Evolution of excited and ground-state species during burst-modeexcitation of a barium vapor laser

We have studied the development of steady-state prepulse conditions in the barium vapor laser (BVL) and the factors that influence steady-state laser output using the “hook” method. The ground-state and excited-state Ba and Ba⁺ species were measured during the afterglow, and the Ba¹P₁ upper laser level during the excitation phase, of selected pulses in a burst of excitation pulses at 8 kHz. Results show that large depletion of the prepulse Ba¹S₀ ground-state density from the axial region of the tube occurs during the burst, and proportional decreases in the peak upper-laser-level density occur during the excitation phase. The prepulse electron density (inferred from the total Ba⁺ density) and lower-laser-level densities do not accumulate during the burst, and it is concluded that ground-state depletion is primarily responsible for the decrease in laser pulse energy that occurs during a burst as reported by Pask and Piper (1994). We observe significantly greater depletion of the ground-state density on-axis in the presence of neon buffer gas than helium, which we attribute primarily to the high ionization observed in neon and the corresponding increase in the effects of ambipolar diffusion. The results highlight the influence of buffer-gas composition on the establishment of steady-state prepulse conditions (and therefore the steady-state laser pulse characteristics). The results have implications to metal vapor lasers in general, including those operating with buffer-gas additives     

Parametric study of the CuBr laser with hydrogen additives

A systematic investigation of the electric parameters of the CuBr laser plasma during the excitation pulse in their correlation to the laser output characteristics is presented. Based on the electric parameters, the temporal course is calculated of the electron density during the discharge. The authors offer a simple mechanism which generally explains how the hydrogen occurred in the copper vapor laser. The mechanism consists of shielding copper ionization by the process of electron detachment from negative hydrogen ions     

Afterglow ground-state copper density behavior in kineticallyenhanced copper vapor lasers

The interferometric “Hook” method has been used to measure the copper ground-state density during the interpulse period for a 38 mm bore diameter copper vapor laser (CVL) operated in kinetically enhanced (KE) mode (Ne-HCl-H₂ gas mixture) and in conventional mode (pure neon and Ne-H₂ gas mixtures). It was found that the rate of regrowth of the axial copper density during the afterglow of the KE-CVL is 3-4 times faster, and the axial prepulse ground-state copper density is 2-3 times higher, than that observed for pure Ne or Ne-HI buffer gases. We conclude that the primary action of the HCl+H₂ additives is to increase the interpulse plasma relaxation rate and to increase the threshold copper density beyond which thermal runaway occurs. These effects are primarily responsible for the elevated pulse rates and increased pulse energies giving improved power scaling characteristics of KE-CVLs     

A self-consistent model for the discharge kinetics in ahigh-repetition-rate copper-vapor laser

A self-consistent computer model has been developed to simulate the discharge kinetics and lasing characteristics of a copper-vapor laser (CVL) for typical operating conditions. Using a detailed rate-equation analysis, the model calculates the spatio-temporal evolution of the population densities of 11 atomic and ionic copper levels, four neon levels, and includes 70 collisional and radiative processes, in addition to radial particle transport. The long-term evolution of the plasma is taken into account by integrating the set of coupled rate equations describing the discharge and electrical circuit through multiple excitation-afterglow cycles. A time-dependent two-electron group model, based on a bi-Maxwellian electron energy distribution function, has been used to evaluate the energy partitioning between the copper vapor and the neon-buffer gas. The behavior of the plasma in the cooler end regions of the discharge tube near the electrodes, where the plasma kinetics are dominated by the buffer gas, has also been modeled. Results from the model have been compared to experimental data for a narrow-bore (φ=1.8 cm) CVL operating under optimum conditions. Close agreement is obtained between the results from the model and experimental data when comparing electrical I-V characteristics of the discharge tube and circuit, and spatio-temporal evolution of the population densities of the laser levels and other excited Cu I and Ne I states, and lasing characteristics. During the period of lasing action, the populations of the laser levels are perturbed by 10-20 percent due to stimulated emission     

