Laser Wakefield Acceleration Using Mid-Infrared Laser Pulses

We study a laser wakefield acceleration driven by mid-infrared （mid-IR） laser pulses through two-dimensional particle-in-cell simulations. Since a mid-IR laser pulse can deliver a larger ponderomotive force as compared with the usual 0.8 μm wavelength laser pulse, it is found that electron self-injection into the wake wave occurs at an earlier time, the plasma density threshold for injection becomes lower, and the electron beam charge is substantially enhanced. Meanwhile, our study also shows that quasimonoenergetic electron beams with a narrow energy-spread can be generated by using mid-IR laser pulses. Such a mid-IR laser pulse can provide a feasible method for obtaining a high quality and high charge electron beam. Therefore, the current efforts on constructing mid-IR terawatt laser systems can greatly benefit the laser wakefield acceleration research.

High-quality micro-shape fabrication of monocrystalline diamond by nanosecond pulsed laser and acid cleaning

The flat plane of small surface roughness below 0.1μm average roughness was obtained for monocrystalline diamond by nanosecond pulsed laser irradiation of 1060 nm and post-process acid cleaning,at a laser fluence around the material removal threshold value.The glossy and flat plane at the bottom of the micro-groove was parallel to the top surface of the specimen,although the round beam of Gaussian mode was irradiated in the direction perpendicular to the top surface of specimen.The square beam of top-hat mode produced a shallower micro-groove with a wider,flatter bottom compared with the round beam in Gaussian mode.The creation method of the flat plane with small surface roughness was discussed in the arrangement strategy of linear micro-grooving by the square beam of top-hat mode.Normal side-by-side repetition of linear micro-grooving did not create a flat plane with constant depth.Therefore,a two-step scanning method was proposed in order to overcome the problem in the normal side-by-side repetition of liner micro-grooving.Non-removal areas were partly retained between the processing lines in the first step,and the laser scanning was conducted on the retained area in the second step.The newly proposed two-step scanning method was practical and useful to create a widely flat plane with small surface roughness,and the two-step scanning method provided superior control over the micro-groove depth.This proposed method can reduce the surface roughness in addition to the shape creation of monocrystalline diamond,and it can be used as a high-quality micro-shape fabrication method of monocrystalline diamond.

Transmission effects of high energy nanosecond lasers in laser-induced air plasma under different pressures

When a high energy nanosecond(ns)laser induces breakdown in the air,the plasma density generated in the rarefied atmosphere is much smaller than that at normal pressure.It is associated with a relatively lower absorption coefficient and reduces energy loss of the laser beam at low pressure.In this paper,the general transmission characterizations of a Joule level 10 ns 1064 nm focused laser beam are investigated both theoretically and experimentally under different pressures.The evolution of the electron density(n_(e)),the changes in electron temperature(T_(e))and the variation of laser intensity(I)are employed for numerical analyses in the simulation model.For experiments,four optical image transfer systems with focal length(f)of 200 mm are placed in a chamber and employed to focus the laser beam and produce plasmas at the focus.The results suggest that the transmittance increases obviously with the decreasing pressure and the plasma channels on the transmission path can be observed by the self-illumination.The simulation results agree well with the experimental data.The numerical model presents that the maximum n_e at the focus can reach 10^(19)cm^(-3),which is far below the critical density(n_(c)).As a result,the laser beam is not completely shielded by the plasmas.

Nanosecond Pulses Generation with Samarium Oxide Film Saturable Absorber

Nanosecond pulse generation is demonstrated in a mode-locked erbium-doped fiber laser(EDFL) utilizing a samarium oxide(Sm2O3) film. The Sm2O3 film exhibits a modulation depth of 33%, which is suitable for modelocking operation. The passively pulsed EDFL operates stably at 1569.8 nm within a pumping power from 109 to 146 m W. The train of generated output pulses has a pulse width of 356 nm repeated at a fundamental frequency of 0.97 MHz. The average output power of 3.91 m W is obtained at a pump power of 146 m W, corresponding to 4.0 nJ pulse energy. The experimental result indicates that the proposed Sm2O3 saturable absorber is viable for the construction of a flexible and reliably stable mode-locked pulsed fiber laser operating in the 1.5 m region.

