An accurate and stable method of array element tiling for high-power laser facilities

Due to laser-induced damage, the aperture of optics is one of the main factors limiting the output capability of highpower laser facilities. Because of the general difficulty in achieving large-aperture optics, an alternative solution is to tile some small-aperture ones together. We propose an accurate, stable, and automatic method of array element tiling and verify it on a double-pass 1 × 2 tiled-grating compressor in the XG-III laser facility. The test results show the accuracy and stability of the method. This research provides an efficient way to obtain large-aperture optics for high-power laser facilities.

Recent diagnostic developments at the 100 kJ-level laser facility in China

A 100 kJ-level laser facility has been designed to study inertial confinement fusion physics in China.This facility incorporates various diagnostic techniques,including optical,x-ray imaging,x-ray spectrum,and fusion product diagnostics,as well as general diagnostics assistance systems and central control and data acquisition systems.This paper describes recent developments in diagnostics at the facility.

Electromagnetic Pulses Generated From Laser Target Interactions at Shenguang Ⅱ Laser Facility

Significant electromagnetic pulses （EMP） can be generated by the intensive laser irradiating solid targets in inertial confinement fusion （ICF）. To evaluate the EMP intensity and distribution in and outside the laser chamber, we designed and fabricated a discone antenna with ultra-wide bands of over 10 GHz. The return loss （Sn parameter） of this antenna was below -10 dB and could even achieve under -30 dB at 3.1 GHz. The EMP intensity in this study at 80 cm and 40 cm away from the target chamber center （TCC） reached 400 kV/m and 2000 kV/m. The current results are expected to offer preliminary information to study physics regarding laser plasma interactions and will also lay experimental foundation for EMI shielding design to protect various diagnostics.

High-Power Single-Spatial-Mode GaSb Tapered Laser around 2.0 μm with Very Small Lateral Beam Divergence

We report high-power single-spatial-mode type-I GaSb-based tapered lasers fabricated on the InGaSb/AlGaAsSb material system. A straight ridge and three different tapered waveguide structures with varying flare angles are fabricated to optimize the output power and spatial-mode performance. The best devices exhibit single-spatial-mode operation with room-temperature output power up to 350？mW in continuous-wave mode at an emission wavelength around 2.0？μm with a very small far-field lateral divergence angle, which is beyond state of the art in terms of single-spatial-mode output power.

A new method for measuring the pulse-front distortion of arbitrary shapes in high-power ultrashort laser systems

We present a new method that can be used to calculate pulse-front distortion by measuring the spectral interference of two point-diffraction fields in their overlapped district. We demonstrate, for the first time, the measurement of the pulse-front distortion of the pulse from a complex multi-pass amplification system, which exists in almost all high-power laser systems, and obtain the irregular pulse-front distribution. The method presented does not need any reference light or assumption about the pulse-front distribution, and has an accuracy of several femtoseconds.

High-Power Passively Q-Switched Nd:YAG Laser at 1112 nm with a Cr^(4+):YAG Saturable Absorber

A high-power passively Q-switched Nd：YAG laser operating at lll2nm with Cr4＋：yAO as a saturable absorber is demonstrated. Under 808 nm diode-direct pumping, the maximum average output power of 2.73 W is achieved at the pump power of 16.65 W, corresponding to an optical-to-optical conversion efficiency of 16.4%. At the same time, the pulse width, pulse repetition rate, single pulse energy and peak power are 27.2ns, 9 kHz, 303.3#3 and 11.2kW, respectively. As far as we know, the result gives the highest average output power at 1112nm generated by an 808 nm diode-end-pumped Nd：YAG laser.

Elucidation of Metallic Plume and Spatter Characteristics Based on SVM During High-Power Disk Laser Welding

During deep penetration laser welding,there exist plume（weak plasma） and spatters,which are the results of weld material ejection due to strong laser heating.The characteristics of plume and spatters are related to welding stability and quality.Characteristics of metallic plume and spatters were investigated during high-power disk laser bead-on-plate welding of Type 304 austenitic stainless steel plates at a continuous wave laser power of 10 kW.An ultraviolet and visible sensitive high-speed camera was used to capture the metallic plume and spatter images.Plume area,laser beam path through the plume,swing angle,distance between laser beam focus and plume image centroid,abscissa of plume centroid and spatter numbers are defined as eigenvalues,and the weld bead width was used as a characteristic parameter that reflected welding stability.Welding status was distinguished by SVM（support vector machine） after data normalization and characteristic analysis.Also,PCA（principal components analysis） feature extraction was used to reduce the dimensions of feature space,and PSO（particle swarm optimization） was used to optimize the parameters of SVM.Finally a classification model based on SVM was established to estimate the weld bead width and welding stability.Experimental results show that the established algorithm based on SVM could effectively distinguish the variation of weld bead width,thus providing an experimental example of monitoring high-power disk laser welding quality.