飞秒激光作用下金属箔材的温度场研究

为分析飞秒激光作用下金属材料的温度场及其影响因素,基于一维双温模型对飞秒激光作用下铜箔的温度场进行数值求解。采用脉宽100～800 fs,能量密度509.30～2 546.48 J/m2的激光参数进行计算,研究电子和晶格温度随时间和深度的变化规律。结果表明,电子温度在10-13 s时间尺度达到峰值,在此期间晶格温度保持不变,此后通过耦合作用二者在几ps时间尺度达到热平衡;脉宽越短,电子温升越快、峰值温度越高、耦合时间越短;而能量密度越大,电子温升越快、峰值温度越高、耦合时间越长。     

Effects of F- ions and F2 molecules on theoscillation process of a discharge-pumped ArF excimer laser

Time-resolved number densities of the fluorine negative ion in a discharge-pumped ArF excimer laser are measured by a dye laser absorption method. The peak density of F^- is 0.93 ×10 ¹⁵ cm^-3 at a total gas pressure of 2.5 atm, a gas mixture ratio of F₂-Ar-He=0.2-10.0-89.8, and a charging voltage of 28 kV for a 68-nF storage capacitor bank. The dependences of the peak F^- density and the ArF laser output power on the F ₂ gas fraction in F₂-Ar-He mixture are investigated. The effects of F^- ions and F₂ molecules on the ArF laser oscillation process are discussed by considering the F₂ mixture-ratio dependences of particle densities, laser output power, mean electron energy, and laser power extraction efficiency. With increasing F₂ mixing ratio, the ArF* excimer formation first increases as F^- increases, but in F₂-rich conditions the laser power decreases because of the laser photon absorption due to F^- ions and quenching of ArF* with F₂ molecules     

Theory of the nitrogen laser excited by a relativistic electron beam

The 3371-Å nitrogen laser power produced by a relativistic electron beam propagating into nitrogen gas is calculated. The excitation contribution of cascade electrons is found to be negligible, compared to the excitation caused by plasma electrons drifting in the inductive electric field produced by the beam. The extensive experimental data on nitrogen are used wherever possible in the calculation, and the sensitivity of the calculation to the various input data is given. The mechanism of excitation by plasma electrons alone yields agreement within experimental uncertainty for such parameters as the peak laser power, the laser pulsewidth and delay, and the dependence of the laser power on gas pressure. For a larger drift tube radius, the plasma electron mechanism predicts higher power and a different pressure dependence, in agreement with experimental trends. In calculating the laser power, the beam propagation details, excitation to the upper and lower laser levels, collisional quenching, a threshold for amplified spontaneous emission, and excitation and deexcitation by plasma electrons are included, but coherent dipole moment effects are neglected.     

Measurement of temporal behavior of electron density in adischarge-pumped ArF excimer laser

Time-resolved number densities of electrons in a discharge-pumped ArF excimer laser are measured by an interferometric method. The peak electron density is 6.7×10¹⁵ cm^-3 at a total gas pressure of 2.5 atom, a gas mixture ratio of F₂-Ar-He=0.2-10.0-89.8, and a charging voltage of 24 kV for a 68-nF storage capacitor bank. The dependences of the electron density and laser output power on the Ar and F₂ fractions in Ar-F₂-He mixture and on the Ne-He mixing ratio in Ar-F₂ -He-Ne mixture are investigated, and the effects of Ar-F₂ -He-Ne mixing ratio on the ArF laser discharge are discussed. The experimental data of the peak electron density are also compared with the results of a computer simulation. A good agreement between them was obtained by considering the fact that the actual discharge volume occupied only part of the electrode width     

Cu HyBrID laser kinetics: optimization of HBr partial pressure and buffer-gas flow rate

