harmonic generation in the relativistic region, driven by an intense STOV beam impinging on a plasma target. It is shown that the plasma surface acts as a spatial–temporal-coupled relativistic oscillating mirror with various frequencies. The spatiotemporal features are satisfactorily transferred to the harmonics such that the TOAM scales with the harmonic order. Benefitting from the ultrahigh damage threshold of the plasma over the optical media, the intensity of the harmonics can reach the relativistic region. This study provides a new approach for generating intense spatiotemporal extreme ultraviolet vortices and investigating STOV light–matter interactions at relativistic intensities.

High Power Laser Science and Engineering
Nov. 15, 2022, Vol. 10 Issue 6 06000e46 (2022)
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