High-Power Continuous-Wave and Acousto-Optical Q-Switched Ho:(Sc_(0.5)Y_(0.5))_2SiO_5 Laser Pumped by Laser Diode

We experimentally investigate the continuous-wave(cw)and acousto-optical(AO)Q-switched performance of a diode-pumped Ho:(Sc_(0.5)Y_(0.5))_2SiO_5(Ho:SYSO)laser.A fiber-coupled laser diode at 1.91m is employed as the pump source.The cw Ho:SYSO laser produces 13.0 W output power at 2097.9 nm and 56.0%slope efficiency with respect to the absorbed pump power.In the AO Q-switched regime,at a pulse repetition frequency of 5 kHz,the Ho:SYSO laser yields 2.1 mJ pulse energy and 21 ns pulse width,resulting in a calculated peak power of 100 k W.In addition,at the maximum output level,the beam quality factor of the Q-switched Ho:SYSO laser is measured to be about 1.6.

Formation of choroidal neovascularization under macular fovea after high-power laser irradiation: a case report

Dear Editor,I am Dr.Shan-Shan Li,from Northern Jiangsu People’s Hospital,Yangzhou,China.I write to present the case of formation of choroidal neovascularization(CNV)under the fovea after high-power laser irradiation.Currently,there is an increasing availability and accessibility to laser instruments,but improper use of these tools can lead to macular damage and irreversible visual impairment.

Ramp-Wave Compression Experiment with Direct Laser Illumination on ShenGuang-Ⅲ Prototype Laser Facility

Ramp-wave compression experiment to balance the high compression pressure generation in aluminum and x-ray blanking effect in transparent window is demonstrated with an imaging velocity interferometer system for any reflector （VISAR） on ShenGuang-Ⅲprototype laser facility. The highest pressure is about 500 GPa after using the multilayer target design Al/Au/Al/LiF and -10^13 W//cm2 laser pulse illuminated on the planar Al target, which generates the spatial uniformity to 〈1% over 500 μm on the ablation layer. A 2-μm-thick Au layer is used to prevent the x-ray from preheating the planar ablation Al layer and window material LiF. The imaging VISAR system can be used to record the abrupt loss of the probe beam （λ= 532 nm） caused by absorption and reflection of 20-μm, 30-μm and 40-μm-thick Al, i.e., the blanking effect. Although there are slight shocks in the target, the peak pressure 500 GPa, which is the highest data up to now, is obtained with ramp-wave compression.

High-Power Dual-End-Pumped Actively Q-Switched Ho:YAG Ceramic Laser

We present a high-power Ho：YAG ceramic laser pumped at 1908nm. Using a dual-end-pumped structure, the maximum continuous-wave output power of 48 W is obtained, corresponding to a slope efficiency of 70.4% with respect to the absorbed pump power. At actively Q-switched mode, the maximum average output power of 46 W and the minimum pulse width of 21 ns are achieved at a pulse repetition frequency of 20 kHz, corresponding to a peak power of approximately 109.5kW. In addition, the beam-quality M2 factor is found to be 1.4 at the maximum output power.

Effective electromagnetic shielding with multi-layer structure material on Shenguang laser facility

Electromagnetic pulses(EMPs)with high intensity and frequency bandwidth can be generated during the intensive laser irradiating solid targets in inertial confinement fusion(ICF).To shield the EMPs radiation and hence protect various diagnostics in and outside the target chamber,we designed a multi-layer structure material to shield the EMFs and demonstrate experimentally and numerically shielding performance of the material structure.The thickness of the multi-layer structure material has a great influence on the EMPs shielding.It is shown that,with the increase of the material thickness,the better shielding performance is obtained,and the material structure with polytetrafluoroethyIene of 0.5 mm,copper of 0.4 mm and lead of 2.4 mm reduces 448 times compared the maximum value of EMPs voltage to that without shielded.The design of multilayer structure material for EMPs shielding provides a promising way to reduce EMPs radiation,which is extremely useful for the diagnostics protection and signal processing in ICF.

Demonstration of a Shock-Timing Experiment in a CH Layer at the Shen Guang Ⅲ Laser Facility

Shock-timing experiments are indispensable to inertial confinement fusion mainly because the timing of multiple shock waves is crucial to understanding the processes of laser irradiation of targets. Investigations into shock waves driven by a two-step radiation pulse in polystyrene（CH） capsule targets are experimentally conducted at the Shen Guang Ⅱ laser facility. Differing from the traditional shock-timing implementation in which one shock wave could catch up with another one in solid CH, in this experiment, the second shock front in a rarefaction CH layer is observed through velocity interferometry. This second shock could also be made to converge with rarefaction waves within only a few micrometers of the CH capsule by designing the two-shock coalescence time. A shock-timing diagnostic technique to tune the multi-shock convergence in the CH capsule can thereby be achieved.The experimental results in the CH layer are quasi-quantitatively interpreted using streamlines simulated with the Multi-1 D program. The experimental results are expected to offer important information for target structure and laser pulse design, both of which are important for realizing inertial confinement fusion.