A detailed investigation into the dependence of the densities of the principal plasma species in the laser discharge on the buffer-gas operating parameters is reported. Simple expressions for the densities of the Cu, Br, and H species are derived by considering their major mechanisms for production and loss. These predict that the atomic Cu and Br densities are proportional to the HBr mass flow rate, whereas the density of H species (i.e., H and H/sub 2/) is proportional to the added HBr partial pressure. The theory agrees well with "Hook" method measurements of Cu density in a 25-mm bore diameter device; the Cu density increases approximately in proportion to the HBr mass flow rate, whereas it depends only weakly on the HBr partial pressure. Measurements of the fraction of Cu atoms excited by the discharge pulse, the rate of regrowth of the ground-state Cu density during the inter-pulse period, and the pre-pulse plasma impedance, are also explained in accordance with the theory. The results show that the plasma conditions for maximum laser output, which are remarkably similar to those of other "halogen enhanced" Cu lasers, can be achieved more directly by adjusting the overall buffer flow rate with the partial pressure of the added HBr fixed at 1-2 mbar. The theory is also useful for predicting optimum buffer-gas conditions for a wide range of Cu HyBrID laser dimensions and operating conditions.     

飞秒脉冲激光烧蚀金属的机理分析

为了深入理解超短脉冲激光烧蚀金属的机理,特别是烧蚀过程中靶面电子发射带来的影响,本文分析了飞秒脉冲激光烧蚀金属的机理,并在此基础上建立了一维热传导双温模型,模型考虑了电子热导率、热容、电子-晶格耦合系数等参数随温度的变化,以及表面热电子发射和多光子电离导致靶面的能量损失。选择波长为 800 nm,FWHM为100 fs,峰值功率密度为1.2×1017 W/m2 的高斯型单脉冲激光辐照铜靶进行数值模拟。并对计算数据进行分析,结果表明:多光子电离所导致的电子发射比热电子发射要强,但是热电子发射持续的时间长;多光子电离导致的电子发射带走的靶面能量比较大,在分析飞秒烧蚀过程中不可忽略。     

用双闸流管运行的高功率铜蒸气激光器电源的研究

介绍了采用双闸流管作主放电开关的高功率铜蒸气激光器电源的原理、实验和结果。输入电源的功率达到10kW，输出激光平均功率最高达到100W，国产闸流管可以有效、正常地工作。     

Effect of preionization of a copper vapor laser

It is shown that the operation of a copper vapor laser (CVL) in exeptionaly lowE/Pis due to preionization. By introducing a wire preionization to a CVL we demonstrate its influence on the laser performance. In a repetitively pulsed CVL a previous pulse can be the preionization source.     

Laser-induced plasma formation in water at nanosecond tofemtosecond time scales: calculation of thresholds, absorptioncoefficients, and energy density

The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density     

Laser Drilling of Copper Foils for Electronics Applications

Single pulse drilling of copper foils and copper-coated dielectric circuit board materials, relevant to applications in micro-electronics packaging, has been investigated here using an enhanced peak power CO ₂-laser. The plasma generated during copper laser ablation, under these conditions, has been found to be self-extinguishing once the copper has been punched through, and does not materially impact the process. The analysis of the undercut formation in the copper coated laminates illustrated a direct link with the energy delivered to the dielectric after the copper has been laser ablated. Holes with zero undercut were obtained by the use of an acousto-optic modulator, used as a pulse shutter, to control the energy delivered to the dielectric. For unmodulated laser pulses, holes with zero undercut were obtained when drilling copper foils 35-mum thick. In general, when drilling copper-coated dielectrics with unmodulated pulses, holes with low undercut were obtained for peak powers <1.2 kW. However, the stochastic nature of copper drilling dominates the process in this regime. At higher peak powers (up to 1.8kW), a yield of 100% holes in copper is obtained, but this also results in significant undercut     

