切向气流作用下玻璃纤维复合材料的激光辐照效应

实验研究了样品表面有切向空气气流、切向氮气气流和无气流时,976 nm连续激光对玻璃纤维增强E-51环氧树脂复合材料的辐照效应。结果表明:无气流时,喷出的热分解产物会对入射激光产生屏蔽作用;有气流时,激光对玻璃纤维的破坏方式是其升温熔化后再被切向气流带走;当激光功率密度较低时,切向空气气流以加强样品表面的对流冷却作用为主,不利于激光对玻璃纤维复合材料的破坏;当激光功率密度较高时,切向空气气流以降低屏蔽作用和提供氧气助燃为主,有利于激光对玻璃纤维复合材料的破坏。三种气流状态下,质量损失随功率密度呈现单调增加趋势,当入射激光功率密度在100~600 W/cm2范围内,随着功率密度的增大,激光能量的利用效率逐渐增大并趋于稳定。     

Formation of laser jet thrust in the supersonic regime

Basic methods for obtaining laser jet thrust in the supersonic regime corresponding to the supersonic flow in the jet nozzle are analyzed. It is shown that the method based on the interaction of a laser ablative jet with the supersonic flow is promising. In this case, laser thrust is formed due to additional acceleration of the flow behind the ablation region. Numerical simulation of the flow in a parabolic nozzle is employed to demonstrate the possibility of effective formation of laser thrust at a level of 3 × 10^?3 N/W.     

激光熔覆中同轴粉末流温度场的数值模拟

在激光同轴送粉熔覆中,由于激光与粉末流相互作用,粉末流整体温度分布直接影响激光熔覆的质量.基于非预混燃烧模型,将激光相处理为连续性介质,粉末颗粒相看作离散相物质,建立了激光作用下粉末流的质量、动量和能量方程.用Fluent软件进行了不同激光功率和粉末流速度条件下粉末流整体温度场数值模拟,讨论了各种参数对温度场分布的影响.为了验证该模型的准确度,利用CCD比色测温方法测量了粉末流整体温度场分布.结果表明,数值模拟与CCD检测结果具有良好的一致性,数值模拟结果对激光熔覆具有指导意义.     

Charateristics of a helical axial flow CO2laser

A novel CO2 laser tube which forms a helical axial gas flow is designed. The influences of such a flow on the output laser beam is discussed.     

黑腔中高斯型激光束偏折的模拟研究

利用三维激光等离子体相互作用流体程序LAP3D数值模拟研究了高斯光束在均匀等离子体传播过程中的激光束偏折现象,分别考察了激光强度变化和等离子体横向流的流速变化对束偏折的影响。模拟结果表明:当流速接近离子声速时,激光束偏折效应最明显,此时激光偏折程度与激光强度成正比;当流速逐渐小于离子声速时,激光束偏折行为受抑制逐渐减弱;而当流速逐渐大于离子声速时,激光光强包络的变化以衍射效应为主。     

Expansion of a laser plume from a silicon wafer in a wide range of ambient gas pressures

Expansion of the laser plume from a silicon wafer into surrounding gas is considered in the range of ambient gas pressure from 0.1 to 1 bar using a kinetic approach. The plume is generated by a nanosecond Gaussian laser pulse. Absorption of laser radiation and heating and melting of the target are described by a two-dimensional thermal model. Axisymmetric flow in the laser plume is calculated by the direct simulation Monte Carlo method. It was found that diffusion of mixture components is significant in the considered time scale, flow is non-equilibrium, and regions of high rarefaction temporally appear in the flow. In atmospheric pressure, the re-deposition of the silicon vapor was observed only in the vicinity of the laser spot.     

