Near infrared lasing transitions in Ar,Kr, and Xe atoms pumped by a coaxial e-beam

Optimizations of gas composition and input energy were performed for gas mixtures containing a buffer gas and either Ar, Kr or Xe as the lasing gas. The total gas pressure was varied between 1 and 14 bar and the input energy from 0.03 to 0.7 J/cm³. The excitation source was a small coaxial electron beam with a pumping length of 20 cm and a pulse length of 30 ns (FWHM). From an active volume of 13.3 cm³ a maximum output energy of 12 mJ was obtained from a gas mixture containing 0.3% Xe in Ar at a total gas pressure of 10 bar. The intrinsic efficiency was 0.9%.     

Spectral line competition in a coaxial electron-beam-pumped high pressure Ar/Xe laser

In order to study the kinetic mechanism of the e-beam pumped Ar/Xe laser, the temporal profiles of individual laser lines during multiline oscillation have been measured as a function of power deposition (1–12MW/cm³) and gas laser pressure (2–14 bar) using a short pulse (30 ns) coaxial electron beam as excitation source. It was found that the optimum output energy at each pressure was obtained at the same specific power deposition.Strong line competition has been observed between the 2.65 and 1.73 m transitions. In order to explain our results we suggest that besides electron collision mixing (ECM) between the 5d and 6p levels of Xe, there is also a redistribution between all 6p levels which strongly favours the lower levels at higher pumping densities.     

On the performance of an ArF and a KrF laser as a function of the preionisation timing and the excitation mode

An X-ray preionised ArF and KrF excimer laser has been studied with three different spiker-sustainer excitation circuits. We observed large differences in the laser performance, when the preionisation delay timing was varied on a nanosecond timescale. The behaviour of both lasers was found to be equivalent. The observations can be understood by considering the effect of the discharge excitation technique on the preionisation process. An excitation mode with a prepulse well above the steady-state voltage V_SS with a subsequent reversed overshoot voltage for initiating the discharge, in combination with a well-timed preionisation pulse is found to give the best results. Optimum output energies of 50 mJ with ArF and 175 mJ with KrF were obtained from an active volume of 60×1.5×1.2 cm in the so-called swing mode, with the preionisation applied 60 ns before the discharge breakdown. Received: 23 February 1999 / Revised version: 4 June 1999 / Published online: 16 September 1999     

Linewidth studies on the Ar+ 476.5 nm and 480.6 nm lines excited in a helium-agron hollow cathode discharge

The spectral line shapes of the Ar⁺ 476.5 nm and 480.6 nm lines, excited in a He-Ar hollow cathode (HC) discharge, were measured using the Fabry-Perot technique. The collisional and Doppler linewidths were determined for the two lines. The collisional broadening constants are estimated to be (5±3) MHz/mbar and (6±3) MHz/mbar, respectively. The temperature obtained from the two Ar ion transitions was found to be 260 K higher than that expected for the rest of the gas mixture from earlier measurements. The possibility is discussed, that this excess temperature is caused by Ar ions partially created in the HC discharge by charge transfer collisions with He ₂ ⁺ molecules.     

反应堆核泵浦3He-Ar-Xe气体激光腔内能量沉积数值模拟研究

在TRIM软件计算结果的基础上利用蒙卡方法编制了能量沉积效率与损失率的EDL计算软件,模拟计算了泵浦腔尺寸、混合气体中3He分压等参数与能量沉积密度之间的关系,研究了反应堆核泵浦激光腔内的能量沉积问题,推导出了能量沉积密度的函数关系式。     

Discharge ignition phenomena in CO2 and N2O lasers with preliminary filling of the interelectrode gap by electrons

A comprehensive model is presented that describes preionization and discharge ignition processes in large-aperture molecular-gas lasers. By initially filling the interelectrode gap with electrons and establishing a space-charge screen at the cathode, electron avalanche begins adjacent to the anode. Cathode-directed ionization waves develop and ignite the discharge. Dynamic profiling of the electric-field distribution and Penning ionization of a readily ionizable additive combine to decrease both the ignition and quasi-steady-state voltages.     

