Self-focusing and defocusing of TEA CO2 laser radiation in NH3

We report on the observation of self-focusing and defocusing of TEA CO₂-laser pulses in NH₃. Self-focusing of the 9R(30) CO₂ laser line resulted in a significant spatial narrowing of the laser beam after propagating through a 100 cm long cell containing NH₃ at a relatively low pressure of 1 Torr. Spatial broadening, characteristics of self-defocusing, has been observed for the 9R(16) CO₂-laser line.     

Multiple breakdown in helium generated by hybrid 10 μm Co2 laser pulses

In this paper we describe the experimental investigation of the post-breakdown development of a helium plasma generated by a hybrid 10 m CO₂ laser pulse. As a result we found that a long tail and a high energy of the CO₂ laser pulse cause multiple successive breakdown in the gas. We have measured the time to the first breakdown and the time between first and second breakdown as functions of gas pressure and laser pulse peak intensity as well as the spatial intensity distribution of the transmitted laser pulse. Among the relevant mechanisms self-focusing is not observed before the second breakdown.     

金属卟啉化合物激发态折射与激发态吸收增强瞬态热折射竞争研究

应用激发态吸收增强瞬态热折射理论结合激发态折射理论研究了新型锌卟啉化合物Zn-porphyrin 中激发态吸收增强瞬态热折射与激发态折射间的相互竞争. 样品在不同脉宽激光激发下呈现自聚焦-自散焦间的转化. 当入射激光脉宽为4 ns时,激发态折射占主要地位,样品呈现自聚焦性质;当入射激光脉宽为8ns时,激发态吸收增强瞬态热折射占主要地位,激发态折射占次要地位,样品呈现自散焦性质. 关键词： 金属卟啉化合物 激发态折射 瞬态热折射     

Dynamical analysis of acousto-optically Q-switched CO2 laser

A compacted size high power CO₂ laser has been developed using an acousto-optically (AO) Q-switch. Performance characteristics have been investigated as a function of output mirror transmittance. The theory of six-temperature model for CO₂ lasers has firstly been utilized to analyze the dynamical process in the AO Q-switched CO₂ laser. This theory perfectly explains the behavior of energy transfer between different molecules in laser gain medium, and describes the shape of pulse laser. The calculated pulse waveforms are in good agreement with the experimental result. Both the experimental and theoretical results present that the optimal value of output mirror transmittance is 39%. Under this condition, the measured peak power is 4750 W and pulsed width is 160 ns, which is consistent with the calculations. Six-temperature model is a perfect theory for CO₂ laser kinetics, which will lay a theoretical foundation for the laser optimum design.     

Comparison of theory and experiment in electron-beam-controlled CO2 laser oscillators

A four-temperature model describing the electron-beam-controlled CO₂ pulsed laser has been developed and used to study the time evolution of a laser pulse and the influence thereon of experimental parameters. This paper explains the experimental results of two different electron-beam CO₂ pulsed laser systems which have been built in the laboratories at Frascati.     

Wavefront study in a Gaussian beam passed through a nonlinear optical medium

We present a numerical model to study the wavefront of a Gaussian laser beam propagated through a nonlinear Kerr media. The model is based on the Gaussian decomposition method. The interaction between a laser beam and a self-focusing or self-defocusing media is discussed from the viewpoint of wavefront distortion. The method is useful for simulation of the wavefront of the beam next to the sample. We also compare our results with those obtained from z-scan method. There is a quite good agreement between data from z-scan method and our results.     

Measurement of nonlinear refractive index of Ag and Au nano-particles using moiré technique

In this paper we have measured second order nonlinear refractive index of Ag and Au nano-particles using the moiré deflectometry with a CW He-Ne laser at wavelength of 632.8 nm. The main advantages of this technique are its extreme experimental simplicity and low sensitivity to external distribution in comparison with other interfrometric methods that need very stable experimental set-up. Because of self-focusing (or self-defocusing) that is due to intensity dependent refractive index, presence of sample in the moiré deflectometry set-up lead to rotation of moiré fringes. By moving the sample in set-up we have measured these rotation angles in different displacements from focus point of lens used in this set-up and then we have calculated the nonlinear refractive index of Ag and Au nano-particles using their values. Also we have obtained the sign of n₂ regard to the direction of rotations. Consequently by using the Mathematica software we have exhibited the suitable fitness of theoretical curve with experimental results.     

