Enhanced performance of a repetitively pulsed 130 mJ KrF laser with improved pre-ionization parameters

Studies related to the effect of pre-ionizer on laser output energy of a repetitively pulsed KrF laser are presented. The dependence of laser output energy, spectral width and beam spot homogeneity on pre-ionization parameters, namely its current and voltage rise time are reported here. Here, effectiveness of pre-ionization is optimized by improving pre-ionization current and rise time of the pump pulse of the automatic UV pre-ionizer KrF laser. It is observed that by increasing pre-ionization current from 6 kA to 10.6 kA, the output energy increases by about 30% (from 100 to 130 mJ). It is also observed that the emission spectral width reduces by almost 60% by increasing the pre-ionization current. Regular homogeneous and well-developed beam spot (nearly Hat-Top profile) was achieved under these optimized conditions.     

Photodynamic Therapy for Cancer Cells Using Metal-Halide Lamps

New metal-halide lamps were developed and their effect on the efficiency of photodynamic therapy (PDT) for cancer cells, murine thymic lymphoma cells (EL-4), was investigated. 5-Aminolevulinic acid-induced protoporphyrin IX was used as a photosensitizer. The metal-halide lamps were made by introducing sodium iodide (Na lamp), lithium iodide (Li lamp), and sodium iodide-lithium iodide mixture (Na-Li lamp) into their discharge tubes. These lamps emitted light in the range of 550 to 750 nm and had specific emission peaks at 580 and 600 nm for the Na lamp, 580, 610, and 680 nm for the Li lamp, and 580, 610, and 675 nm for the Na-Li lamp. Changes in the survival rate of EL-4 with increasing irradiation time indicated that PDT efficiency of the lamps increased in the order Li lamp < Na lamp < Na-Li lamp. We also found that a dark interval during irradiation of the light with the Na-Li lamp enhanced PDT efficiency.     

Interaction of excess electrons with water molecules at the early stage of laser-induced plasma generation in water

Forward and backward stimulated Raman scattering (SRS) of OH stretching vibrations are measured when an intense 40 ps pulsed beam is focused into water. Characteristic features related to the interaction of OH groups with excess electrons generated by the strong laser irradiation are observed. The SRS spectra are indicative of the important role which excess electrons play in transient enhancement of the SRS at the initial stage of laser-induced plasma generation in water. The hydration structures around the excess electrons in water are also discussed.     

横向电容自耦式紫外预电离开关

对紫外预电离技术在气体开关方面的影响进行原理研究,分析并实验验证电容并联的横向辅助电极对主开关进行紫外预电离的可行性。辅助电极击穿释放紫外光,通过光电效应在主开关表面产生初始电子,以减小主开关的击穿电压离散度。预电离效果与辅助电极击穿导通电流的峰值强度、辅助电极两端的电压以及预电离储能电容有关,优化这些参数可以提高光照强度来增强预电离效果。实验结果表明:当主开关内部充满氮气、分压电容为pF量级时,距离主电极15 cm远的横向辅助电极可以产生预电离效应。在此基础上,提出一种新型的横向型电容自耦式紫外预电离开关的设计。     

Fabrication of an integrated optical Mach-Zehnder interferometer based on refractive index modification of polymethylmethacrylate by krypton fluoride excimer laser radiation

It is known that deep ultraviolet (UV) radiation induces a refractive index increase in the surface layer of polymethylmethacrylate (PMMA) samples. This effect can be used for the fabrication of integrated optical waveguides. PMMA is of considerable interest for bio and chemical sensing applications because it is biocompatible and can be micromachined by several methods, e.g. structuring by photolithography, ablation and hot embossing. In the presented work direct UV irradiation of a common PMMA substrate by a krypton fluoride excimer laser beam through a contact mask has been used to write integrated optical Mach-Zehnder interferometers (MZI). MZI are used as sensitive bio and chemical sensors. The aim was to determine contact mask design and laser irradiation parameters for fabricating single-mode MZI for the infrared region from 1.30 μm to 1.62 μm. Straight and curved waveguides have been generated and characterized to determine the optical losses. The generation of channel waveguide structures has been optimized by a two step irradiation process to minimize the lithographic writing time and optical loss. By flood exposure to UV laser radiation in the first step the optical absorption of PMMA can be increased in the irradiated region. The required refractive index profile is then achieved with a second lithographic irradiation. The spectral behaviour of an unbalanced, integrated optical MZI fabricated by this excimer laser based contact mask method is shown for the first time. Further the optical intensity at the output port of a MZI has been measured while the optical path length difference was tuned by creating a temperature difference between the two arms of the MZI.     

