Parametric generation of short acoustic pulses

A method of parametric generation of short sound pulses with controllable duration upon excitation of steady-state vibrations of the acoustic field in the resonator under conditions of modulation of the amplitude of high-frequency variations in the speed of sound in a resonator material was proposed and theoretically studied.     

PIC simulation of effect of energy-dependent foil transparency in an axially-extracted vircator

We have performed two-dimensional, relativistic, electromagnetic, particle-in-cell simulations for an axially extracted vircator. With a copper foil anode, the output frequency of the vircator is found to be strongly dependent upon the variation of foil transparency with electron energy. Using an average transparency for all electron energies yields results that are markedly different from those obtained using the actual variation. However, the output power shows only mild sensitivity. Using the full energy-dependent form of the transparency, we get fairly good agreement with published experimental results. Increasing the energy resolution for the calculation of foil transparency increases the accuracy of calculation of the dominant frequency, although the frequency tends to saturate beyond a certain resolution. However, an increase in the resolution must necessarily be accompanied by an increase in the number of simulation particles, in order to limit statistical fluctuations to an acceptable level. A physical explanation has also been provided for these trends.     

Influence of spectral phase on cross-polarized wave generation with short femtosecond pulses

In this paper we present the first comprehensive study of the role of spectral phase on cross-polarized wave (XPW) generation using sub-30 femtosecond (fs) laser pulses. XPW generation improves the temporal contrast and shortens the pulse duration of fs chirped pulse amplification (CPA) lasers. For Ti:Sa lasers, compression below 30?fs is non-trivial and therefore never perfect. We therefore systematically analyze the effect of an arbitrary input spectral phase on the output spectrum and efficiency of the XPW process, both theoretically and experimentally. We derive the maximum acceptable value of residual phase for a given initial pulse duration in order to efficiently drive the XPW process for pulse shortening and contrast improvement.     

Numerical simulation of the generation of gain switched laser pulses

Gain switched laser pulses with FWHM of 32 ps having an optical peak power exceeding 400 W are generated using low cost single heterostructure (SH) Fabry–Perot laser diodes. To examine and to optimize this effect in detail, a thorough physical device simulation is done. Maxwell's equations together with a simplified version of the Boltzmann transport equation (BTE) and various parameters like carrier recombination and material gain, form a coupled set of nonlinear, inhomogeneous partial differential equations, that are discretized using the finite difference scheme and solved selfconsistently.     

Self-similar counter-propagating beams of electromagnetic waves in nonlinear cubic media

We study the structure of the self-similar beams of counter-propagating electromagnetic waves in nonlinear cubic media. Eigenfrequencies of two-mirror quasi-optical resonators, whose eigen-modes are self-similar beams of counter-propagating waves, are found.     

Numerical simulation of process dynamics during laser beam drilling with short pulses

In the last years, laser beam drilling became increasingly important for many technical applications as it allows the contactless production of high quality drill holes. So far, mainly short laser pulses are of industrial relevance, as they offer a good compromise between precision and efficiency and combine high ablation efficiency with low thermal damage of the workpiece. Laser beam drilling in this pulse length range is still a highly thermal process. There are two ablation mechanisms: evaporation and melt expulsion. In order to achieve high quality processing results, a basic process understanding is absolutely necessary. Yet, process observations in laser beam drilling suffer from both the short time scales and the restricted accessibility of the interaction zone. Numerical simulations offer the possibility to acquire additional knowledge of the process as they allow a direct look into the drill hole during the ablation process. In this contribution, a numerical finite volume multi-phase simulation model for laser beam drilling with short laser pulses shall be presented. The model is applied for a basic study of the ablation process with μs and ns laser pulses. The obtained results show good qualitative correspondence with experimental data.     

Efficient generation of isolated attosecond pulses with high beam quality by two-color Bessel-Gauss beams

The generation of isolated attosecond pulses with high efficiency and high beam quality is essential for attosecond spectroscopy. We numerically investigate the supercontinuum generation in a neutral rare-gas medium driven by a two-color Bessel-Gauss beam. The results show that an efficient smooth supercontinuum in the plateau is obtained after propagation and the spatial profile of the generated attosecond pulse is Gaussian-like with the divergence angle of 0.1° in the far-field. This bright source with high beam quality is beneficial for detecting and controlling the microscopic processes on attosecond time scale.     

The dynamics of transients in an overdamped Josephson junction biased by trains of short current pulses with controllable parameters

The paper discusses the development of the method for studying the dynamics of an overdamped Josephson junction biased by a periodic current with controlled parameters, suggested earlier by the authors. The results obtained allow several known experimental observations to be explained. Analytic expressions for the rate of transient process damping and energy expenditures necessary for switching a Josephson junction from one quantum state to another are derived. Problems of interactions between the bias current and supercurrent and the influence of cos?-type terms on the rates of transient process damping and energy conversion are discussed. The results for junction biasing by short (uni-or bipolar) δ function-shaped pulses are obtained in the form of exact analytic expressions. Diagrams illustrating the dependence of the Shapiro step width on the shape of biasing pulses are given.     

Collisions of counter-propagating pulses in coupled complex cubic-quintic Ginzburg–Landau equations

We discuss the results of the interaction of counter-propagating pulses for two coupled complex cubic-quintic Ginzburg–Landau equations as they arise near the onset of a weakly inverted Hopf bifurcation. As a result of the interaction of the pulses we find in 1D for periodic boundary conditions (corresponding to an annular geometry) many different possible outcomes. These are summarized in two phase diagrams using the approach velocity, v, and the real part of the cubic cross-coupling, c_r, of the counter-propagating waves as variables while keeping all other parameters fixed. The novel phase diagram in the limit v ↦0, c_r ↦0 turns out to be particularly rich and includes bound pairs of 2 π holes as well as zigzag bound pairs of pulses.     

