Effect of the pulse leading-edge time on the dielectric strength of a vacuum gap

Optimal regimes for electrode conditioning in a vacuum by applying voltage pulses with different waveforms are considered. For nanosecond pulses with a constant duration (t _p = const), the impulse dielectric strength for an oblique voltage wave is shown to be more than four times higher than for a rectangular pulse with an infinitely short leading-edge duration. The dependences of the dielectric strength on the conditioning pulse duration in the range 10^?10 < t _p < 10^?3 s for pulses with different rise rates are obtained. The dielectric strength increases from 2 × 10⁷ V/m for microsecond pulses to 10¹⁰ V/m for subnanosecond pulses.     

Second-harmonic generation taking into account dispersion of nonlinear susceptibility

Second-harmonic generation in the field of an ultrashort pulse and the propagation of extremely short pulses in a medium with quadratic nonlinearity are analyzed. Second-harmonic generation is analyzed taking into account the effect of second-and third-order group velocity dispersion and dispersion of nonlinear susceptibility up to the second order. Corrections, whose order of smallness is determined by the parameter (ω_L t _p)^?1, where t _p is the pulse duration and ω_L is the carrier frequency of the pump wave, are obtained. For a large phase mismatch, two new solutions are found that describe the stationary evolution of solitary pump and second-harmonic waves in the regions of both anomalous and normal group velocity dispersions.     

Cascaded compression of the first and second Stokes pulses during forward transient stimulated Raman amplification

The results of numerical modelling of cascaded compression of the first and second Stokes pulses during regenerative regime of the forward transient stimulated Raman amplification are presented for the case when the walk-off length of the first Stokes pulse due to group velocity mismatch is shorter than the length of the nonlinear medium. The influence of the initial amplitudes of the seed first Stokes pulses, its durations and its time delay with respect to the pump pulse, the Kerr nonlinearity of the medium on the conversion efficiency, duration and propagation factor M² of the first and second Stokes pulse are studied. It is demonstrated that for the pump pulse duration of 1 ps the duration of the compressed second Stokes pulses in a KGW crystal near the beam axis may be approximately 14 times shorter than the pump pulse duration. It is shown that the propagation factor of the compressed pulses increases significantly because of complex spatial-temporal dynamics of compression and the influence of Kerr nonlinearity of Raman medium.     

Quantitative infrared photochemistry with CO2 laser of different temporal shape: Dissociation of CF3I with nanosecond pulses

Infrared laser pulses of different temporal pulse shape with a length of 2–50 ns and a nominal power of 100–1000 MW are generated by a TEA CO₂ laser oscillator combination using saturable intracavity absorbers and a fast CdTe electro-optical switch. The multiphoton dissociation of CF₃I + nhv → CF₃ + I at 1074.6 cm⁻¹ and p(CF₃I) = 10 Pa was studied using these pulses. The product yields and the absolute rate constants were determined for different temporal pulse profiles, showing clear intensity effects at low intensities and a transition to a linear intensity regime at higher intensities.     

Theory of synchronously pumped dye lasers

An analysis of cw synchronously pumped dye lasers is presented. Under the assumption that the cavity (tuning element) bandwidth is much wider than the bandwidth of the transform limited pulses generated, the pulse forming dynamics is rigorously treated. It is shown that for a finite mismatch between the lengths of the dye and the pump lasers, a steady-state pulse develops in the dye laser cavity with a conserved pulseshape. The characteristics (energy, shape, peak power, duration) of these pulses of ultimate width are quantitatively determined as a function of cavity mismatch. An analytical solution for the pulse envelope is determined, which yieldsI(t)∝Sech²(t/t _p ) to a good approximation.     

Photolytical XeF(C–A) laser amplifier of femtosecond optical pulses: gain measurements and pump efficiency

Gain characteristics of a photolytically driven XeF(C–A) laser amplifier are studied experimentally in the unsaturated amplification regime. The gaseous active medium is optically pumped by vacuum-ultraviolet (VUV) radiation from two large-area multichannel surface discharges initiated along opposite walls of the amplifier chamber. A total gain factor of 10² is obtained for the ultrashort optical pulses under multipass amplification in the active volume of 40?×?18?×?4 cm³ dimensions with a spatially homogeneous gain distribution. Spectral measurements reveal a good conservation of the seed pulse spectrum. Small-signal gain reaching 2×10^-3 cm^-1 is observed for the blue-green seed pulses of 150 fs duration, as well as for the continuous seed radiation at 488 nm. The obtained gain values, being compared with the gain calculated for the measured pumping radiation power, indicate that the quantum yield of the XeF(B) formation as a result of the XeF₂ photodissociation is high and approaches unity within the spectral band of the XeF₂ VUV photodissociation continuum.     

