Improving thermal efficiency and stability of laser welding process for magnesium alloy by combining power modulation and subatmospheric pressure environment

The laser welding(LW)process of highly reflective materials presents low thermal efficiency and poor stability.To solve the problem,the effects of subatmospheric environment on LW process,technological parameters in subatmospheric environment on weld formation and welding with sinusoidal modulation of laser power on the stability of LW process in subatmospheric environment were explored.The AZ31magnesium(Mg)alloy was used as the test materials.The test result revealed that the weld penetration in subatmospheric environment can increase by more than ten times compared with that under normal pressure.After the keyhole depth greatly rises,significantly periodic local bulge is observed on the backwall surface of the keyhole and the position of the bulge shifts along the direction of the keyhole depth.Eventually,the hump-shaped surface morphology of the welded seam is formed;moreover,the weld width in local zones in the lower part of the welded seam remarkably grows.During LW in subatmospheric environment,the weld penetration can be further greatly increased through power modulation.Besides,power modulation can inhibit the occurrence of bulges in local zones on the backwall of the keyhole during LW in subatmospheric environment,thus further curbing the significant growth of the weld widths of hump-shaped welding beads and local zones in the lower part of welded seams.Finally,the mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials through power modulation in subatmospheric environment was illustrated.This was conducted according to theoretical analysis of recoil pressure and observation results of dynamic behaviors of laser induced plasma clouds and keyholes in the molten pool through high speed photography.

A high-power high-stability Q-switched green laser with intracavity frequency doubling using a diode-pumped composite ceramic Nd:YAG laser

We successfully obtain a high-average-power high-stability Q-switched green laser based on diode-side-pumped composite ceramic Nd：YAG in a straight piano-concave cavity. The temperature distribution in composite ceramic Nd：YAG crystal is numerically analyzed and compared with that of conventional Nd：YAG crystal. By using a composite ceramic Nd：YAG rod and a type-II high gray track resistance KTP （HGTR-KTP） crystal, a green laser with an average output power of 165 W is obtained at a repetition rate of 25 kHz, with a diode-to-green optical conversion of 14.68%, and a pulse width of 162 ns. To the best of our knowledge, both the output power and optical-to-optical efficiency are the highest values for green laser systems with intracavity frequency doubling of this novel composite ceramic Nd：YAG laser to date. The power fluctuation at around 160 W is lower than 0.3% in 2.5 hours.

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.

可应用于光纤激光器的高稳定度泵浦源系统研制

为了提升光纤激光器的输出性能,研制了一个高稳定度泵浦源系统,包括激光泵浦模块和外围控制系统。泵浦源系统测试结果表明,输出光功率与驱动电流之间具有良好的线性关系,输出光功率的短期稳定度为0.0613%,长期稳定度为0.1080%,而且输出光谱的中心波长在长时间持续运行中稳定不变。将该泵浦源系统应用于光纤激光器,经过连续测试,输出光谱的中心波长仅发生4 pm的偏移。

高功率激光-MAG复合焊接成形稳定性

采用激光-MAG(Metal active gas arc welding)复合焊接工艺,以焊缝表面成形、焊缝纵截面形貌和熔深波动程度为工艺稳定性评价依据,借助高速摄像系统和图像处理方法,对焊接过程中飞溅和等离子体两种关键过程信息进行特征识别和定量化表征,系统地研究激光功率从5 kW提高到30 kW时,焊接过程关键特征信息与焊接过程稳定性之间的关系。结果表明,随着激光功率的增加,焊缝表面成形出现周期性“上凸-下凹”现象,焊缝内部裂纹和熔深变化特征也随之周期性变化;等离子体面积和飞溅面积均随激光功率的提高呈增加趋势,且两者波动程度和熔深波动程度均呈正相关;等离子体面积增加会导致激光传输过程中能量衰减程度的加剧,使焊缝熔深增加趋势逐渐变缓,其波动程度是影响焊接过程稳定性的关键因素之一。

可调谐激光器多闭环控制驱动系统设计

为满足可调谐激光器输出激光的高控制稳定性需求,设计并实现了应用于可调谐激光器的多闭环控制驱动系统。首先,为实现高稳定性电流驱动,基于深度负反馈控制原理设计了电流控制内环,并通过积分补偿和RC滤波回路削弱了噪声和电流波动对激光二极管(LD)的干扰;其次,为了防止恒定电流驱动下LD仍存在的功率衰减问题,利用积分电路和光功率二极管(PD)设计了功率控制内环,并应用仪表放大电路消除了环内共模噪声干扰并实现了线性控制;最后,为了实现驱动电流和输出功率的实时调节,应用位置式PID对系统进行外环控制,与内环共同构成了多闭环控制系统。实验结果表明,电流控制稳定度均优于200 ppm,功率控制稳定度优于0.005 dB/h,激光波长稳定度均优于1 pm/h,能够满足可调谐激光器的高控制稳定性需求。