Laser operation and Raman self-frequency conversion in Yb:KYW microchip laser

We present a diode-pumped Yb:KYW microchip laser. The passive Q-switchedand CW regimes of operation for the Yb:KYW microchip laser have been investigated. An efficiency for CW operation of up to 10% with regard to incident pump has been obtained. Raman self-frequency conversion in the Q-switched regime has been observed. Received: 27 May 2002 / Revised version: 4 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +375-17/2840-879, E-mail: asg@dragon.bas-net.by     

Complete two-electron spectra in double photoionization: the rare gases Ar,Kr, and Xe

A new spectroscopic technique, giving complete two-dimensional e(-)-e(-) coincidence spectra in single photon double photoionization, is presented. The technique resolves the states of doubly charged ions and provides spectra of the individual electrons emitted in formation of each final dication state. Complete spectra for double photoionization of Xe, Kr, and Ar at photon energies up to 51 eV have been recorded. Overall and surprisingly, the np(4) 3P, 1D, and 1S states are populated according to their statistical weights. When the evident autoionization is excluded, the supposedly favored 3P states are in fact disfavored. Detailed information on the autoionization processes is also made available.     

A high power high energy pure N2 laser in the first and second positive systems

Simultaneous high energy lasing in the N₂ first and second positive systems is reported. 20.5 mJ (1.5 MW) UV and 5 mJ (0.4 MW) IR laser pulses were produced in pure N₂. The design is open and easy to improve.     

LD泵浦高功率高斜效率Nd:YVO4声光调Q激光器

 报道了采用激光二极管(LD)端面泵浦Nd:YVO₄双端键合晶体高平均功率高斜效率1 064 nm声光调Q激光器。通过对大功率泵浦情况下激光晶体热透镜效应进行分析和估算,优化了模式匹配及热稳腔结构参数,实现了稳定的高功率高斜效率准连续脉冲激光输出。在泵浦功率46.8 W、最高重复频率50 kHz下,获得最大平均输出功率17.6 W,光-光转换效率为37.6%,斜效率达70.1%,脉冲宽度51.3 ns;在最低重复频率10 kHz下,获得最大单脉冲能量0.91 mJ,峰值功率为46.2 kW,脉冲宽度为19.8 ns。     

Generation and Modulation of Phase Difference of Output Intensities in a Feedback Nd:YAG Laser with an Extracavity Waveplate Rotated

External anisotropic feedback effects on the phase difference behaviour of output intensities in a microchip Nd：YAG laser are presented. By rotating a quarter wave plate placed in the external cavity, the angle between laser initial polarization direction and o-axis of the wave plate is tuned from -45°to 45°, which results in variable extra-cavity birefringence along two orthogonal detection directions. With only one optical path and one wave plate, laser intensities of the two orthogonal directions, both modulated by the external cavity length, are output with a tunable phase difference, which can be continuously changed from zero to twice as large as that of the waveplate. Experimental results as well as a theoretical analysis based on Fabry-Perot cavity equivalent model and the refractive index ellipsoid, are presented. The potential applications of this phenomenon are also discussed.     

Total photoionization cross-sections of Ar and Xe in the energy range of 2.1–6.0 keV

Total photoionization cross-sections of Ar and Xe have been accurately measured in the photon energy region of 2.1–6.0 keV, using a four-electrode ion chamber with monochromatized synchrotron radiation. Experimental data are presented in both graphical and tabular forms and compared with previous data reported in the literatures.     

Calculation of the Spatio-Temporal Dynamics of Q-switched Yb^3＋：YAG Laser with an Unstable Cavity and a Super-Gaussian Mirror

A numerical simulation used to compute the spatio-temporal dynamics of pulse formation of diode-pumped Q- switched Yb： YA G laser is carried out. The model takes the laser amplification and gain saturation, the properties of the laser cavity, and the diffractive effects of the laser disc into account. The numerical calculation is performed for a confocal positive-branch unstable resonator with a super Gaussian coupling mirror. The simulation results show that the laser pulse starts from a Gaussian intensity distribution and becomes rapidly non-Gaussian. The corresponding beam quality M^2 factor is seen to vary approximately from 1.5 at the beginning of the formation of pulse to more than 10 in the tail of the pulse, with a value of 11.6 at the peak of the pulse.     

Pressure-induced ultraviolet absorption in rare gases: Absorption coefficients for mixtures of Xe and Ar at pressures up to 40 atm in the vicinity of 147 nm

Absorption coefficient κ_υ for mixtures of Xe (0.2–20 kg/cm²) and Ar (5–40 kg/cm²) has been measured in the range 0.1 cm^-1 < κ_υ < 40 cm^-1. Exponential and power dependencies of κ_υ on wavenumber were observed and are discussed. The κ_υ-dependencies on the pressure of mixture components are explained by photon absorption at multiparticle collisions of Xe and Ar atoms.     