A reliable thyratron-switched flashlamp-pumped dye laser

A thyratron-switched flashlamp-pumped dye laser capable of an average power of approximately 1 watt operating at 40 pulses/sec with a bandwidth of several GHz is described. The lamps are run in a high-current simmering mode to decrease lamp inductance and extend lamp life. In heavy use the laser system has proven to be very reliable with lamp life-times in excess of 10⁷ shots and half-power dye lifetimes of better than 10⁶ J/1 for Rhodamine 6G.     

触发预电离一体化脉冲氙灯电源设计

采用高压触发预电离为一体+脉冲形成网络触发氙灯的技术路线,分别设计了脉冲氙灯电源系统中高压触发、预燃电流维持两个功能单元。根据脉冲氙灯160mm×9mm、极间距70mm的技术指标,成功设计了一台具有手动和电脉冲触发功能的方波脉冲氙灯电源。氙灯充电电压300~900V可调,光脉冲宽度为150μs±20μs,光脉冲上升时间不超过90μs,触发延迟不超过90μs,触发抖动在±2μs范围内。给出了单次触发情况下脉冲氙灯电源输出的实验结果,验证了所采用的设计原理和技术方法的可行性。     

Triggering of high voltage discharge by femtosecond laser filaments on different wavelengths

The triggering and guiding of high voltage discharge by plasma filaments generated by 400 nm and 800 nm femtosecond laser were studied experimentally. The detailed diagnostics for characteristics of filaments, such as amount of free electrons, diameter and electron density, were performed using sonographic method, fluorescence imaging and resistivity measurement. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filaments was observed. It is shown that the 400 nm laser pulses demonstrated stronger triggering ability than 800 nm laser pulse under same pulse energy. This behavior of 400 nm femtosecond pulse is connected to the rich population of highly excited particles in filaments.     

Photo-differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5?seconds and a low intensity of 13.80?mW/cm².     

A comparison of the emission characteristics of UV-LEDs and fluorescent lamps for polymerisation applications

Ultraviolet (UV) fluorescent lamps are widely used in the manufacturing process of biomaterials. The possibility of replacing these lamps with ultraviolet light emitting diodes (UV-LEDs) was investigated and the results are presented here. A number of emission characteristics, including the spectral output and intensity of both light sources were measured and compared. The warm up time of the UV-LED was found to be faster than that of the fluorescent lamp while their stabilities were found to be comparable. The ability of each source to initiate photopolymerisation in a biomaterial sample was monitored using Fourier Transform Infrared spectroscopy and the percentage polymerisation calculated. The results presented here show that UV-LEDs are a viable alternative to UV fluorescent lamps in the manufacturing process of biomaterials.     

高峰值功率KDP晶体四倍频266nm紫外激光器

报道了一种灯泵浦结构的Nd:YAG晶体电光调Q高峰值功率266nm紫外激光器。结合磷酸二氢钾(KDP)晶体性质,基于倍频理论,分析了考虑走离效应情况下存在相位失配量时KDP晶体长度对转换效率的影响。该激光器采用紧凑的平平腔结构,灯泵浦Nd:YAG晶体电光调Q 1064nm激光作为基频光,腔外采用Ⅱ类匹配磷酸钛氧钾(KTP)和Ⅰ类匹配KDP分别作为二倍频和四倍频晶体。利用能量计、示波器等仪器进行测量,激光器重复频率1Hz时,获得脉宽6.0ns,单脉冲能量35mJ的266nm紫外激光输出,峰值功率高达5.83 MW;当重复频率10Hz时,获得单脉冲能量28.9mJ的266nm紫外激光。532~266nm转换效率最高可达31.9%。利用该高峰值功率、窄脉宽266nm紫外激光器,能够实现激光打标、激光雕刻。     

Theoretical investigation of dielectric corona pre-ionization TEA nitrogen laser based on transmission line method

This paper models the dielectric corona pre-ionization, capacitor transfer type of flat-plane transmission line traveling wave transverse excited atmospheric pressure nitrogen laser by a non-linear lumped RLC electric circuit. The flat-plane transmission line and the pre-ionizer dielectric are modeled by a lumped linear RLC and time-dependent non-linear RC circuit, respectively. The main discharge region is considered as a time-dependent non-linear RLC circuit where its resistance value is also depends on the radiated pre-ionization ultra violet (UV) intensity. The UV radiation is radiated by the resistance due to the surface plasma on the pre-ionizer dielectric. The theoretical predictions are in a very good agreement with the experimental observations. The electric circuit equations (including the ionization rate equations), the equations of laser levels population densities and propagation equation of laser intensities, are solved numerically. As a result, the effects of pre-ionizer dielectric parameters on the electrical behavior and output laser intensity are obtained.     