The ablation characteristics of laser-assisted pulsed plasma thruster with metal propellant

In this study,a laser-assisted pulsed plasma thruster(LA-PPT)with a novel configuration is proposed as an electric propulsion thruster which separates laser ablation and electromagnetic acceleration.Owing to the unique structure of the thruster,metals can also be used as propellants,and a higher specific impulse is expected.The ablation quality,morphology,and plume distribution of various metals(aluminium alloy,red copper,and carbon steel)with different laser energies were studied experimentally.The ablation morphology and plume distribution of red copper were more uniform,as compared to those of other metals,and the ablation quality was higher,indicating its greater suitability for LA-PPT.The plume generated by nanosecond laser ablation of aluminium alloy expanded faster,which indicated that the response time of the thruster with aluminium alloy as the propellant was shorter.In addition,when the background pressure was 0.005 Pa,an obvious plume splitting phenomenon was observed in the ablation plume of the pulsed laser irradiating aluminium alloy,which may significantly reduce the utilisation rate of the propellant.

Laser chirp controlled relativistic few-cycle mid-infrared pulse generation

Relativistic few-cycle mid-infrared(mid-IR)pulses are unique tools for strong-field physics and ultrafast science,but are difficult to generate with traditional nonlinear optical methods.Here,we propose a scheme to generate such pulses with high efficiency via plasma-based frequency modulation with a negatively chirped laser pulse(NCLP).The NCLP is rapidly compressed longitudinally due to dispersion and plasma etching,and its central frequency is downshifted via photon deceleration due to the enhanced laser intensity and plasma density modulations.Simulation results show that few-cycle mid-IR pulses with the maximum center wavelength of 7.9µm and pulse intensity of a_(MIR)=2.9 can be generated under a proper chirp parameter.Further,the maximum energy conversion efficiency can approach 5.0%.Such a relativistic mid-IR source is promising for a wide range of applications.

2-μm mode-locked nanosecond fiber laser based on MoS_2 saturable absorber

We demonstrated a 2-μm passively mode-locked nanosecond fiber laser based on a MoS2 saturable absorber（SA）.Owing to the effect of nonlinear absorption in the MoS2 SA, the pulse width decreased from 64.7 to 13.8 ns with increasing pump power from 1.10 to 1.45 W. The use of a narrow-bandwidth fiber Bragg grating resulted in a central wavelength and 3-dB spectral bandwidth of 2010.16 and 0.15 nm, respectively. Experimental results show that MoS2 is a promising material for a 2-μm mode-locked fiber laser.

Oxidation mechanism of high-volume fraction SiCp/Al composite under laser irradiation and subsequent machining

Conventional mechanical machining of a composite material comprising an aluminum matrix reinforced with a high volume fraction of SiC particles(hereinafter referred to as an SiCp/Al composite)faces problems such as rapid tool wear,high specific cutting force,and poor surface integrity.Instead,a promising method for solving these problems is laser-induced oxidation-assisted milling(LOAM):under laser irradiation,the local workpiece material reacts with oxygen,thus forming loose and porous oxides that are easily removed.In the present work,the oxidation mechanism of SiCp/Al irradiated by a nanosecond pulsed laser is studied to better understand the laser-induced oxidation behavior and control the characteristics of the oxides,with laser irradiation experiments performed on a 65%SiCp/Al composite with various laser parameters and auxiliary gases(oxygen,nitrogen,and argon).With increasing laser pulse energy density,both the ablated groove depth and the width of the heat-affected zone increase.When oxygen is used as the auxiliary gas,an oxide layer composed of SiO_(2)and Al2O3 forms,and CO_(2)is produced and escapes from the material,thereby forming pores in the oxides.However,when nitrogen or argon is used as the auxiliary gas,a recast layer is produced that is relatively difficult to remove.Under laser irradiation,the sputtered material reacts with oxygen to form oxides on both sides of the ablated groove,and as the laser scanning path advances,the produced oxides accumulate to form an oxide layer.LOAM and conventional milling are compared using the same milling parameters,and LOAM is found to be better for reduced milling force and tool wear and improved machined surface quality.

Towards all-fiber structure pulsed mid-infrared laser by gas-filled hollow-core fibers

We report here on a diode-pumped pulsed mid-infrared laser source based on gas-filled hollow-core fibers(HCFs)towards an all-fiber structure by the tapering method. The pump laser is coupled into an acetylene-filled HCF through a tapered single-mode fiber. By precisely tuning the wavelength of the diode to match different absorption lines of acetylene near 1.5 μm, mid-infrared emission around 3.1–3.2 μm is generated. With 2 m HCFs and3 mbar acetylene gas, a maximum average power of 130 m W is obtained with a laser slope efficiency of ~24%.This work provides a potential scheme for all-fiber mid-infrared fiber gas lasers.

