准分子激光束的喇曼整形

受激喇曼散射可以将紫外准分子激光辐射频移到特定的近紫外和可见光波长.采用喇曼整形技术还可以获得衍射极限发散角的斯托克斯输出.本文研究了注入光束质量对喇曼整形的影响,并求得不同氢压力下的喇曼增益系数和饱和参量.     

准分子激光束的喇曼组束

受激喇曼散射可以将紫外准分子激光辐射频移到特定的近紫外或可见光波长,采用喇曼整形技术可以改善斯托克斯光的光束质量,本文报道喇曼组束提高喇曼整形效率的实验结果.     

A broadband excimer source of visible radiation with barrier discharge pumping

We investigate the characteristics of a broadband cylindrical excimer source of visible radiation with a surface area of 230 cm² excited by a pulse-periodic barrier discharge based on multicomponent mixtures (mercury diiodide and dibromide with helium and admixtures of molecular nitrogen and xenon). The working mixture components were excited by a pulse-periodic (pulse repetition rate 500–5000 Hz, pulse duration ～150 ns) barrier discharge. We detected radiation from excimer HgI* and HgBr* molecules, the second positive system of molecular nitrogen, and mercury and xenon atoms. The amplitude, duration, and trailing edge of the radiation pulses in the HgI₂:HgBr₂:Xe:He and HgI₂: HgBr₂:N₂:He mixtures with admixtures of xenon and molecular nitrogen were found to change compared to the HgI₂:HgBr₂:He mixture. The optimal partial pressure of helium lies within the range 162–195 kPa. The most intense radiation from HgI* and HgBr* molecules (in a ratio of more than 3: 1) is observed in the HgI₂:HgBr₂: Xe:He mixture. The mean and pulse radiation powers are 45 W and 93 kW, respectively, at a pumping pulse repetition rate of 5000 Hz and an efficiency of 30%. We discuss the spectral and temporal characteristics of the radiation source and the dependence of the radiation intensity of excimer molecules of mercury monoiodide and monobromide on the partial pressures of xenon and nitrogen. We point out that the radiation source is of considerable interest for applications in biotechnology and medicine.     

kHz准分子激光器全固态脉冲激励源

由于寿命制约,准分子激光器使用的传统放电开关闸流管不能满足准分子激光器高重复频率长期稳定工作运行的要求。设计了基于两级磁脉冲压缩技术的全固态脉冲功率模块,使用大功率半导体开关结合脉冲升压变压器产生s级的高压脉冲,利用磁脉冲压缩技术将上升时间为s级高压脉冲压缩至满足准分子激光器使用的上升时间为0.1 s级高压脉冲。在ArF准分子激光头上放电,获得激励脉冲的上升时间约为90 ns,放电电压16.5 kV,重复频率达到1 kHz,两级磁脉冲压缩开关能量传递效率达59.1%。     

kHz准分子激光器全固态脉冲激励源

由于寿命制约,准分子激光器使用的传统放电开关闸流管不能满足准分子激光器高重复频率长期稳定工作运行的要求。设计了基于两级磁脉冲压缩技术的全固态脉冲功率模块,使用大功率半导体开关结合脉冲升压变压器产生s级的高压脉冲,利用磁脉冲压缩技术将上升时间为s级高压脉冲压缩至满足准分子激光器使用的上升时间为0.1 s级高压脉冲。在ArF准分子激光头上放电,获得激励脉冲的上升时间约为90 ns,放电电压16.5 kV,重复频率达到1 kHz,两级磁脉冲压缩开关能量传递效率达59.1%。     

Transformation of spatial characteristics of excimer laser radiation

An optical attachment of an excimer laser beam that enables the transformation and optimization of spatial characteristics of a coherent UV beam was studied in an experimental setting. Experiments revealed that a rectangular 3 × 20 mm laser beam with the a divergence of 2 × 5 mrad attains a square shape of 20 × 20 mm and a divergence of 5 × 5 mrad in orthogonal directions after passing through the beam attachment. It is demonstrated that the application of a beam attachment in installations for material microprocessing simplifies optical layout of the illumination module allows one to obtain optimal homogeneity of sample illumination (under 2%) upon the projection of the image on the processing plane of the sample with submicron precision and to reduce the radiation load on optical elements via the elimination of hot spots.     

A laser plasma source of EUV radiation for projection nanolithography

A laboratory laser plasma source of extreme ultraviolet radiation optimized for use on a nanolithography test bench at a wavelength of 13.5 nm is described. The key characteristics of the source are presented. When operating with a molybdenum target, the measured ratio of laser pulse energy conversion into radiation with a central wavelength of 13.5 nm into the half-space and in the 2% spectral band is 0.03%.     

