Spatiotemporal instabilities from a transversely excited atmospheric CO2 laser

Local spatiotemporal instabilities in a TEA CO₂ laser have been studied. Their evolution was studied from the beginning of the gain switch peak to the quasistationary state in the collisional transfer tail of the pulse.The dynamics varies when the diameter of an intracavity aperture is changed. In the TEM₀₀ mode an ordered axial mode beating is observed. In transverse multimode regime irregular fluctuations are observed. In this case signals from two different points taken in the near field show a low cross-correlation. The application of the Grassberger-Procaccia algorithm to the irregular signals shows a good slope convergence towards a value of 2.5, pointing to a chaotic deterministic behaviour.     

可调谐TEA CO2激光器多频输出

为了满足多频红外激光分离同位素的需要,采用平行的双光栅腔结构,在最佳混合气体体积分数、总压强50.7 kPa、电压27 kV条件下,成功得到了TEA CO2激光器常规带多组双频和三频激光输出。输出的双频波（或三频波）具有良好的空间重叠性和时间同步性,输出波长可调谐且调谐范围较大,激光从一光栅旁边近似地沿平行于腔轴的方向输出,可以满足对一些物质双频（或多频）激发或分离同位素的需要。     

Properties of kinetics of chemiluminescence of luminol excited by a nitrogen laser

The kinetics of chemiluminescence of luminol C₈H₇N₃O₂ excited by a pulsed nitrogen laser is studied The kinetics of chemiluminescence of free luminol molecules is compared with that of luminol molecules in the complex UO₂(C₈H₆N₃O₂)₂. Photochemical consequences of excitation of the UO₂(C₈H₆N₃O₂)₂ are considered.     

Time-resolved studies of emission excited in sodium vapour by 330 nm laser pulses

Received: 21 June 1996/Accepted: 4 November 1996     

激光诱导氮气等离子体时间分辨光谱研究及温度和电子密度测量

激光诱导击穿光谱(LIBS)作为一种重要的分析手段被广泛应用于材料分析、环境监测等领域.特别是随着大气污染问题的日趋严重,基于LIBS的大气污染在线监测分析技术快速发展,氮气等离子体特性的时间演化规律对研究激光诱导大气等离子体动力学和发展大气污染监测的LIBS技术具有重要意义.而温度和电子数密度作为表征等离子体状态最重要的参数,直接影响着等离子体形成、膨胀和退化中的动力学过程以及等离子体中的能量传输效率.本文利用等离子体时间分辨光谱,研究了连续背景辐射、分子谱线强度及信背比(分子谱线与连续背景辐射的比值)在等离子体演化过程中的变化规律,结果显示连续背景辐射寿命在700 ns左右,N_2~+(B~2Σ_u~+-X~2Σ_g~+,v:0-0)跃迁谱线强度在12—15μs范围内达到最大值,信背比随时间呈现上升、稳定的趋势,因此利用N+2分子离子第一负带系(B~2Σ_u~+-X~2Σ_g~+)研究等离子体温度的观测窗口应选择在10—25μs之间;基于双原子光谱理论,通过拟合实测光谱和仿真光谱研究了大气压下激光诱导氮气等离子体温度随时间的演化趋势,由于辐射损耗远小于碰撞作用,在10—28μs内等离子体温度从约10000 K按指数衰减到约6000 K;在准确测定仪器展宽线型的基础上,利用Nelder-Mead单纯形算法,研究了N原子746.831 nm谱线的Stark展宽和位移随时间的演化趋势,计算了等离子体中电子数密度随时间在10~(17)—10~(16)cm~(-3)量级间衰减,通过分析发现造成等离子体中电子数衰减的主要机理是三体碰撞复合.     

Time-resolved spectroscopy analysis of Ga atom in laser induced plasma

Time-resolved optical emission spectroscopy was used to characterize Ga atom in plasma produced by 1064 nm, 10 ns pulsed laser. The evolution and the character of the spectra were analyzed. The line width and shift of Ga lines were detected. The Ga lines have two distinct line shift—“red shift” and “blue shift.” It attributed to their level states and the quadratic stark effect. The strong lines at around 425 nm whose upper energy level could be regard as high excited states can be used to explain why the intensities of Ga I spectral line 403.3 and 417.2 nm begin with a increasing dependence on time followed by a decrease. In addition, the effects of materials, laser power densities and the environmental pressure on the plasma were also detected and analyzed.     