Promoting the Range and Range Resolution of a LIDAR (DIAL) System Using a Suitable Pinhole Plasma Shutter

Pulsed transversely exited atmospheric (TEA) CO₂ lasers, employed extensively in various applications such as light detection and ranging (LIDAR), have a pulse duration of about a microsecond due totheir nitrogen tail. In order to promote the measurement accuracy and the mean power of the laser pulse, the pulse duration should be shortened. In this research, we present the details of making a passive pinhole plasma shutter for a LIDAR (DIAL) system, which shortens the pulse duration of CO₂ lasers from 1.5 μs to 25 ns in air at atmospheric pressure. This instrument increases the range resolution of the LIDAR system from 225 to 3.75 m. Also we show the results of investigation of the clipped pulse duration of the microsecond CO₂ laser pulse using aluminum and copper pinhole metal targets with different pinhole diameters (1.5 and 1.8 mm) and at various laser output energies (338 and 309 mJ). Our experimental results show that the aluminum pinhole is more suitable than the copper pinhole for shortening the nitrogen tail of the CO₂ laser pulse with a larger output average power. Thus, the range of the LIDAR system, which is proportional to the logarithm of the output pulse power, is increased.     

All-optical limiting using nonlinear directional couplers composed of a self-focusing and a self-defocusing waveguide

We numerically demonstrate the all-optical limiting features in a mismatched nonlinear directional coupler (NLDC) composed of a self-focusing and a self-defocusing waveguide for both continuous wave and pulse cases. The working conditions required are analyzed. To obtain the limiting feature, the propagation constant of the self-focusing waveguide should not be larger than that of the self-defocusing waveguide. Cascaded asymmetric NLDCs are investigated to improve the limiting characteristics. The limiting threshold and the limiting output power can be adjusted by varying the coupler length or the ratio of the nonlinearity coefficients of the self-defocusing and self-focusing waveguides. Analytical solutions are presented in the case of a continuous wave. For the pulse case, numerical solutions show that the top part of the output pulse, if it exceeds the limiting threshold, will be tailored, while the rising and falling edges of the output pulse are almost the same as the input pulse. There is almost no pulse breakup. The influence of the second order dispersion and the intermodal dispersion on the limiting characteristics are analyzed. PACS 42.79.Gn; 42.79.Ta     

光生伏打LiNbO3:Fe晶体从自散焦到等效“自聚焦”的动态转换

实验研究了在一定条件下LiNbO₃:Fe晶体中从自散焦到等效“自聚焦”转换的动态行为,提出了出现自聚焦的物理机理为双相位共轭的法布里珀罗干涉腔中的多光束干涉与光折变效应的共同结果.该效应有希望在光折变自散焦介质中形成亮空间孤子 关键词： 自散焦 自聚焦 光折变     

Study of third-and fifth-order nonlinear optical processes in thin C<Subscript>60</Subscript> films

The nonlinear refraction in thin films of fullerene C₆₀ (100 nm) is studied by the Z-and RZ-scan methods using the second harmonic of a picosecond Nd:YAG laser (λ = 532 nm, τ = 55 ps). The combined effect of n₂ (self-focusing of laser radiation) and n₄ (self-defocusing) is analyzed. Mechanisms responsible for the nonlinear refraction in films are discussed.     

Numerical solution of Boltzmann tranport equation for TEA CO2 laser having nitrogen-lean gas mixtures to predict laser characteristics and gas lifetime

Selective laser isotope separation by TEA CO₂ laser often needs short tail-free pulses. Using laser mixtures having very little nitrogen almost tail free laser pulses can be generated. The laser pulse characteristics and its gas lifetime is an important issue for long-term laser operation. Boltzmann transport equation is therefore solved numerically for TEA CO₂ laser gas mixtures having very little nitrogen to predict electron energy distribution function (EEDF). The distribution function is used to calculate various excitation and dissociation rate of CO₂ to predict laser pulse characteristics and laser gas lifetime, respectively.Laser rate equations have been solved with the calculated excitation rates for numerically evaluated discharge current and voltage profiles to calculate laser pulse shape. The calculated laser pulse shape and duration are in good agreement with the measured laser characteristics. The gas lifetime is estimated by integrating the equation governing the dissociation of CO₂. An experimental study of gas lifetime was carried out using quadrapole mass analyzer for such mixtures to estimate the O₂ being produced due to dissociation of CO₂ in the pulse discharge. The theoretically calculated O₂ concentration in the laser gas mixture matches with experimentally observed value. In the present TEA CO₂ laser system, for stable discharge the O₂ concentration should be below 0.2%.     