Numerical stability for laser radiation computations

The numerical stability for solving the coupled differential equations for laser radiation intensity and gain is investigated. Simplified lasing conditions are considered under which a closed-form solution is obtained and numerical stability can be determined. The analytical solution illustrates the effect of the important physical parameters, and it provides a basis for evaluation of numerical methods. It is shown that the explicit numerical procedure is inherently unstable, and that stability requires an implicit approach. Partially-implicit methods, which are conditionally stable, are also discussed.     

Anomalous burn-through of thin foils by high-intensity laser radiation

The paper presents results of experiments performed on the Pico facility in which foils were heated by laser radiation, and anomalously fast burn-through of foils by a structured laser beam was detected. Comparison with two-dimensional calculations has allowed us to suggest a tentative mechanism for the effect under investigation. The targets in the experiments were thin aluminum foils of thickness 3 to 40 μm. The flux density of laser radiation on the target surface varied between 10¹³ and 10¹⁴ W/cm². We detected a strong dependence of the transmitted energy on the foil thickness and the shortening of the transmitted laser pulse. Penetration of laser radiation through foils with thicknesses considerably larger than 3 μm has been observed, although it was stated in earlier publications [V. V. Ivanov, A. K. Knyazev, A. V. Kutsenko, et al., Kratk. Soobshch. Fiz. FIAN No. 7–8, 37 (1997)]; A. é. Bugrov, I. N. Burdonskii, V. V. Gol’tsov et al., Zh. éksp. Teor. Fiz. 111, 903 (1997) [JETP 84, 903 (1997)] that, at the laser radiation parameters used in our experiment, the evaporated layer of the foil could not be thicker than 2 μm. Two-dimensional calculations have allowed us to interpret this effect in terms of local “piercing” of the target at spots on the target surface where the radiation intensity has its peaks. The possibility of reducing these peaks by using a symmetrizing prepulse is discussed in the paper. Zh. éksp. Teor. Fiz. 116, 1287–1299 (October 1999)     

Performance of optical fibers for transmission of high-peak-power XeCl excimer laser pulses

The simple and efficient fiber delivery of 5-ns pulses from a XeCl excimer laser operating at a wavelength of 308 nm is demonstrated. The coupling scheme uses all of the output energy of the XeCl excimer laser and benefits from a simple and easy-to-adjust fiber coupling. Experiments on the 308-nm fiber delivery for more than 2.5 million laser pulses of 8-ns pulse width (FWHM) and up to 8-mJ stabilized pulse energy are performed. The long-time pulsed UV laser transmission is found to be different for individual samples of optical fibers that perform very similarly in low-intensity UV light applications. For applications with strict demands on the long-time stability, a critical evaluation of the fiber performance with the 308-nm laser under operating conditions is necessary. Measurements between 1 and 200 Hz show a negligible dependence of the fiber delivery performance on the repetition rate of the transmitted laser pulses. PACS 42.55.Lt; 42.81.Cn; 07.69.Vg     

Interaction of 157-nm excimer laser radiation with fluorocarbon polymers

Two important fluoropolymers, polytetrafluoroethylene [PTFE—(C₂F₄)_N] and polyvinylidene fluoride [PVDF—(C₂H₂F₂)_N], respond to 157-nm laser radiation in dramatically different ways. At fluences sufficient to produce rapid etching, the volatile emissions from PTFE are dominated by (CF₂)_N fragments. The velocities of the fastest (CF₂)_N molecules at each mass are consistent with kinetic energies on the order of an electron volt—and change little with fluence. This fluence independence suggests that the velocities are not affected by collisions after emission. To account for the high kinetic energies and the unusual, half-monomer mass distribution, we propose that these fragments are produced by photochemical scission of the polymer backbone, and that a fraction of the excitation energy is delivered to each fragment as kinetic energy. In contrast, the principle neutral species from PVDF is HF. HF is produced by the scission of C-F bonds, followed by chemical reactions with nearby hydrogen. This process is accompanied by the conjugation of backbone C-C bonds. The photochemical cleavage of C-C bonds in PTFE and C-F bonds in PVDF is consistent with the lower C-C bond energy of PTFE.     