A systematic investigation of high harmonic generation using mid-infrared driving laser pulses

We report on a systematic investigation of the influences of gas pressure,focal position and focusing geometry on high harmonic generation by use of mid-infrared femtosecond laser pulses. We also discuss the spatial characteristics of harmonics under different focusing conditions. By optimizing the parameters,we experimentally observed the generation of 1 kHz,low divergence coherent X-ray beams in the water-window region.

LEO航天器太阳电池阵激光诱导电弧放电试验与分析

基于太阳电池阵空间电弧放电理论,对纳秒脉冲激光诱导太阳电池阵电弧放电机理进行分析,采用空间放电模拟试验系统,试验研究了低地球轨道(LEO)等离子体环境下纳秒脉冲激光诱导太阳电池阵电弧放电的功率密度阈值及其放电特性。试验结果表明:LEO等离子体环境下激光诱导太阳电池阵电弧放电的功率密度阈值约为1.08×10^(8)W/cm^(2),与无激光诱导相比,放电持续时间增加了20%,且每施加1次纳秒脉冲激光诱导电弧放电概率约为60%。对比分析不同工况下的太阳电池阵损伤效应可得:纳秒脉冲激光诱导电弧放电破坏绝缘层最严重,容易造成太阳电池阵中部分PN结短路失效,其损坏程度远远大于空间环境太阳电池串间电弧放电烧蚀损坏效应和盖玻片表面热应力损伤造成的PN结断路失效。此结论可为航天器太阳电池阵放电试验的措施及试验参数选取提供参考。

脉冲激光改性碳纤维复合材料及其金属化研究

目的选择性地实现碳纤维复合材料(Carbon fiber reinforced plastic,CFRP)表面良好的导电性,满足它在共形天线上的应用。方法采用波长为1064 nm的纳秒脉冲激光器对惰性极高的CFRP表面进行表面改性处理,并结合化学镀铜技术,制备激光改性碳纤维复合材料表面金属层。利用扫描电镜、X射线光电子能谱仪对改性后的材料表面进行表征,利用焊点拉脱方法表征金属层结合力。结果研究表明,激光能量密度越高,则CFRP基材表面树脂被去除得越多;激光搭接率越大,则碳纤维束表面越粗糙;在适当的激光能量密度(60 J/cm^(2))和横/纵向搭接率(50%)下,会产生大量的极性化学基团。当激光能量密度为10~100 J/cm^(2)、激光搭接率为0%时,CFRP表面镀铜层的结合强度为0.57~3.16 MPa,且激光能量密度与镀层结合强度呈正相关。当激光能量密度为60 J/cm^(2)、激光搭接率为−100%~90%时,CFRP表面镀铜层的结合强度为0.19~3.24 MPa,且激光搭接率与镀层结合强度呈正相关。结论脉冲红外纳秒激光在一定能量密度和搭接率的条件下,可改变碳纤维表面形貌、化学成分,实现金属层的制备。

无保护层纳秒脉冲激光强化对DD5单晶合金耐磨性能的影响

目的提高发动机涡轮叶片榫头材料DD5单晶高温合金的高温耐磨性能。方法采用不同能量的无保护层纳秒脉冲激光对DD5单晶合金试样表面进行处理,利用摩擦磨损试验机获得DD5单晶合金在650℃下的摩擦磨损性能参数,并通过显微硬度计、扫描电子显微镜、能谱仪、共聚焦三维表面轮廓仪对磨损前后的DD5单晶试件进行表征分析,揭示纳秒脉冲激光强化技术提高单晶高温合金耐磨性能的机理。结果采用纳秒脉冲激光处理DD5单晶高温合金后,材料表层发生了重熔现象,并在重熔层中生成了弥散分布的铝的氧化物颗粒,其粒径为5~50nm;纳秒脉冲激光处理后产生的弥散分布的氧化铝颗粒为硬质相,它提升了单晶表面的显微硬度;经纳秒脉冲激光处理后,试样的磨损率降低了19.7%~37.6%,单晶合金的磨损机制以磨粒磨损为主。结论经纳秒脉冲激光处理后,单晶表面生成了含有弥散分布的纳米氧化铝颗粒的重熔层,在它与激光诱导的等离子体冲击波产生的硬化层的共同作用下,提升了镍基单晶高温合金在高温下的耐磨性能。