Time-resolved dynamics analysis of nanoparticles applying dual femtosecond laser pulses

The evolution of a fs laser generated plume on copper was studied by dual laser pulses. The plume generated by the first pulse (620 nm) was excited by the second, delayed pulse (310 nm). The actual state of the plume was monitored by detecting the intensities of the emitted light and the reflected delayed pulse from the interaction range, i.e. 0–150 μm above the surface. Four peaks were observed in the 0–1400 ps time range and assigned to appearance of electrons, ions, atomic species and nanoparticles. Accordingly, the timing of the creation and ejection of these species was determined. By integrating the intensity of the nanoparticle peak, the production yield of the nanoparticles and its dependence on the ablating laser fluence was calculated. The ablation products were deposited on a Si substrate, too, and analyzed by atomic force microscopy and scanning electron microscopy. The results of the deposition and the dual-pulse experiments are in good agreement with the theory. PACS 52.38.Mf; 79.20.Ds; 61.80.Ba     

Time-resolved phase measurement of a self-amplified free-electron laser

We report on the first time-resolved phase measurement on self-amplified spontaneous emission (SASE) free-electron laser (FEL) pulses. We observed that the spikes in the output of such free-electron laser pulses have an intrinsic positive chirp. We also observed that the energy chirp in the electron bunch mapped directly into the FEL output. Under certain conditions, the two chirps cancel each other. The experimental result was compared with simulations and interpreted with SASE theory.     

On the repetitive operation of a self-switched transversely excited atmosphere CO<Subscript>2</Subscript> laser

The repetition rate capability of self-switched transversely excited atmosphere (TEA) CO₂ laser was studied for different gas flow configurations. For an optimized gas flow configuration, repetitive operation was achieved at a much smaller gas replenishment factor between two successive pulses when compared with repetitive systems energized by conventional pulsers.     

Infrared dye laser excited by a diode laser

A waveguide dye laser with an inner diameter of 30 μm and a length of 1.0 nm has been excited with a pulsed GaAlAs diode laser. The infrared-absorbing dye 5,5′-dichloro-11-diphenylamino-3,3′-diethyl-10,12-ethylenethiatri- carbocyanine perchlorate was used in tetramethylene sulfoxide solution. By changing dye concentration, laser wavelengths from 950 to 1005 nm were obtained.     

Time-resolved analysis of nonlinear optical limiting for laser synthesized carbon nanoparticles

Nonlinear optical limiting materials have attracted much research interest in recent years. Carbon nanoparticles suspended in liquids show a strong nonlinear optical limiting function. It is important to investigate the nonlinear optical limiting process of carbon nanoparticles for further improving their nonlinear optical limiting performance. In this study, carbon nanoparticles were prepared by laser ablation of a carbon target in tetrahydrofuran (THF). Optical limiting properties of the samples were studied with 532-nm laser light, which is in the most sensitive wavelength band for human eyes. The shape of the laser pulse plays an important role for initializing the nonlinear optical limiting effect. Time-resolved analysis of laser pulses discovered 3 fluence stages of optical limiting. Theoretical simulation indicates that the optical limiting is initialized by a near-field optical enhancement effect.     

A novel double-pulse laser plasma spectroscopic technique for H analysis in metal samples utilizing transversely excited atmospheric-pressure CO2 laser-induced metastable He atoms

The analysis of hydrogen in a metal sample (zircaloy-4), which is usually difficult to perform using conventional laser-induced breakdown spectroscopy (LIBS) techniques, has been achieved using a double-pulse technique under He gas at atmospheric pressure. In this technique, a transversely excited atmospheric-pressure (TEA) CO₂ laser (1.5 J, 200 ns) was focused onto the metal surface to induce a strong He gas plasma whilst simultaneously focussing a Nd-doped yttrium aluminum garnet (Nd:YAG) laser (120 mJ, 8 ns), synchronized with the TEA CO2 laser, onto the metal to ablate atoms into the resulting He gas plasma. The emission spectrum obtained shows a narrow H linewidth with low background intensity and long lifetime emission, thereby indicating that excitation takes place via metastable He atoms. The H emission from H₂O can be suppressed by a careful pretreatment involving heating the sample in a vacuum chamber.     

Operational characteristics and power scaling of a transverse flow transversely excited CW CO<Subscript>2</Subscript> laser

Transverse flow transversely excited (TFTE) CO₂ lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO₂ laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO₂ laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.     

FIR NH3 cascade laser excited by a Q-switch laser

Short FIR laser pulses of high repetition rate are obtained by pumping NH₃ with the pulses of a passively Q-switched IR laser. The two cascade laser transitions observed show coupled relaxation oscillations. The pulse shapes and delay times are qualitatively in agreement with a four-level rate-equation model.Partially supported by Deutsche ForschungsgemeinschaftOn leave from Korea Standards Research Institute, Dae Deog Dan Ji, P.O. Box 3, Choong Nam, Korea, supported by Korea Science and Engineering Foundation