The influence of ultra-fast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation

The effect of ultra-short laser-induced morphological changes upon irradiation of silicon with double pulse sequences is investigated under conditions that lead to mass removal. The temporal delay between 12 double and equal-energy pulses (E _p=0.24 J/cm² each, with pulse duration t _p=430 fs, 800 nm laser wavelength) was varied between 0 and 14 ps and a decrease of the damaged area, crater depth size and periodicity of the induced subwavelength ripples (by 3–4 %) was observed with increasing pulse delay. The proposed underlying mechanism is based on the combination of carrier excitation and energy thermalization and capillary wave solidification and aims to provide an alternative explanation of the control of ripple periodicity by temporal pulse tailoring. This work demonstrates the potential of pulse shaping technology to improve ultra-fast laser-assisted micro/nanoprocessing.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Multiline short pulse amplification and compression in high gain CO2 laser amplifiers

Theoretical studies are presented for the amplification and compression of multiline short pulses of CO₂ laser radiation at 10.6 μ in a high gain, CO₂ laser amplifier. A method of efficiently generating high energy, subnanosecond pulses of CO₂ laser radiation is proposed utilizing a pulse tailoring technique recently disclosed by Figueira and Sutphin.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

Auditory detection of paired pulses by a dolphin in the presence of a pulse jam

From behavioral studies of a bottlenose dolphin (Tursiops truncatus), the audibility thresholds were measured for a single pair of equal-amplitude pulses, i.e., clicks, presented to the dolphin in combination with a pulse jam. The pulse jam consisted of pairs of identical pulses with a pulse spacing τ_j within the pairs and a pair repetition rate f _j. Series of pulses were interrupted by a pause R>1/f _j, within which the pulse jam was absent while a pair of test pulses was supplied to one of the two channels at random. Each series had a duration T, and the total stimulation cycle was J=T+R. The dependence of the test pair detection threshold on the pulse spacing τ_j was studied at different fixed values of the pulse spacing in the test pair: τ_t=50, 100, 200, and 500 µs. Preliminary measurements performed with τ_j=τ_t=100 µs were used to adjust the parameters of the pulse jam. The threshold shift at τ_j=τ_t=100 µs reached 35 dB above the audibility threshold of the test pair in the absence of the pulse jam. On both sides of the point τ_j=τ_t=100 µs the thresholds decreased with varying τ_j to approximately 20 dB above the detection threshold of the test pair in the absence of the jam. However, in the course of training, the threshold curves gradually shifted downwards approaching the detection level of the test pair in the absence of the jam and becoming progressively flatter (the selectivity with respect to the pulse jam vanished). A decrease in the pause duration R restored the dependence of the test pair detection threshold on τ_j. In this case, a statistically significant maximum was obtained at τ_j=τ_t for τ_j within the critical interval (for τ_t<500 µs). Beyond the critical interval (for τ_t>500 µs), even with the smallest pause duration (R=15 ms), no dependence of the test pair detection thresholds on τ_j could be observed.     

Molecular dissociation of SF6 by ultra-short CO2 laser pulses

We have measured the absolute dissociation probability of SF₆ gas irradiated by CO₂ laser pulses varying in duration from 0.5 nsec to 100 nsec. We find a threshold for dissociation of ? 1.4 J/cm², independent of pulse duration. For fixed energy density, the dissociation probability increases as the pulse duration decreases, but not nearly to the degree expected from theory. We have also determined the dissociation probability in the ν₂ + ν₆ combination band and find it to be 10³ times less than in the ν₃ band, in contradiction to some conclusions recently reported by Ambartzumian et al. The dissociation mechanism takes place without collisions, and if collisions occur, they tend to be detrimental to the dissociation rate.     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Multifilamentation of femtosecond laser pulses propagating in?turbulent air near the ground

The propagation of femtosecond laser pulses in turbulent air near the ground is analyzed. Confining to a power regime distinctly above the critical power for self-focusing, i.e. P≈100P _cr, and concentrating on initial peak intensities around 2.5×10¹¹W/cm², the onset and early evolution of multiple filaments are addressed. Making use of the turbulence phase-screen method, numerical simulations of the pulse propagation indicate that turbulence fields with spatial scales below 6 mm are able to induce the onset of multifilamentation. An analytical linear plane wave perturbation model of the underlying modulation instability of the pulse front is introduced in support of the computational results. By this means, insight into the amplification of an initial perturbation of the pulse front from the point of view of the spatial frequency domain is given.     

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%.     