激光能量密度对CMOS探测器的感光面热应力的影响研究

开展了入射脉冲激光能量密度对纳秒脉冲激光辐照CMOS(complementary metal oxide semiconductor)光电探测器的感光面(二氧化硅层/硅层交界面)处热应力的影响研究。建立了CMOS光电探测器的仿真几何结构模型,基于傅里叶热传导方程及热力耦合方程组对纳秒脉冲激光辐照下CMOS光电探测器感光面中心点温升和热应力进行了仿真计算,并对入射脉冲激光能量密度对温升时间演化过程以及热应力的空间分布进行了探讨。仿真计算结果表明,随着入射脉冲激光能量密度增大,CMOS光电探测器的感光面处峰值温度增加以及热应力增大。在纳秒脉冲激光辐照CMOS光电探测器时,感光面处存在的拉应力使CMOS光电探测器先发生力学损伤,随着激光能量密度增大,再发生热学损伤。研究结果对于纳秒脉冲激光诱导CMOS光电探测器损伤机制以及损伤效果的研究具有一定的理论支持。

激光一次性去除铝合金飞机蒙皮涂层实验与数值研究

目的研究激光的频率、功率及扫描速度等参数对脉冲激光清洗航空铝合金表面S06-0215油漆涂层的影响,分析脉冲激光清洗的机制,优化工艺参数组合,并设计航空铝合金表面涂层一次性去除方法。方法以航空铝合金2A12为基材,开展脉冲激光的频率、功率和扫描速度等参数对基材表面涂层烧蚀深度的影响研究,以及烧蚀过程中基材表面最高温度的模拟研究。同时,以表面粗糙度和去除深度为评价指标,对2A12铝合金飞机蒙皮表面涂层进行清洗实验,对采用优化参数清洗后的蒙皮表面进行粗糙度测量、元素含量以及组成成分分析。结果脉冲激光的扫描速度和频率变大,以烧蚀为主导作用的去除机制逐渐减弱,同时振动机制逐渐增强,并最终转变为主导地位。影响激光去除深度的参数,按权重大小依次为扫描速度、功率、频率。结合模拟与实验结果发现,激光频率125kHz、功率70W和速度50mm/s为表面涂层S06-0215最佳的一次性去除工艺参数组合,此时能量密度大小为1.47J/cm^(2),清洗过程中,不损伤2A12铝合金飞机蒙皮基材。通过XRD、SEM以及EDS表征分析表明,氧化膜仅被部分去除。结论激光清洗铝合金表面油漆涂层采用合适的参数组合可以保留氧化膜,可以实现一次性地去除铝合金表面油漆涂层,同时对基材不会造成损伤。

飞秒-纳秒双脉冲激光诱导击穿光谱(LIBS)技术对合金的定量分析

激光诱导击穿光谱(Laser-induced breakdown spectroscopy,LIBS)技术几乎不受聚变环境中的强磁场影响,是一种最有希望实现托卡马克装置中面向等离子体材料(Plasma facing materials,PFMs)原位在线诊断的技术,已被用于多个托卡马克PFMs壁诊断。然而,LIBS技术对PFMs表面元素的探测限、定量分析以及PFMs的服役状态判定依旧面临很大挑战。采用同轴飞秒-纳秒激光协同技术,建立了飞秒-纳秒双脉冲激光诱导击穿光谱(fs-ns-DP-LIBS)技术,通过高峰值功率、低激光能量的飞秒激光诱导等离子体,再用纳秒激光增强常规单脉冲LIBS技术信号发射强度,进而提升常规单脉冲LIBS的探测灵敏度,同时结合6种合金标准样品,采用fs-ns-DP-LIBS技术对样品中的主要元素进行了定量分析,并进一步结合机器学习方法对6种合金进行种类判别。结果显示:在纳秒单脉冲和飞秒单脉冲LIBS检测中,Ni、Fe和Mo在400~800 nm波段没有观察到明显特征峰,仅观察到Cr的特征峰;在飞秒-纳秒脉冲间2μs延时,NiⅠ498.02 nm、FeⅠ517.16 nm、FeⅠ523.85 nm、MoⅠ588.83 nm和MoⅠ603.07 nm均被激发,且Cr的特征峰强度增强明显;并且相比对纳秒单脉冲LIBS技术,Cr的信号强度提高约7倍。定量分析结果显示,fs-ns-DP-LIBS技术绘制的定标曲线拟合度(R ^(2))更高,且Cr的探测限提高约3.5倍。进一步采用决策树、最近邻、线性判别和支持向量机等算法对6种合金进行了分类研究,线性判别和支持向量机的分类预测准确率大于99%。研究有望为原位LIBS技术诊断PFMs壁表面元素、定量分析及其服役状态判定提供检测方法。