The thermal effect analysis in the high power frequency-doubled Nd:YAG Laser

In the high power frequency-doubled Nd:YAG Laser, the temperature inside a nonlinear crystal will become non-uniform due to the absorbtion of the fundamental and second harmonic waves, leading to phase mismatch, conversion efficiency reduction and output power instability. This phenomenon appears severe in the high-power high-repetition rate laser system. In this paper, temperature distribution inside a KTP crystal was analyzed by solving the thermal conductivity equation. According to the temperature distribution of the KTP, we have theoretically calculated the optimal phase matching angles, tolerance angles and walk-off angles of type II KTP crystal as a function of temperature. __________ Translated from Chinese Journal of Quantum Electronics, 2005,22 (4) (in Chinese)     

Magnetic scattering and unusual behavior of the critical fields and other parameters: Application to borocarbides

The pair-breaking effect and its impact on the properties of borocarbides is studied. The pair-breaking effect caused by localized magnetic moments drastically affects the superconducting properties. Interaction between the magnetic moments and the resulting ordering trend lead to a behavior entirely different from the conventional picture. The main focus is on the behavior of the upper () and lower () critical fields. In addition, the temperature dependence of several quantities (penetration depth, coherence length, etc.) has been calculated. The theory has been applied to the borocarbide LuNi₂B₂C and is in very good agreement with the recent experimental data. Received 29 June 1999     

Ab initio study of Nb2SC and Nb2S2C: Differences in coupling between the S and Nb-C layers

Using ab initio calculations, we have studied the structurally related compounds Nb₂SC and Nb₂S₂C. In Nb₂S₂C (space group , prototype Bi₂Te₃), S atoms are nearest neighbours, while in Nb₂SC (space group P6₃/mmc, prototype Cr₂AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6-3.7% to previously-published experimental data and the bulk modulus-to-c₄₄ ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb₂S₂C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb₂SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.     

Brownian particles in random and quasicrystalline potentials: How they approach the equilibrium

We study the Brownian motion of an ensemble of single colloidal particles in a random square and a quasicrystalline potential when they start from non-equlibrium. For both potentials, Brownian dynamics simulations reveal a widespread subdiffusive regime before the diffusive long-time limit is reached in thermal equilibrium. We develop a random trap model based on a distribution for the depths of trapping sites that reproduces the results of the simulations in detail. Especially, it gives analytic formulas for the long-time diffusion constant and the relaxation time into the diffusive regime. Aside from detailed differences, our work demonstrates that quasicrystalline potentials can be used to mimic aspects of random potentials.     

The Rydberg matter laser: excitation, delays and mode effects in the laser cavity medium

Temporal and temperature effects are studied in Rydberg matter (RM) formed from K atoms and N₂ molecules as the active medium in a cavity. The function of this setup as a laser was recently described. Temperature-variation studies show that the photons re-exciting the RM clusters usually have a longer wavelength than the photons emitted in the stimulated emission process in the cavity. The deficit is probably covered by background photons. Very long time constants observed after emitter temperature changes indicate that long-wavelength photon energy is accumulated in the RM clusters. Long-wavelength modes are located farther from the RM emitter. The modal structure can be TEM₀₁ or TEM₀₀, as observed clearly by the spatial structure in rapid pulsing experiments. The in-cavity chopped beam signal is delayed by approximately 50 μs. The initial growth rate of the signal during chopping is temperature dependent. Tailing is also observed by chopping, but rapid pulsing of the beam with a spinning mirror does not show any delay of the start of the lasing. The conclusion is that delays exist in the stimulated emission process. The broad intense band appearing at 11 000 nm is shown to be formed partly by light in the range 3500–5000 nm, probably by standing wave interaction at the grating surface (grating bands).     

Application of power ultrasound in freezing and thawing Processes: Effect on process efficiency and product quality

Freezing is one of the most efficient preservation approaches applied to food products and thawing is the reverse process of freezing. However, traditional freezing / thawing methods have low process efficiency. The application of ultrasound is a potential supplementary technique to improve the performance of both freezing and thawing processes of foods. Application of power ultrasound is able to better maintain the microstructure, reduce drip loss, decrease color and texture changes and retain some natural nutrients of foods during freezing. Meanwhile, quality improvement is also observed in food items thawed by ultrasound-assisted thawing methods. The fundamentals and the influences of ultrasound on the freezing and thawing processes of foods are demonstrated in this review article, from the aspects of efficiency enhancement and quality improvement.