高功率准分子激光主振荡功率放大系统光学元件的稳定性

为了分析高功率准分子激光主振荡功率放大(MOPA)系统中各光学元件稳定性对靶面光斑定位精度的影响,建立了分析模型,利用分析结果指导高稳定性镜架设计以满足系统实验需求。根据高功率准分子激光MOPA系统特点,有效地简化了系统光路,建立了系统光路模型;按照系统打靶精度要求,利用三维坐标变换和光线追迹法,计算得到了系统单个光学元件稳定性对靶面光斑定位精度的影响规律;最后,对自行设计的镜架进行了稳定性测量。计算结果表明,反射镜的旋转变化和透镜垂直光轴的平移变化是影响靶面光斑定位精度的主要因素,且主放大光路中反射镜在X方向和Y方向上最大的变化范围分别不能超过0.8和1.6μrad。实际测量结果表明,设计的镜架在X方向和Y方向最大的变化范围分别为0.6和0.81μrad,满足系统实验要求。     

环形分布激光束引起光学窗口镜热变形理论分析

 用有限Hankel变换就常见的环形分布激光束情形，推导得到了激光束辐照透过窗口镜引起热传导温度场和热变形分布的理论解，以此计算结果与实验值相比较，分析了白宝石、石英两种窗口镜在激光辐照下的热变形情况。分析结果表明，在激光束辐照的4s时段内，温升场主要集中在激光束辐照的环形区内，在镜面内远未拉平，由此引起的厚度方向热膨胀变形也很不均匀。材料对激光的吸收系数和热膨胀系数以及热扩散率分别是决定热变形量值大小和空间分布的最主要的物性参数。     

空间层叠矩形阵列式角多路解码光路设计

根据角多路技术原理,提出了一种矩形阵列空间层叠形式的解码光路结构和先分组解码再组内解码的两步解码方式,并针对本实验室的高功率准分子激光系统给出了具体的解码光路设计实例。该方法具有解码精度高,设计误差小,与光路准直、激光参数测量系统等兼容性好,便于加工制作和安装调试等优点。     

高功率准分子激光主振荡功率放大系统光学元件的稳定性

为了分析高功率准分子激光主振荡功率放大(MOPA)系统中各光学元件稳定性对靶面光斑定位精度的影响,建立了分析模型,利用分析结果指导高稳定性镜架设计以满足系统实验需求。根据高功率准分子激光MOPA系统特点,有效地简化了系统光路,建立了系统光路模型;按照系统打靶精度要求,利用三维坐标变换和光线追迹法,计算得到了系统单个光学元件稳定性对靶面光斑定位精度的影响规律;最后,对自行设计的镜架进行了稳定性测量。计算结果表明,反射镜的旋转变化和透镜垂直光轴的平移变化是影响靶面光斑定位精度的主要因素,且主放大光路中反射镜在X方向和Y方向上最大的变化范围分别不能超过0.8和1.6μrad。实际测量结果表明,设计的镜架在X方向和Y方向最大的变化范围分别为0.6和0.81μrad,满足系统实验要求。     

空间层叠矩形阵列式角多路解码光路设计

根据角多路技术原理,提出了一种矩形阵列空间层叠形式的解码光路结构和先分组解码再组内解码的两步解码方式,并针对本实验室的高功率准分子激光系统给出了具体的解码光路设计实例。该方法具有解码精度高,设计误差小,与光路准直、激光参数测量系统等兼容性好,便于加工制作和安装调试等优点。     

用于XeF蓝绿激光器的表面放电光泵浦源

基于Al2O3陶瓷、BN陶瓷和聚四氟乙烯三种基底建立了分段表面放电光泵浦源,对比研究了这三种表面放电光泵浦源的电学特性、辐射特性和烧蚀特性。利用放电波形计算了表面放电光泵浦源的等效电感、等效电阻和沉积效率,应用光谱法比较了它们的紫外辐射强度,并采用平均线烧蚀率评估了三种泵浦源的耐烧蚀性能。通过比较研究发现,在充电电压为13.5～26.8 kV、间隙长度为8 cm、放电室内混合气体气压为100 kPa条件下,三种泵浦源中 Al2O3陶瓷表面放电光泵浦源的沉积效率最高,大于82%;辐射光谱具有紫外增强效应,紫外辐射最强;平均线烧蚀率最小(小于0.15 m/shot),耐烧蚀性能最好。研究结果表明采用Al2O3陶瓷表面放电光泵浦源作为大功率重频XeF蓝绿激光器的泵浦源,可提高XeF蓝绿激光器的寿命。     

Generation of high peak power excimer laser radiation by pulse shortening

Techniques for shortening the pulses of a commercial oscillator-amplifier excimer laser have been investigated. By a combination of a H₂-Raman cell and a saturable absorber dye jet the oscillator pulses are shortened from about 25 ns to typically 1.5 ns. Upon amplification pulses around 3.5 ns with peak powers of more than 100 MW for KrF (248 nm) and 25 MW for XeCl (308 nm) are obtained.