Lichtelektrische Leitung und Polarisation durch Laserstrahlung im Volumen von ZnO-Kristallen

In the volume of ZnO-single crystals a high concentration of free electrons was generated by two-photon absorption of ruby laser pulses. With regard to the different lifetimes of the moved electrons parallel and perpendicular to thec-axis influenced charges could be measured, which result only from the thermal motion of the electrons. With an applied voltage the Schubweg per unit electric field (called range) was investigated as a function of the concentration of absorbed photons. During irradiation with the laser pulse a pressure wave is caused by the thermal diffusion of the electrons; the arrival of the pressure wave on the electrodes in thec-direction was measured by the piezoelectric effect.     

One-step two-dimensional rainbow-holographic technique with a nonlaser light source

A technique for recording two-dimensional rainbow holograms by use of a nonlaser light source is described. Multicolor holograms are recorded in one step. The optical setup is lensless. The recording light source is an InGa lamp that provides a spectrum from UV to blue. The slit effect is generated by replication from the slit master plates.     

Pulsed mercury capillary lamps for dye laser pumping: Spectral measurements

Calibrated spectral measurements of the light emitted by a pulsed high-pressure mercury capillary lamp are presented. These pulsed lamps have been used in a previous work to pump a Rhodamine 6G laser. The measurements presented here show a very high efficiency in the blue and near UV part of the spectrum, suggesting that these lamps represent a very attractive pumping source for dye lasers emitting in the blue. In particular, as an example, a computation has been made for a basic solution of 4-MU yielding a spectral efficiency of 24%.     

Experimental studies of a XeCl laser having UV preionization perpendicular and parallel to the electrode surfaces

This paper reports on the results of an experimental study of a XeCl laser having two independent preionizators which generated UV radiation into the active medium in the perpendicular and parallel directions to the electrode surfaces. Output energy and other parameters were measured using a single preionizator and two preionizators, changing the power supply polarity and varying the delay time between the preionization and the main discharge. When the preionization system generated photons only in the direction perpendicular to the electrode surfaces, a high discharge current and a high output laser energy were obtained. Besides, the diffusion coefficient of the photo-preionized electrons, played an important role in the laser behaviour, particularly when a consistent delay time between the onset time of the preionization and the main discharge was present.     

强激光能源系统新型放电回路研究

 从理论上分析了强激光能源系统新型放电回路能量传输效率和氙灯能量分配均匀性问题，给出了预电离回路的实验研究结果，此外还介绍了新型大尺寸氙灯爆炸能量的测试情况。     

Realizing a laser-driven electron source applicable for radiobiological tumor irradiation

Laser-accelerated electron pulses have been used to irradiate human tumors grown on mice’s ears during radiobiological experiments. These experiments have been carried out with the JETI laser system at the Institute of Optics and Quantum Electronics in Jena, Germany. To treat a total of more than 50 mice, a stable and reliable operation of the laser-electron accelerator with a dose rate exceeding 1 Gy/min was necessary. To achieve this, a sufficient number of electrons at energies in excess of 5 MeV had to be generated. The irradiation time for a single mouse was a few minutes. Furthermore, the particle pulses’ parameters needed to remain achievable for a time period of several weeks. Due to the online detection of the radiation dose, the unavoidable shot-to-shot fluctuations, currently still typical for laser-based particle accelerators, could be compensated. The results demonstrate that particle pulses generated with laser-based accelerators have the potential to be a future alternative for conventional particle accelerators used for the irradiation of tumors.     

双预电离对TEA CO2激光器放电特性的影响

一些低增益弱线CO2激光光谱在分离某种同位素方面是非常有用的。在成比例的混合气体中很难获得这些谱线的输出,须适当提高CO2和N2的含量,但结果给主电极的辉光放电带来了困难。通过对改装后的双预电离放电回路的参量进行调节,实现了对预电离的单独控制,有利于增强预电离效果,使高含量的CO2混合气体的放电和出光成为可能。理论和实验证明了双预电离能较好地改善TEA CO2激光器的放电性能,尤其是有望实现用于分离某些同位素的弱线输出。     

Modeling of high pressure discharge lamps including electrodes

Electrode behavior in high pressure discharge lamps (HID), although understood qualitatively, is still not well understood quantitatively. Traditional models using different layers in front of the cathode are based on the assumption that the electrons gain substantial kinetic energy in the cathode fall to ionize the gas. For many HID lamps (e.g., p>1 MPa) this assumption may not be fulfilled. This paper uses a different model first proposed by Fischer (1987). We present modeling results for a Xe lamp with special emphasis on the cathode region