Status and development of high-power laser facilities at the NLHPLP

In this paper, we review the status of the multifunctional experimental platform at the National Laboratory of High Power Laser and Physics(NLHPLP). The platform, including the SG-II laser facility, SG-II 9th beam, SG-II upgrade(SG-II UP) facility, and SG-II 5 PW facility, is operational and available for interested scientists studying inertial confinement fusion(ICF) and a broad range of high-energy-density physics. These facilities can provide important experimental capabilities by combining different pulse widths of nanosecond, picosecond, and femtosecond scales. In addition, the SG-II UP facility, consisting of a single petawatt system and an eight-beam nanosecond system, is introduced including several laser technologies that have been developed to ensure the performance of the facility. Recent developments of the SG-II 5 PW facility are also presented.

高功率固体激光装置负载问题研究进展

高功率固体激光装置的负载问题是制约装置建设与运行的瓶颈问题。在高通量紫外纳秒激光辐照下,熔石英后表面的损伤不断产生和增长,严重限制了装置的负载能力。在提升熔石英抗损伤性能的基础上修复既有损伤,循环使用光学元件,是现阶段提升装置负载能力的主要手段。主要介绍了国内外近年来在熔石英损伤的规律与机制、光学元件循环处理的支撑技术以及提升负载能力的新材料与新技术方面所取得的重要进展。

2 kW single-mode fiber laser employing bidirectional-pump scheme

A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers(CLSs) has been demonstrated by employing bidirectional-pump scheme.2.009 k W signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%.Raman Stokes light is less than-47 d B at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 mm/400 mm.The beam quality M^2≤1.2 and the spectral FWHM bandwidth is 4.34 nm.There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.

大型激光装置脉冲波形调控技术

精密高效的脉冲波形调控是大型激光装置满足惯性约束聚变实验需求的重要技术之一。脉冲波形生成原理是通过编辑任意波形发生器中每个子脉冲的电压值,经电光调制器转换为光脉冲强度形成任意形状的激光脉冲轮廓。在电光转换过程中,各子束响应过程并非线性且子束间存在个性差异。为实现此条件下精密高效的脉冲波形调控,制定并开发了基于闭环迭代思路的激光脉冲波形快速调控方法。实验结果表明,算法可在10 min内实现任意脉冲波形整形,并具备23∶1高对比度脉冲波形调控能力,调控精度均优于10%(rms),满足常规物理实验运行条件下对激光参数调控的精度和效率需求。

Design and performance of final optics assembly in SG-ⅡUpgrade laser facility

In high power laser facility for inertial confinement fusion research, final optics assembly(FOA) plays a critical role in the frequency conversion, beam focusing, color separation, beam sampling and debris shielding. The design and performance of FOA in SG-II Upgrade laser facility are mainly introduced here. Due to the limited space and short focal length, a coaxial aspheric wedged focus lens is designed and applied in the FOA configuration. Then the ghost image analysis,the focus characteristic analysis, the B integral control design and the optomechanical design are carried out in the FOA design phase. In order to ensure the FOA performance, two key technologies are developed including measurement and adjustment technique of the wedged focus lens and the stray light management technique based on ground glass.Experimental results show that the design specifications including laser fluence, frequency conversion efficiency and perforation efficiency of the focus spot have been achieved, which meet the requirements of physical experiments well.

基于调制转移的大功率半导体激光器频率稳定控制

为了提高大功率半导体激光器输出频率稳定控制的效果,提出基于调制转移的大功率半导体激光器频率稳定控制方法。首先分析大功率半导体激光器工作原理,利用光电调制器对激光器泵浦光相位进行调制,获取大功率半导体激光器光波附加相位和泵浦光电场矢量;然后以附加相位和泵浦光电场矢量为基础,使用调制转移技术建立调制转移光谱锁频环路模型,计算大功率半导体激光器探测光和边带拍频信号;最后建立调制转移光谱锁频环路模型表达式,通过求解该表达式,得到大功率半导体激光器的锁频数值,实现激光器输出频率的有效控制。实验表明,文中方法计算大功率半导体激光器拍频信号较为准确,降低其输出拍频信号区间,响应时间最大为0.07 s,响应较为迅速,控制精度较高,具备较好的频率稳定控制能力,控制效果好。

大型激光装置光学元件的稳定性设计

大型高功率固体激光装置通常要求总的打靶精度为几十微米,这对结构的稳定性设计提出了很高的要求。本文以一大型激光装置为背景,以关键光机系统为例进行稳定性设计。通过采用钢筋混凝土建筑结构、一体化结构和桁架结构、隔离振动源增加结构刚度等方式减小光机结构对环境振动的响应,提高结构的稳定性。然后,应用有限元理论对关键光学元件及其支撑结构的稳定性进行分析,以验证光机结构设计的合理性。分析结果显示,该系统中腔镜的最大转角为0.14μrad,注入系统角漂元件的最大转角为0.423μrad,光束切换模块反射镜的最大转角为0.394μrad。计算结果表明:设计的结构合理可行,关键光学元件均满足稳定性设计指标的要求,且裕度系数>1.5。

