激光脉冲能量对激光诱导Al等离子体辐射特征的影响

使用Nd：YAG激光器烧蚀金属Al靶获得等离子体,利用光谱时-空分辨技 术,在52 mJ～145 mL/pulse激光脉冲能量范围内,关于激光脉冲能量对激光诱导等离子体 辐射特征的影响进行了研究。使用的气体是Ar气,压强为10 kPa。结果发现,激光脉冲能量 升高,引起特征谱线强度增加,连续谱强度也增加;但能量过高,会击穿周围气体,产生气 体微等离子体。此时,特征谱和连续谱几乎不再增强;最大特征辐射强度在145 mJ、10 kPa 、靶前0.1 mm处、延时180 ns获得;同一条件下获得最强背景连续谱,而信号-背景差是在 145 mJ、10 kPa、靶前1.0 mm处、延时450 ns达到最大值。基于Al等离子体不同激光脉冲能 量下的时间-空间分辨谱,对结果进行了简单的讨论。并分别确定了获得最大特征辐射和信 号-背景差的条件。     

Dual sub-picosecond and sub-nanosecond laser system

A high power laser system delivering a 20-TW, 0.5-0.8 ps ultra-short laser pulse and a 20-J, 500-ps long pulse simultaneously in one shot is completed. This two-beam laser operates at the wavelength of 1053 nm and uses Nd doped glass as the gain media of the main amplification chain. The chirped-pulse amplification (CPA) technology is used to compress the stretched laser pulse. After compression, the ultra-short laser pulse is measured: energy above 16.0 J, S/N contrast ratio-10~5 : 1, filling factor->52.7%. Another long pulse beam is a non-compressed chirped laser pulse, which is measured: energy-20 J, pulse duration 500 ps. The two beams are directed onto the target surface at an angle of 15°.     

飞焦级脉冲激光能量测量方法研究

为解决1 fJ～1 pJ脉冲激光能量的测量问题,提出了一种基于时域波形积分的飞焦级脉冲激光能量测量方法。该方法采用光电倍增管（PMT）获得飞焦级脉冲激光的响应信号,该微弱响应信号经放大、校准与激光脉冲时域波形积分后实现飞焦级脉冲激光能量测量。根据该方法设计了飞焦级脉冲激光能量测量装置,并分析了该装置的测量不确定度。实验表明,该装置实现了波长1 064 nm、脉冲宽度5 ns～1 s、能量范围1 fJ～1 pJ的脉冲激光光源的能量测量,测量不确定度为15.8%。     

Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser

Mechanical properties of engineering material can be improved by introducing compressive residual stress on the material surface and refinement of their microstructure. Variety of mechanical process such as shot peening, water jet peening, ultrasonic peening, laser shot peening were developed in the last decades on this contrast. Among these, lasers shot peening emerged as a novel industrial treatment to improve the crack resistance of turbine blades and the stress corrosion cracking (SCC) of austenic stainless steel in power plants. In this study we successfully performed laser shot peening on precipitation hardened aluminum alloy 6061-T6 with low energy (300 mJ, 1064 nm) Nd:YAG laser using different pulse densities of 22 pulses/mm² and 32 pulses/mm². Residual stress evaluation based on X-ray diffraction sin² ψ method indicates a maximum of 190% percentage increase on surface compressive stress. Depth profile of micro-hardness shows the impact of laser generated shock wave up to 1.2 mm from the surface. Apart from that, the crystalline size and micro-strain on the laser shot peened surfaces have been investigated and compared with the unpeened surface using X-ray diffraction in conjunction with line broadening analysis through the Williamson-Hall plot.     

一种基于增益调制技术的全光纤化脉冲Yb光纤激光器

以波长为975 nm的半导体激光器作为泵浦源,周期性地脉冲泵浦一个包含Yb掺杂光纤和光纤光栅对的Yb光纤激光器,实现了基于增益调制技术的全光纤化高功率Yb光纤激光器的稳定脉冲输出.在50 kHz重频下,采用20 W的泵浦功率和2.4 μs的泵浦脉冲宽度,获得了1 060 nm波长脉冲宽度仅100 ns的稳定脉冲激光输出,单脉冲激光能量约为20 μJ.以此作为脉冲激光种子进行功率放大,获得了性能稳定的全光纤结构高功率脉冲激光输出,放大后单脉冲能量超过200 μJ,激光放大器斜率效率达到60%.     

Design and construction of Q-switched Nd:YAG laser system for LIBS measurements

A passive, Q-switched pulsed, Nd:YAG laser system was designed and built, which can provide a potential compact robust laser source for portable laser induced breakdown spectroscopy systems.The developed laser system operates at 1064 nm. Each laser shot contains a train of pulses having maximum total output energy of 170 mJ. The number of pulses varies from 1–6 pulses in each laser shot depending on the pump energy. The pulse width of each pulse ranges from 20 to 30 ns. The total duration of the output pulse train is within 300 μs. The multi-pulse nature of the laser shots was employed to enhance the LIBS signal. To validate the system, LIBS measurements and analysis were performed on ancient ceramic samples collected from Al-Fustat excavation in Old Cairo. The samples belong to different Islamic periods in Egypt history. The results obtained are highly indicative that useful information can be provided to archeologists for use in restoring and repairing of precious archeological objects.     