二极管双侧抽运横流连续液体激光系统性能模拟

为解决无机液体激光系统的热效应问题,采用激光二极管双侧抽运横向流动的Nd3 :POCl3:ZrCl4溶液以很好地减小热效应,实现液体激光系统高功率高光束质量输出。建立了液体激光理论模型,分析了工作参量对系统能量转换效率与介质热效应的影响;模拟了系统在不同吸收系数和不同流速下的能量转换效率,远场光斑分布以及激光束亮度分布。模拟结果表明:采用二极管作为抽运源可以获得很高的能量转换效率,而且光束质量较好;在给定抽运体积和抽运强度为800 W/cm2时,介质的吸收系数位于2.5~3.0 cm-1,流速约为25 m/s时,输出功率与光束质量实现最佳匹配,激光束亮度最高,系统性能达到最佳。     

Phase transition model of water flow irradiated by high-energy laser in a chamber

In the absorption chamber of a high-energy laser energy meter, water is directly used as an absorbing medium and the interaction of the high-power laser and the water flow can produce a variety of physical phenomena such as phase transitions. The unit difference method is adopted to deduce the phase transition model for water flow irradiated by a high-energy laser. In addition, the model is simulated and verified through experiments. Among them, the experimental verification uses the photographic method, shooting the distribution and the form of the air mass of water flow in different operating conditions, which are compared with the simulation results. The research shows that it is achievable to reduce the intensity of the phase transition by increasing the water flow, reducing the power intensity of the beam, shortening the distance the beam covers, reducing the initial water temperature or adopting a shorter wavelength laser. The study＇s results will provide the reference for the design of a water-direct-absorption-type high-energy laser energy meter as well as an analysis of the interaction processes of other similar high-power lasers and water flow.     

激光能量对飞灰颗粒流中未燃碳LIBS检测影响研究

飞灰含碳量是评价锅炉燃烧效率的重要指标之一,对其进行在线测量有助于实时进行燃烧优化调整,从而提高整个机组的经济性和安全性。利用螺杆式给粉机搭建飞灰颗粒流含碳量测量台架,将脉冲激光直接作用于飞灰颗粒流,形成等离子体,利用激光诱导击穿光谱技术测量飞灰颗粒流中的含碳量信息。重点研究了激光能量对飞灰颗粒流中未燃碳有效激发和测量的影响规律。研究结果显示,在40～130 mJ能量的脉冲激光作用下,碳谱线强度随着激光能量的增大线性增强,而其信噪比则先增加后趋于稳定,无效光谱的剔除率则呈下降趋势。本实验条件下,激光能量在90～100 mJ之间,可得到较强的等离子体发射信号和较优的光谱数据利用率。因而激光能量与颗粒流的激发状态、碳元素特征谱线强度等密切相关。合理的激光能量有利于保证飞灰颗粒流的有效稳定激发,并获得具有良好信噪比的等离子体光谱信号。     

氧碘化学激光器输出功率的估算

 建立了激光在工作时的动态模型,即假设在单位时间内,激光介质的粒子数全部被泵浦到上能级。引入激光介质重复泵浦次数的概念,根据能量守恒原理,通过分析氧碘化学激光器(COIL)的传能机理,计算了沿气流方向变化的碘原子上能级的弛豫时间及沿气流方向变化的碰撞传能时间,并且计算了碘原子在出光区内被单重态氧反复泵浦的次数及相应的残余单重态氧数目,从而计算出碘原子在出光区的反复泵浦次数,修正了原计算COIL输出功率的公式。理论计算与实验结果均表明：当碘流量过低时,增益小于损耗,激光器不能起振;当增大碘流量时,激光输出功率也逐步增大,反复泵浦次数逐渐减少;当碘流量继续增大时,激光输出功率达到最大,且在一定碘流量变化范围内基本保持稳定;但随着碘流量的进一步增大,激光输出功率却逐步下降。     