空间滤波器阵列结构优化设计

给出了新型大型激光装置阵列化空间滤波器结构优化设计。对大口径阵列化光束超长程传输、光路控制、光束质量控制,以及支撑稳定性、震动隔离、离轴调节等关键技术进行了分析研究,对物理方案和参数进行分解。运用Pro/E软件建立阵列化空间滤波器三维实体模型,给出了阵列化空间滤波器的总体结构解决方案。对关键部件和技术指标进行了详细分析,采取了新颖的结构和相应的技术措施;运用ANSYS分析软件对结构刚性、强度、振型等指标进行分析,通过有限元仿真与CAD环境的双向嵌入优化结构;为给装置结构的适应性找到可靠的评估依据,对影响稳定性较大的器件和单元进行了微振动测试和分析。理论分析和测试结果表明,该方案达到了优化设计,是可行的。

8路神光Ⅱ装置在基频和三倍频条件下工作研究进展(英文)

简要描述在神光 装置上进行的部分物理实验 ,为神光 激光装置提供光学性能指标。神光 激光装置已经能在三倍频、外转换效率高于 6 0 %的条件下常规输出三倍频能量。穿孔实验表明 ,蓝光 (3ω0 )的光束质量 ,特别是远场旁瓣分布质量 ,甚至于要好于红光 (1ω0 光 )。用 10 0 ps脉冲宽度的红光输出打爆推内爆的氘氚球靶 ,获得单发最高中子产额 4× 10 9个。基频线聚焦打靶 ,获得类 Ni银 X光激光饱和输出 ,并成功应用于激光等离子体密度测量 ,观测到莫尔条纹移动。

表面热透镜技术测量3.8μm和2.8μm激光薄膜的微弱吸收

采用表面热透镜技术,对3.8μm和2.8μm激光辐照下镀制在Si基底上的单层ZnS,YbF3和YBC薄膜及不同膜系的YbF3/ZnS多层分光膜和多层高反膜,以及镀制在CaF2基底上的增透膜进行了吸收测量,并对3.8μm和2.8μm激光的测量结果进行了比较分析。实验结果表明,2.8μm波长下的吸收比3.8μm的大得多,两者之间约相差一个量级,测得的多层高反膜YbF3/ZnS薄膜在的3.8μm处的最低吸收为4.57×10-4,测量系统的灵敏度约为10-5。

高功率激光装置光传输管道污染规律及对光学表面损伤性能的影响

研究高功率激光装置光传输管道内部洁净度变化规律,分析其对内部重要光学元件光学性能的影响规律,提出污染控制措施。对光传输管道内部的气溶胶进行采样,并利用空气品质分析仪及扫描电镜对其进行分析,得到光传输管道内部洁净度变化规律和污染源;采用内部放置透射膜元件的方法,研究洁净度等级水平对透射膜的微观结构和透射率的影响,并利用"1-on-1"的测试方式进行透射膜元件的损伤阈值测试。研究结果表明:光传输管道内部的洁净度在激光辐照后迅速上升至万级水平,透射膜元件在此环境下其透过率严重下降,下降幅度为2.5%,且表面微观形貌发生变化。光学透射薄膜表面损伤阈值随表面污染水平呈现线性下降规律,最大下降幅度约为10%。污染监测和成分分析结果表明管道内部灰尘及杂散光或者鬼光束辐照金属产生的等离子体是管道内污染的主要源头,在此基础上提出了正压密封保持的技术手段确保内部光学表面洁净度水平,延长使用寿命。

“神光-Ⅱ”装置第九路靶场终端光学组件的研制

针对“神光-Ⅱ”装置第九路系统主激光瞄准精度小于等于30μm和大焦斑辐照均匀性优于10%的要求,提出了靶场终端光学组件的设计结构。应用有限元法对组件关键机械元件和ICF靶室整体进行动静态分析,优化了设计参数。同时与聚焦透镜配合进行数值分析列阵透镜,确定了单元数、曲率和厚度以及单元长和宽等参数。经过实验测试,主激光瞄准精度达到28.9μm,大焦斑辐照的形状为1000μm×500μm,均匀性为12.0%。

高功率激光装置中透镜一阶鬼点形成规律分析

为了得到一阶鬼点形成的一般规律,从成像公式出发,利用薄透镜近似,对在高功率激光装置中所使用的各种形状的透镜,各表面剩余反射形成的一阶鬼点位置与其焦距的关系进行了推导,给出了两种入射方式下的关系式,为高功率激光装置中透镜的设计提供了一定的参考。

多程放大系统中自激振荡的分析和抑制

根据离轴多程放大系统的光路特点,对产生谐振腔最可能的光路部分进行了自激振荡形成条件的分析,根据分析结果并结合实际情况采取了一些抑制自激振荡的措施。