High-Power Continuous-Wave Nd:GdVO4 Solid-State Laser Dual-End-Pumped at 880 nm

A high-power cw all-solid-state Nd：GdVO4 laser operating at 88Onto is reported. The laser consists of a low doped level Nd：GdV04 crystal dual-end-pumped by two high-power diode lasers and a compact negative confocM unstable-stable hybrid resonator. At an incident pump power of 820 W, a maximum cw output of 240 W at 1064nm is obtained. The optical-to-optical efficiency and Mope efficiency are 40.7% and 53.2%, respectively. The M2 factors in the unstable direction and in the stable direction are 4.38 and 5.44, respectively.

High-Power and High-Efficiency Operation of Terahertz Quantum Cascade Lasers at 3.3 THz

A high-power and high-effciency GaAs/A1GaAs-based terahertz （THz） quantum cascade laser structure emitting at 3.3 THz is presented. The structure is based on a hybrid bound-to-continuum transition and resonant-phonon extraction active region combined with a semi-insulating surface-plasmon waveguide. By optimizing material structure and device processing, the peak optical output power of 758mW with a threshold current density of 120 A/cm2 and a wall-plug effciency of 0.92% at 10K and 404mW at 77K are obtained in pulsed operation. The maximum operating temperature is as high as llS K. In the cw mode, a record optical output power of 160roW with a threshold current density of 178A/cm2 and a wall-plug efficiency of 1.32% is achieved at 1OK.

New Laser Facility SG-Ⅱ Established

A novel laser facility, named SG-Ⅱ (Shenguang, or Divine Light), has recently passed the appraisal of a panel of experts from the Chinese Academy of Sciences (CAS), China Academy of Engineering Physics, and State High-tech Development Program (National 863 Program). ……

Effect of the high-power electromagnetic pulses on the reactivity of the coal-water slurry in hot environment

An effect of the high-power electromagnetic pulses onto the droplet of coal-water slurry inside the furnace was investigated.In contrary to the previously investigated laser-induced fuel atomization that occurs at the room temperature,the pre-heated(to 400 K)slurry becomes dry enough to prevent the explosion-like steam formation.Thus,fuel does not atomize and the ignition does not accelerate.Furthermore,the absorption of several laser pulses leads to evident sintering of irradiated surface with following increase of the ignition delay time for up to 24%.Variation of the pulse energy in range 48-118 mJ(corresponding intensity up to 2.4 J·cm^-2)leads to certain variation of the increase of ignition delay.The strong pulsed overheating of the coal water slurry which does not initiate the fine atomization of the fuel generally makes its ignition longer.

Control systems and data management for high-power laser facilities

The next generation of high-power lasers enables repetition of experiments at orders of magnitude higher frequency than what was possible using the prior generation.Facilities requiring human intervention between laser repetitions need to adapt in order to keep pace with the new laser technology.A distributed networked control system can enable laboratory-wide automation and feedback control loops.These higher-repetition-rate experiments will create enormous quantities of data.A consistent approach to managing data can increase data accessibility,reduce repetitive data-software development and mitigate poorly organized metadata.An opportunity arises to share knowledge of improvements to control and data infrastructure currently being undertaken.We compare platforms and approaches to state-of-the-art control systems and data management at high-power laser facilities,and we illustrate these topics with case studies from our community.

High-power femtosecond laser generation from an all-fiber linearly polarized chirped pulse amplifier

An all-fiber high-power linearly polarized chirped pulse amplification(CPA)system is experimentally demonstrated.Through stretching the pulse duration to a full width of approximately 2 ns with two cascaded chirped fiber Bragg gratings(CFBGs),a maximum average output power of 612 W is achieved from a high-gain Yb-doped fiber that has a core diameter of 20μm with a slope efficiency of approximately 68%at the repetition rate of 80 MHz.At the maximum output power,the polarization degree is 92.5%and the M^(2)factor of the output beam quality is approximately 1.29;the slight performance degradations are attributed to the thermal effects in the main amplifier.By optimizing the B-integral of the amplifier and finely adjusting the higher-order dispersion of one of the CFBGs,the pulse width is compressed to 863 fs at the highest power with a compression efficiency of 72%,corresponding to a maximum compressed average power of 440.6 W,single pulse energy of 5.5μJ and peak power of about 4.67 MW.To the best of our knowledge,this is the highest average power of a femtosecond laser directly generated from an all-fiber linearly polarized CPA system.

中国光子大科学装置的发展

光子大科学装置主要分为同步辐射光源和自由电子激光装置两大类,可以产生光子能量范围从红外到硬X射线的高通量、高亮度、光子能量连续可调的光,在过去半个多世纪里已经发展成为服务于物理学、化学、材料科学、生命科学、能源、环境科学等众多学科的综合性科研中心。光子大科学装置在我国经过了40多年的发展,已经建立起较完整的体系,达到世界先进水平。文章通过对已有的和建设中的装置的描述分别介绍同步辐射光源和自由电子激光在国内的发展历程和现状,并对未来发展趋势进行展望。