激光诱导炽光法与消光法诊断烧蚀羽烟消光性能

为了研究流场中碳纤维增强环氧树脂复合材料在激光辐照时产生的烧蚀羽烟对入射激光的屏蔽效应,通过对朗伯-比尔定律进行分析,得到了评价羽烟消光性能的平均质量消光系数的表达式,其与羽烟场浓度和激光透过率相关。采用激光诱导炽光法(LII)和激光消光法,搭建了羽烟消光性能联合诊断实验平台,使待测激光落于LII的激发光平面上,通过同步采集待测激光的透过率和LII信号,获得激发光平面上羽烟浓度场和激光消光比,得到羽烟在不同气流速度下的平均质量消光系数。实验得到气流速度为7,10,20m/s时羽烟对1064nm激光的归一化质量消光系数分别为2.51,1.08,1.00。实验发现,质量消光系数受到气流速度影响,当气流速度较低时质量消光系数曲线波动幅度大,且曲线均值较大;当气流速度较高时质量消光系数趋于稳定且均值较小。     

空气环境下煤粉流LIBS多元素同时检测中激光能量研究

考察了大气环境下激光诱导击穿光谱适用于煤粉流多元素同时检测的激光能量范围,分析了造成煤粉流测量谱线信号波动的原因,得到了适用于煤粉流多元素同时检测的激发区域功率密度范围和最佳功率密度。实验选取能量范围为20～160 mJ,粒径小于200 μm煤粉颗粒经下料口自由下落形成煤粉流束,通过螺杆式给粉机控制流量,波长1 064 nm脉冲激光聚焦后作用于下降的煤粉流束上,产生等离子体,光谱仪采集等离子体发射光谱信号,分析结果表明：实验台架下适于煤粉流LIBS检测的能量范围为30～60 mJ,对应激发前沿功率密度选取范围14.4～34.4 GW·cm-2,最佳测量功率密度19.5 GW·cm-2。     

The effect of varying the hydraulic parameters of a flow cell in a dye laser pumped by a copper vapor laser

A simple method for measuring hydrodynamic parameters that influence the copper vapor laser (CVL) pumped dye laser stability is reported. A specially designed converging diverging curved flow duct is employed. The flow gap at the laser pump region is varied from 0.4 to 0.6 mm and the Reynolds number is varied from 1000 to 20,000. As a result, the channel aspect ratio and effective curvature ratio are varied from 41.6 to 62.5, and 69 to 102, respectively, at the CVL pump region. The variation in the intensity of a He-Ne laser beam transmitted through the water flowing in the test dye cell at different flow rates correlated with the numerically estimated hydrodynamic parameters. The impacts of these hydrodynamic parameters on a 5.5 kHz CVL pumped dye laser output are discussed.     

激光推进流场演化的高速阴影测量方法

 介绍了高速阴影技术在激光推进流场演化诊断过程中的应用。采用了阴影技术和高速摄影相结合的方法,对激光推进流场演化过程进行了考察。由高速相机提供时序零时,控制激光器和闪光源的延迟时间,再对高速相机进行同步曝光,解决了高速阴影测量过程中高速相机、闪光仪和激光器的时序控制问题,实验获得了清晰的流场演化图像,为进一步研究提供了实验依据和便利条件。     

基于增益负反馈的COIL自脉冲方法

针对超音速化学氧碘激光器增益介质的横流特性,设计了具有增益负反馈的光学谐振腔结构,通过化学反应的超音速流场和光场的耦合仿真,根据具体的激光器运行参数,论证了这种结构实现自脉冲激光的可行性。结果表明:振荡激光在增益区上游放大次数越多,对上游增益介质能量提取能力越强,脉冲调制作用越显著。采用激光光斑缩束再放大的结构设计,脉冲激光的峰值功率提高10~20倍,从理论上证明了增益负反馈的调制方法能够实现横流激光器脉冲激光输出。     

Computational fluid dynamic modeling of gas flow characteristics of the high-power CW CO2 laser

To increase the photoelectronic conversion efficiency of the single discharge tube and to meet the requirements of the laser cutting system,optimization of the discharge tube structure and gas flow field is necessary.We present a computational fluid dynamic model to predict the gas flow characteristics of high-power fast-axial flow CO 2 laser.A set of differential equations is used to describe the operation of the laser.Gas flow characteristics,are calculated.The effects of gas velocity and turbulence intensity on discharge stability are studied.Computational results are compared with experimental values,and a good agreement is observed.The method presented and the results obtained can make the design process more efficient.     