Display glass cutting by femtosecond laser induced single shot periodic void array

We propose an idea of fast cutting a display glass plate where the sample is pre-processed micromachining single shot rear-surface and internal void arrays aligned on working plane prior to glass cleaving. Single shot void morphology is investigated varying input pulse energy, focusing depth, and scanning speed. A femtosecond laser with pulse duration of 172 fs, central wavelength of 780 nm, and repetition rate of 1 kHz is used to fabricate voids.     

Noise amplification during supercontinuum generation in microstructure fiber

Supercontinua generated by femtosecond pulses launched in microstructure fiber can exhibit significant low-frequency (<1-MHz) amplitude noise on the output pulse train. We show that this low-frequency noise is an amplified version of the amplitude noise that is already present on the input laser pulse train. Through both experimental measurements and numerical simulations, we quantify the noise amplification factor and its dependence on the supercontinuum wavelength and on the energy and duration of the input pulse. Interestingly, the dependence differs significantly from that of the broadband white-noise component, which arises from amplification of the input laser shot noise.     

Self-Q-switched diode-end-pumped Nd: BGO laser

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新型LD泵浦振荡器与多程放大系统联机实验

 新型LD泵浦振荡器采用环形结构，具有单纵模几率高、时间波形和幅度高度稳定等特点，这些特点使其与多程放大器的联机非常方便，没有系统多脉冲注入破坏风险和能量不稳破坏风险。联机实验取得了显著成果，输出能量达1.252kJ，是当前国内固体激光器单束输出最高能量，此时输出脉冲脉宽为1.26ns，光束口径为20cm×20cm，平均功率密度达2.5GW/cm²。     

阵列口径Nd：YAG片状激光器重复频率输出特性研究

 介绍了一种新型阵列口径的固体片状激光器的设计及初步实验，研究了其激光输出特性。激光器采用2×1阵列的以布儒斯特角放置的片状Nd:YAG激光头模块，并采用氙灯泵浦方式，由平均功率为60 kW电池组电源供电；电源采用大电流恒流放电，对输出放电进行可编程控制。实验得到32 J的单脉冲输出，小信号增益系数为0.047 5 cm^-1；泵浦单脉冲运行，脉宽为1~5 ms时，输出能量呈线性增长；当重复频率为20 Hz时，5 ms脉宽比1 ms脉宽输出功率下降30%，其原因是布儒斯特角结构存在时，由热应力引起的热致双折射损耗造成输出下降。     

Generation of 1.5-12 ns width-tunable 532 nm pulses by adopting laser-induced plasma shutter technique

The pulse-shaping technique has found widespread applications in nonlinear optics and material processing. Experimental research on laser-induced plasma shutter to control the 532 nm pulse width is conducted. The impacts of the total pulse output energy on pulse compression are investigated, and a useful conclusion can be drawn that there exists an optimal value of pulse energy at which the shortest output pulse of 3.23 ns can be obtained without a device for delay-time. Once the device for delay-time is employed to change the optical differences between two laser paths, the pulse width can be further shortened to 1.51 ns. In short, the 1.5-12 ns width-tunable 532 nm laser pulses have been obtained by adopting the laser-induced plasma shutter technique.     

斩波法在激光微能量计标定中的应用

针对激光微能量测量的问题,提出一种激光微能量计标定新方法与装置。研究内容包括使用稳功率连续激光作为光源,通过基于斩波的脉冲发生器组件产生微能量的脉冲激光,分别使用脉冲宽度测量组件与功率测量组件进行采集、软件分析,计算微能量值及通过比对给出待标定能量计的修正系数,计算机自动控制。结果表明,该方法获得的脉冲激光波形稳定,脉冲宽度测量不确定度为0.08%,激光功率测量不确定度为0.25%,实现了对0.1 pJ～1 mJ的激光能量进行准确复现和传递。     

Electron Spectrum of Nonlinear Cold Emission from a Metal under the Action of a Laser Shot

The nonlinear emission of electrons from a metal under the action of a femtosecond moderate-intensity laser pulse (laser shot) has been studied. A theoretical model of the process has been constructed based on the 1D nonstationary Schrödinger equation in the vacuum half-space with given boundary conditions for the electron wavefunction. This equation has been solved using the Laplace transformation. It has been assumed that the states of free electrons in a metal, which are described by the Sommerfeld theory of metals, are insignificantly influenced by the laser field. The energy spectrum of emitted electrons has been obtained, and its dependence on the parameters of the lased shot has been found. The calculated spectrum of nonlinear electron emission from a tungsten nanotip under the action of a 6.5-fs-long laser shot generating a field of 9.26 V/nm agrees with the experimental data.     