Self-focusing and the initial stages of plasma generation by short laser pulses

When a short high power laser pulse interacts with a transparent solid target in vacuum, radiation in the rising edge of the pulse can self-focus in the solid and cause “damage tracks”. The self-focusing stage of the laser/solid interaction has been studied, a method has been found to suppress the “damage tracks”, and information about the penetration of laser radiation through the critical layer of plasma, in the early stages of the interaction, has been obtained.     

The thermal properties of a single-heterostructure laser diode supplied with short current pulses

The analysis of the heat spreading in the single-heterostructure GaAs-Ga₁-x Al _x As laser diode supplied with short current pulses (in the case, however, when the adiabatic approximation is no longer valid) at room temperature is presented in this paper. Relations are derived, describing the time-dependent temperature rise within the volume of the laser diode. The calculations are carried out for a typical SH laser diode. It turns out that in the duration of the short current pulses (t _I=200 ns,j=1.5 × 10⁴A cm^–2) the increase in junction temperature of the typical SH laser diode amounts to about 6.1 K. This increase leads to an increase of about 9% in the threshold current, to a decrease of about 18% in the laser radiation intensity, and to a shift of the spontaneous radiation band and of the stimulated radiation modes of about 1.9 nm and 0.22 nm, respectively, during each current pulse.     

Strong-interaction approximation in the theory of second harmonic generation of intense short laser pulses

The method of strong interaction of nonlinear waves has been developed to analyse the second harmonic generation of intense laser radiation in the transient regime. Analytic solution for frequency conversion efficiency – taking into account the pump radiation depletion, influence of phase mismatch, dispersion of group velocities and higher nonlinearities – has been first obtained. A comparison of theoretically derived results and known experimental data has been conducted, and a general agreement of these results has been obtained. The optimal conditions of the second harmonic generation of neodymium laser radiation into femtosecond pulse duration range have been determined.     

All-optical processing of RZ-DPSK signals using counter-propagating pulses in a saturated SOA

We demonstrate three novel all-optical signal processing methods for RZ-DPSK signals using a saturated SOA. The two logically opposite pulse streams from a demodulated noisy RZ-DPSK signal counter-propagate in the SOA. This configuration decreases the fluctuations in the demodulated marks while preventing the spaces from getting amplified. Photonic balancing provides functionality analogous to electrical balanced detection by reducing the required OSNR, and saturated asymmetric filtering removes the in-band ASE noise before detection. These methods are demonstrated experimentally by observing eye diagrams and BER measurements at 10.7 Gb/s and 22.3 Gb/s.     

Efficient generation of short terahertz pulses via stimulated Raman adiabatic passage

We have analyzed the efficiency of coherent scattering of infrared radiation in molecular gases for the production of intense, short terahertz (THz) pulses by using stimulated Raman adiabatic passage for the preparation of coherence. We show that coherently driven molecular media potentially yield strong, controllable, short pulses of THz radiation. The pulses have energies ranging from several nanojoules to microjoules and time durations from several femtoseconds to nanoseconds at room temperature.     

Molding loose metal particles into briquettes with the use of short pulses of high density current

Theoretical estimates and experimental investigations were carried out on the process of molding a mixture of metal particles and a dielectric (air) into briquettes with the use of short pulses of high density current. Four groups of specimens were studied consisting of metal fragments and the dielectric in various proportions. In the experiments and the current and voltage oscillograms, the electrical resistance and the temperature of the specimens before and after passing the current were measured and their ultimate strength was determined. It has been found that the processes affecting the distribution of the current over the specimen cross section and the specimen inductance during the current pulse play an important role in molding briquettes of sufficient strength for transportation and processing.     

Nonlinear conductivity in CaRuO3 thin films measured by short current pulses

We investigate the effects of carbon (C) on hydrogen (H) solubility in copper (Cu) using a first-principles method. We show C can increase the solution energy of H in the bulk Cu originated from the charge density redistribution, which leads to a weak repulsion between H and C in Cu. On the contrary, we demonstrate the C-vacancy (C-V) complex can serve as a trapping centre of H, and one C-V complex can hold up to six H atoms. Moreover, it is found that C can effectively decrease the solution energy of a single H in the vacancy, 0.68 eV lower than that of H in the C-free vacancy, changing the solution process of H in the vacancy from endothermic to exothermic. This can be attributed to the strong bonding interaction between H and C in the vacancy. Based on analyzing the role of C in different metals, we propose that the effects of C on the H solubility in the vacancy mainly depend on the difference between the H-C interaction and the C-metal atom interaction. These indicate that C plays a key role in H trapping behavior in Cu.     

Processing of metals by double pulses with short laser pulses

Experimental results related to the influence of time delayed pulses for ablation efficiency with short multi pulses (pulse duration of 5 ps) are reported. A significant improvement of the micro structuring quality at relatively high fluence regime in metals is obtained. Less removed or recast matter is observed and the processed surface appears to be smoother with better roughness. Ablation depths and burr heights are compared for single pulses and double pulses in steel, Al and Cu as a function of scans number. Best results are obtained for weak time delays, typically less than 1 ps. PACS 79.20.Ds; 42.62.Cf; 81.65.Cf