用H-S波前传感器测量穿过超声速流场的激光像差特性

 用Hartmann-Shack(H-S)波前传感器可以准确测量穿过超声速流场的激光波面及其变化过程,进一步计算可以获得光束质心漂移、远场分布等数据。给出了H-S方法测量穿过超声速流场激光波面的原理,采用模式法进行了波前重构,计算了在几种流场条件下的激光波面像差特性PV值,RMS值和Zernike像差系数、Strehl比和环围能量曲线等。结果表明,H-S法可以很好地反映流场建立、稳定和结束过程中Zernike像差的变化。比较无流场和给定参数的超声速流场,激光穿越后产生的最明显的像差变化为离焦和低阶像散的增大。在相同流场参数下,无模型时光束质量好于有模型时光束质量。     

Laser Doppler imaging through tissues phantoms by using self-mixing interferometry with a laser diode

Laser Doppler imaging has been widely used for the evaluation of cutaneous blood flow. We report on how the self-mixing interferometry configuration with a laser diode is explored for what is believed to be the first time to generate flow maps. The experiment was carried out by sensing the laser intensity power spectrum at each pixel as the laser was scanned over a model that mimics the properties of skin and circulating blood.     

高焓流场氧原子激光诱导荧光技术初步研究

基于双光子吸收激光诱导荧光(TALIF)技术,在纯净的高焓流场环境中,将脉冲激光从垂直于流场方向射入流场,并通过布置在风洞试验段外与流场和激光形成的平面向垂直方向上的ICCD测量获取到了二维氧原子的荧光信号,该信号可以反映氧原子相对浓度。为保证实验中使用到的激发激光波长为最佳激发波长,分别对氧原子基态处在不同角量子数的情况进行了测试,最终确定J=2时,波长为225.584 nm为最终实验激发波长,为了保证所获取的氧原子荧光信号在非饱和线性区,在同种状态下,将激发激光能量从小到大调整并进行荧光信号的测试,获取了激光能量在3.4 mJ以下的线性区域。为确保获取清晰对比度最优的ICCD荧光图像,选择了Nikon f=105 mm F/2.8镜头为实验测试镜头, 同时将50次曝光的累计结果作为图像输出。对所获取的荧光图像进行分析,实验结果可以清晰地看到超声速流场中压缩波形成的距中心线约±50 mm位置处左右各一个的小峰,亚声速流场中中心处氧原子浓度有一个约60 mm宽的均匀区域,区域以外氧原子浓度急剧下降,这些结果符合实验风洞特性,此测试方法可以很好地运用到流场测量中。     

High-power transverse flow CW CO2 laser for material processing applications

A transverse flow transversely excited (TFTE) CW CO₂ with a maximum output power about 15 kW has been developed. This is excited by pulser sustained DC discharge applied between a pair of multi-pins anodes and a common tubular cathode. Though the laser power in convective cooled CO₂ laser scales proportionally with the volumetric gas flow, it did not increase in this laser when the volumetric gas flow was increased by increasing the electrode separation keeping the flow velocity constant. The discharge voltage too remained almost unchanged with increase of the electrode separation. These observations are explained considering the electrical discharge being controlled by ionization instability. Laser materials processing applications often demand programming facilities for laser power modulation. A four-stage cascaded multilevel DC–DC converter-based high-frequency switch mode power supply has been developed to modulate the output power of the laser. Laser was operated up to 15 kW output power in four different modes viz. continuous wave mode, pulse periodic mode, single shot mode and processing velocity-dependent power mode with 1.2 kHz modulation bandwidth. We describe briefly the laser system, the SMPS, and the temporal behavior of laser beam.