Change in the resonance frequency of surface plasmons in copper nanoparticles excited by femtosecond laser pulses

Changes in the optical density ΔD(t), halfwidth ΔH/2(t), and spectral position of the maximum Δλ_SP(t) of the surface plasmon band in Cu nanoparticles after their excitation by femtosecond laser pulses have been investigated. The Δλ_SP(t) dependence appears to be alternating and is accompanied by a nonmonotonic variation in ΔH/2(t) in the time interval 0–5 ps. The results are explained in a model based on the evolution of the dielectric response of such a composite medium excited by intense laser pulses.     

水中受激布里渊散射微弱Stokes信号光的高增益放大

采用脉冲宽度为7.2 ns的种子光注入式倍频Nd:YAG脉冲激光器,以CS₂为放大介质,实验并理论研究了水中受激布里渊散射微弱Stokes光的信号增益随延迟时间、放大器池长、抽运光能量的变化规律. 结果表明,当抽运光脉冲相对信号光脉冲延迟进入放大器,且延迟时间为脉冲宽度的一半,抽运光能量略低于介质受激布里渊散射阈值,选择合适的放大器池长可获得最佳的信号增益. 适当选择抽运光能量,亦可实现微弱信号光的线性放大. 实验中采用独立双池放大系统,当水中Stokes信号光的能量为1 pJ时,信 关键词： 布里渊放大器 信号增益 延迟时间 抽运光能量     

Time-resolved spatial structure of TEA CO2 laser pulses

The evolution of the intensity profile of TEA CO₂ laser pulses along the pulse length is investigated both analytically and experimentally. A simple scalar model of the pulse amplitude is introduced, which gives the loaded-cavity modes as a linear combination of bidimensional Hermite-Gauss functions. According to this model a number of equations are derived which link the time-varying spatial structure of the pulse to its time-resolved second-order intensity moments, namely, the beam width and the M² parameter.     

Generation regimes of a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity

The generation regimes in a pulsed Nd:YAG laser with transverse LED pumping and multiloop self-pumped phase-conjugate cavity on the gain gratings are studied. The differential efficiency of laser is 27% in the free-running regime at a pulse energy of up to 1 J and quality parameter M ² of no greater than 1.5. The pulse energy under passive Q-switching is no less than 60% of the pulse energy in the free-running regime at the same beam quality. The generation of the narrow-band radiation is demonstrated. A generation band of no greater than 1.2 GHz corresponds to the primary single-frequency high-power laser pulse in the free-running mode under conditions for self-Q-switching on the gain gratings. When additional elements (F ₂ ^? :LiF and Cr⁴⁺:YAG crystals) are introduced in the optical scheme of the phase-conjugate cavity, similar narrowband single-mode generation is observed in the passive Q-switching regime as a pulse train or monopulse. The laser pulse power is up to 2 MW at a pulse duration of 20 ns.     

Theoretical investigations of the processes of selective laser interaction with melanin granules in pigmented tissues for laser applications in medicine

Theoretical investigations and the results of computer modeling of optical, thermophysical, thermochemical, and hydrodynamical processes during selective laser interaction with melanoprotein granules (melanosomes) in heterogeneous pigmented tissues (retinal pigment epithelium) are reviewed in this paper. Physico-mathematical models and system of equations are formulated which describe interaction processes for “short” laser pulses of duration t _p < 10^?6 s and for “long” pulses of duration t _p > 10^?6 s. The results of numerical simulation of the processes give the space-time distributions of temperature and degrees of thermodenaturation of the protein molecules inside and around melanosomes and in the volume of irradiated tissue. Energy absorption, heat transfer, and thermochemical processes occurring during the interaction of laser pulses with pigmented spherical and spheroidal granules in heterogeneous tissues are theoretically investigated. The possibility for selective interaction of short laser pulses with pigmented granules, which results in the formation of denaturation microregions inside and near the pigmented granules (granular thermodenaturation) without origination of a continuous macroscopic thermodenaturation lesion in tissue, is discussed. An analytical model of heating of a single spherical and spheroidal granule by a laser pulse is presented. Simple equations for the time dependences of particle temperature are obtained. Vapor generation under the action of a laser pulse on pigmented spherical granules in a water-containing tissue and the formation and dynamics of a vapor blanket are theoretically investigated. The values of pulse energy which give rise to granular and ophthalmoscopically visible thermodenaturation lesions on the retina and to vapor generation are discussed, as well as laser-induced breakdown on granules in pigmented tissues, on the basis of experimental results and numerical and analytical calculations. The comparison and agreement of the numerical results with the experimental data validate the models and techniques developed. The presented results are of essential interest for laser applications in ophthalmology and can be used to investigate laser interaction with heterogeneous tissues in dermatology and various fields of laser medicine.