CO2-laser-induced multiphoton absorption of CF2Cl2

Energy absorption by CF₂Cl₂ from an intense TEA CO₂ laser pulse is measured as a function of the pressure of CF₂Cl₂ and the pressure of Ar bath gas for different laser energy fluences. The fraction of the molecules excited by the laser field and the average energy of the molecular ensemble are determined by a simple experimental method.     

富勒烯衍生物中的自散焦、自聚焦及其相互转化

用Z扫描方法研究了富勒烯有机铬衍生物fac- and mer- [bis (1,2-diphenylphosphino) ethane] (tricarbonyl) (η²-fullerene-C₆₀) chromium的非线性折射特性;用速率方程理论分析了激发态光学非线性材料非线性折射行为,以及大光强下高阶激发态折射度如何引起材料自散焦-自聚焦相互转化.计算出了转变光强.并数值模拟了不同光强下Z扫描实验曲线的变化过程.理论计算与实验符合得较好. 关键词： 自散焦 自聚焦 富勒烯衍生物 Z扫描     

脉冲CO2激光器的多频动力学模型

建立了适用于各种气压下(20×133—20×10⁵ Pa)的脉冲CO₂激光器的六温度多频动力学模型，该模型考虑了增益谱线重叠效应，序列带、热带的影响，以及非洛伦兹线型效应.对模型进行数值求解可以预言和解释不同气压下的脉冲CO₂激光器的输出特性，有助于评价不同的抽运设计和研究可调谐特性，为设计脉冲CO₂激光系统提供理论支持. 关键词： 2激光器')" href="#">CO₂激光器 激光动力学 六温度模型 增益谱线重叠     

Investigation of kerr-lens mode-locking of a flashlamp-pumped Nd:YAG laser

In this paper, we report that mode-locked operation is realized in a flashlamp-pumped Nd:YAG laser with a nearly critical stable resonator, using LiF:F₂⁻ color center crystal as a modulation device. A single pulse train with the energy of 25 mJ and the pulse duration of 82 ps is obtained. Both the theoretical and experimental results have shown that the mode-locked operation has resulted from kerr-lens effect formed in combination with self-focusing within Nd:YAG and an aperture at a specific point, but not directly from the saturable absorption of LiF:F₂⁻ crystal.     

Shifting bright spatial solitons in LiNbO₃

We observe spatial bright solitons in a transversely shifting LiNbO₃ crystal. In this scheme, the photovoltaic nonlinearity in LiNbO₃ can be switched from self-defocusing to self-focusing by continuous shifting of the beam. Moreover, the shifting velocity has a strong influence on the wave shape of the bright spatial solitons.     

Nonlinear refractive index measurements and self-action effects in Roselle-Hibiscus Sabdariffa solutions

We report the observation of self-action phenomena, such as self-focusing, self-defocusing, self-phase modulation and beam fanning in Roselle-Hibiscus Sabdariffa solutions. This material is found to be a new type of natural nonlinear media, and the nonlinear reflective index coefficient has been determined using a Z-scan technique and by measuring the critical power for the self-trapping effect. Z-scan measurements show that this material has a large negative nonlinear refractive index, n ₂ = 1 × 10^?4 esu. A comparison between the experimental n ₂ values and the calculated thermal value for n ₂ suggests that the major contribution to nonlinear response is of thermal origin.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Single longitudinal mode pulse from a TEA CO<Subscript>2</Subscript> laser by using a three-mirror resonator with a Fabry–Pérot etalon

We present a single longitudinal mode (SLM) TEA CO₂ laser oscillation by using a three-mirror resonator with a Fabry–Pérot etalon. The etalon was inserted in the optical path taken out from the main resonator of the CO₂ laser for protecting the etalon from damage on the surface. A modified numerical model of the three- mirror resonator was investigated for design the laser. SLM pulse from the TEA CO₂ laser was achieved, and the experimentally measure values were found to have good agreement with the numerical model. The maximum pulse energy of reliable SLM emission is obtained in excess of 200 mJ at 9.57 μm. The reliability of producing SLM pulses was higher than 90%, and there was no damage on the etalon PACS  42.55.Lt; 42.60.Fc