可调谐DFB激光器泵浦的PPLN绿色倍频光源

为研制脉冲宽度、重复频率以及输出功率可调谐至单光子级别的可见光波段高重频纳秒级窄脉冲光源,提出了一种基于增益调制型DFB激光器和PPLN晶体的绿色倍频光源设计方案。通过自制的纳秒级高速驱动电路以脉冲方式驱动DFB激光器产生固定波长的泵浦源;利用光纤半导体衰减器调节衰减量,调制泵浦光强度;采用PPLN晶体作为非线性晶体材料,通过其可调谐特性得到准相位匹配的绿色倍频激光输出。实验结果表明,DFB激光器输出的泵浦光功率和中心波长稳定,脉冲宽度1.02 ns~10.25 ns、工作频率1 Hz~10 MHz连续独立可调,泵浦光功率衰减至-44.61 dB时工作于532.96 nm波段的绿色倍频光输出脉冲平均光子数约为384 N,倍频光-光转换效率9.3%。

激光烧蚀反应烧结碳化硅表面形貌特征及亚表面损伤研究

反应烧结碳化硅(RB-SiC/Si)因具有耐高温、耐腐蚀、比刚度高、热膨胀小、耐磨等优点而广泛应用于大口径空间望远镜、卫星遥感、航空发动机零部件等领域。然而高强度高硬度的材料特性,也导致反应烧结碳化硅呈现出难加工的特点。激光束加工具有能量密度高、加工范围广等特点,适用于加工硬脆材料,但同时也存在表面热损伤严重的缺点。本文研究了激光参数对反应烧结碳化硅加工表面形貌特征的影响,分析了纳秒激光与反应烧结碳化硅之间的作用机理。在反应烧结碳化硅烧蚀产物分析基础上确定了高温氧化过程及氧化产物类型。通过角度抛光法研究激光加工单沟槽产生的亚表面损伤,以及激光加工沟槽组之间产生的大尺寸亚表层损伤,建立了激光加工参数与亚表面损伤深度间映射关系。本文的研究成果可为反应烧结碳化硅的高效精密低损伤加工奠定理论基础和提供技术支持。

纳秒脉冲激光降低超声速流场波阻机理研究

纳秒脉冲激光能量沉积在减小超声速波阻方面具有显著的优势和潜在应用价值。为揭示纳秒脉冲激光降低超声速流场波阻机理,利用激光能量非对称沉积模型,建立了在Fluent软件中自定义激光能量沉积区域初始温度的数值模拟方法,研究了在气流马赫数为1.92、圆柱形钝头体直径为20mm、沉积激光重复频率为80kHz时的减阻过程。结果表明:高重频激光在超声速流场中沉积形成一系列低压涡环,涡环合并构成一个“激光空气锥”结构覆盖在钝头体表面,使得波阻减小,在高激光频率的条件下,“激光空气锥”还可以抑制透射激波的再压缩过程,减阻效果稳定。

纳秒脉冲激光清洗紧固件表面污垢试验

针对锈蚀油垢易导致紧固件表面局部腐蚀问题,采用正交试验法对紧固件表面污垢进行纳秒脉冲激光清洗试验。利用极差分析得出激光功率、扫描速度、激光频率及扫描间距与试样清洗效果的显著性关系及最优参数组合,并将清洗前后试样宏观与微观表面形貌、粗糙度大小、元素成分3方面进行对比分析,验证试验的合理性。结果表明,当激光功率为20 W、扫描速度为300 mm·s^(-1)、激光频率为75 kHz、扫描间距为0.04 mm时,能有效去除紧固件表面锈蚀油垢的同时不损伤基底,表面粗糙度由清洗前的8.342μm降至2.972μm,表面Fe元素含量由清洗前的27.5%升至75.7%。