脉宽对激光撞击电子辐射峰值影响的模拟计算

为了研究激光脉宽对撞击电子后产生的辐射能量分布, 采用模拟计算的方法, 以Lorentz方程以及电子辐射方程为基础, 建立了紧聚焦激光作用于静止单电子模型, 并通过MATLAB软件模拟了不同脉宽下的激光脉冲与电子作用后产生的电子辐射能量分布, 对飞秒紧聚焦椭圆偏振激光脉冲的脉宽与电子间的辐射功率峰值进行了深入研究。结果表明, 当紧聚焦激光脉冲遇到静止单电子, 在激光脉冲撞击电子时, 电子会发出辐射; 散射辐射在散射方向的中心呈尖锥状积累; 随着激光脉宽的增加, 辐射功率分布逐渐呈现出双峰形; 脉冲宽度越宽, 电子辐射功率峰值越小, 脉宽为10λ₀时的峰值功率仅为脉宽为0.1λ₀时峰值功率的1%(初始脉宽λ₀=3.33fs), 同时辐射功率达到峰值所需时间越长, 最高峰的持续时间越长, 频谱函数的截止频率越低, 高频分量变少, 谐波次数增加。该结果对激光空气等离子体诊断方面具有重要意义。     

高频飞秒激光对硅材料烧蚀的热积累效应

对硅材料飞秒激光损伤进行了数值模拟。首先,在飞秒激光与硅相互作用的双温模型中引入能带电子激发、双光子吸收,俄歇复合等一系列非线性项,通过每一个光场半周期中的麦克斯韦电磁场公式,得出材料中不同时间的光场强度,作为描述材料对光强的响应物质方程的输入项。通过麦克斯韦方程与物质方程间的迭代可以得到烧蚀的硅的一系列参数,如电子浓度、晶格温度等。在此基础上分析了皮秒、亚皮秒量级双脉冲和三脉冲飞秒激光作用下,激光能量密度和脉冲延迟时间对热积累效应的影响。     

铜蒸汽激光器双闸流管触发系统的研究

研制成用于钢蒸汽激光器的双间流管触发系统。用两个闸流管并联进行交替触发，总输入电功率达7．8kW。低功率运行时，激光器的效率为0．6％与单间流管激光器的效率一致。     

溴化亚铜脉冲激光研究

本文应用电子温度、电子密度实验值,通过数值求解CuBr激光能级粒子数速率方程,得到了脉冲期间随时间变化的铜原子能级粒子数和激光脉冲功率,结果表明:计算的激光脉冲与实验观测基本相符;电子温度峰值和电子温度在较高能量区的滞留时间,是决定激光行为的两个重要因素。     

小型磁脉冲压缩器在铜蒸气激光器上的应用

介绍了一种用小镍-锌磁杯组成的小型磁脉冲压缩器,并将它成功地用于铜蒸气激光器电路中。同时给出了它的基本原理和理论设计,并且对实验结果进行了分析。     

Modeling the plasma kinetics in a kinetically enhanced copper vaporlaser utilizing HCl+H2 admixtures

A detailed computer model has been used to simulate the plasma kinetics and lasing characteristics in a kinetically enhanced copper vapor laser (KE-CVL) which utilizes Ne-H₂-HCl buffer gas mixtures. The model reproduces key features of the observed operating characteristics of the KE-CVL-in particular, relating to the electrical characteristics of the plasma tube, time evolution of Cu 4s²S _1/2 ground state density, and formation of the laser output. It is shown that the principal role of the HCl additive is to increase the electron loss rate during the interpulse period via dissociative attachment reactions between free electrons and vibrationally excited HCl (ν=1,2) molecules. This leads to a reduction of the prepulse electron density establishing more favorable prepulse conditions for laser action during the subsequent excitation phase. In the KE-CVL, the plasma skin effect governing the development of the radial electric field is greatly reduced compared to conventional CVL's, altering the spatio-temporal evolution of the optical gain and laser field intensities to substantially enhance high-beam-quality output. Comparisons between model results and experimental data for the decay rate of the Cu 4s^{2 2}D_3/2 metastable lower laser level in the early afterglow suggest that there may be an additional de-excitation mechanism for the ²D_3/2,5/2 levels in the KE-CVL plasma which has yet to be identified