Ablation induced by single-and multiple-femtosecond laser pulses in lithium niobate

We present results on the surface-damage threshold of lithium niobate after single-and multiple-femtosecond laser pulse irradiation at the pulse duration of 80 fs, with a 800-nm wavelength, and a repetition rate of 1 kHz. The surface-ablation threshold was found to decrease significantly with an increase in the pulse number applied to the surface until reaching an almost constant level due to an incubation effect, which is attributed to the laser-induced defect formation. The threshold of lithium niobate under a single shot is found to be 2.82 J/cm², and the threshold fluence for an infinite number of pulses F _th(∞) converges to a common value of 0.52 ± 0.06 J/cm² for N > 80. The results have the potential for application in laser micromachining and the fabrication of related optical devices and applications in frequency conversion by a femtosecond laser in lithium niobate.     

A high power N2 laser using water filled strip lines

A fast Blümlein circuit with a water strip line having an impedance of Z=0.078 is used to excite a TE high power nitrogen laser at =337 nm. A pseudospark switch works as a high power switch. The laser operates without any preionization and delivers an energy of 15 mJ per pulse (7.5 ns FWHM) at 10 kPa nitrogen pressure. The shot to shot reproducibility is better than 5%.     

Deep drilling on a silicon plate with a femtosecond laser: experiment and model analysis

The ablation rate when drilling fine holes having large aspect ratios in silicon substrates with femtosecond laser pulses was estimated from mechanically ground cross sections of the ablated holes. The ablation rate shows a dramatic change at the depth at which the laser pulse reaches a certain fluence, which is nearly constant when the initial laser fluence was varied from 14.5 to 59.4 J/cm². The ablation rate, threshold fluence, in three fluence domains, and the transition fluences at which the ablation rate shows a dramatic change, were derived. However, when a pulse energy of 200 μJ was used a much greater ablation rate was obtained, suggesting that another fluence domain for larger ablation rates exists. The experimentally obtained hole depths as a function of shot numbers were reproduced by a theoretical model, which incorporates laser pulse attenuation in the holes that is the same as that in waveguides for some attenuation coefficient and ablation rates for three fluence domains. PACS 42.62.-b; 42.65.Re; 78.40.Fy; 78.47.+p; 81.20.Wk     

荧光激光雷达技术探测水面油污染系统仿真研究

为实现对水面油污染的探测, 根据荧光激光雷达系统的发展趋势, 采用激光诱导荧光技术, 建立了适用于水面油污染探测荧光激光雷达的系统模型. 提出采用单激光器结合增强电荷耦合器件的小型荧光激光雷达探测系统, 通过分析激光器单脉冲能量与探测浓度之间的关系, 结合荧光激光雷达系统参数, 对系统模型的探测能力与信噪比等进行了数值仿真. 结果表明, 系统选用单脉冲能量50 μJ的355 nm Nd:YAG激光器作为激发光源, 白天在7 m的距离处探测信噪比可以达到10, 满足实验室搭建模拟系统的要求. 针对实际探测水面油污染, 提出采用增大激光器功率的方法, 并通过模拟计算验证了采用50 mJ的单脉冲能量激光器在230 m的探测距离处可得到与实验室相同的结果, 对实际系统的搭建具有指导意义.     

Optodynamic energy conversion efficiency during laser ablation on metal surfaces measured by shadow photography

In this investigation, we used high-speed shadow photography to observe fast optodynamic phenomena such as shock waves and the ablation of flat metal surfaces. These phenomena were induced in air by a Q-switched Nd:YAG laser (λ = 1,064 nm) with a pulse duration of 4 ns and an excitation pulse energy between 10 and 55 mJ. For a good spatial resolution of the shadowgraphs, we used short illumination pulses (30 ps) from a frequency-doubled Nd:YAG laser (λ = 532 nm). Using the shadowgraphs of the shock wave expansion into a half-space, we measured the optodynamic energy conversion efficiency, defined as the ratio between the mechanical energy of the shock wave and the excitation pulse energy. This efficiency increases with an increasing excitation pulse energy. We also present the characteristic shadowgraphs of the ablation of a black-painted metal surface, where the macroscopic material particles are clearly visible. They follow the shock wave and eventually overtake it. As a result, the shape of the shock wave, which normally expands concentrically into the half-space, has an altered form. The presented results reveal the phenomenon of the laser ablation of coated metal surfaces.     

Characterization of a high efficiency, ultrashort pulse shaper incorporating a reflective 4096-element spatial light modulator

We demonstrate pulse shaping via arbitrary phase modulation with a reflective, 1 × 4096 element, liquid crystal spatial light modulator (SLM). The unique construction of this device provides a very high efficiency when the device is used for phase modulation only in a prism based pulse shaper, namely 85%. We also present a single shot characterization of the SLM in the spatial domain and a single shot characterization of the pulse shaper in the spectral domain. These characterization methods provide a detailed picture of how the SLM modifies the spectral phase of an ultrashort pulse.