Wavelength tunable ultra-short pulses based on a flat broadband spectrum generated in a nonlinear ytterbium-doped fiber amplifier

We present a nonlinear ytterbium-doped fiber amplifier based on enhanced nonlinear effects that can produce a flat broadband spectrum ranging from 1050–1225 nm with a maximum average output power of 7.8 W at 14 W pump power.Its repetition rate is 89 MHz. Using a pair of gratings and two knife edges as a filter, wavelength tunable picosecond pulses of tens to hundreds of milliwatts can be obtained in the broadband spectrum range. The output power, pulse width, and spectrum（center wavelength and linewidth） are adjusted by tuning the distance of the grating pair and/or the knife edges.Fixing the distance between the two gratings at 15 mm and keeping the output spectrum linewidth at approximately 20 nm,the shortest pulse width obtained is less than 1 ps centered at 1080 nm. The longest wavelength of the short pulses is around1200 nm, and its output power and pulse width are 40 m W and 5.79 ps, respectively. The generation of a flat broadband spectrum is also discussed in this paper.

Generation of time-dependent ultra-short optical pulse trains in the presence of self-steepening effect

Starting from the extended nonlinear Schrodinger equation in which the self-steepening effect is included, the evolution and the splitting processes of continuous optical wave whose amplitude is perturbed into time related ultra-short optical pulse trains in an optical fibre are numerically simulated by adopting the split-step Fourier algorithm. The results show that the self-steepening effect can cause the characteristic of the pulse trains to vary with time, which is different from the self-steepening-free case where the generated pulse trains consist of single pulses which are identical in width, intensity, and interval, namely when pulses move a certain distance, they turn into the pulse trains within a certain time range. Moreover, each single pulse may split into several sub-pulses. And as time goes on, the number of the sub-pulses will decrease gradually and the pulse width and the pulse intensity will change too. With the increase of the self-steepening parameter, the distance needed to generate time-dependent pulse trains will shorten. In addition, for a large self-steepening parameter and at the distance where more sub-pulses appear, the corresponding frequency spectra of pulse trains are also wider.

Study on Ultra-Short Laser Pulse Ablation of Metals by Molecular Dynamics Simulation

The dynamical progresses involved in ultra-short laser pulse ablation of face-centered cubic metals under stress confinement condition are described completely using molecular dynamics method. The laser beam absorption and thermal energy turning into kinetics energy of. atoms are taken into account to give a detailed picture of laser metal interaction. Superheating phenomenon is observed, and the phase change from solid to liquid is characterized by a destroyed atom configuration and a decreased number density. The steep velocity gradients are found in the systems of Cu and Ni after pulse in consequence of located heating and exponential decrease of fluences following the Lambert-Beer expression. The shock wave velocities are predicted to be about 5 000 m/s in Cu and 7 200 m/s in Ni. The higher ablation rates are obtained from simulations compared with experimental data as a result of a well-defined crystalline surface irradiated by a single pulse. Simulation results show that the main mechanisms of ablation are evaporation and thermoelastic stress due to located heating.

Techniques and physical properties of 10MHz short pulse focused ultrasonic transducer

A focused ultrasonic transducer used for biomedical purposes with a fundamental frequency of 10MHz and a pulse width of one and a half periods is described in this paper. Its physical properties are given including (1) focused acoustic field recorded by an optical means, (2) electric waveform for triggering the transducer and the corresponding waveform of the wave received by another transducer, and (3) result of tests on a sample object.

Effect of microstructure on short pulse duration shock initiation of TATB and initial response mechanism

Micro-TATB particles with different sizes and 3D nanoporous TATB architectures with different specific surface areas were prepared through recrystallization to study short pulse duration shock initiation properties by electric gun technology.For micro-TATB,the initiation threshold significantly decreases with TATB average size ranging from 79.7μm to 0.5μm.For 3D nanoporous TATB architecture,the initiation threshold decreases and then increases with specific surface areas increased from 9.6 m^2/g to36.2 m^2/g.The lowest initiation thresholds are obtained for the micro-TATB with average sizes of 1.3μm and 0.5μm,and 3D nanoporous TATB architecture with specific surface area of 22.4 m^2/g.The shock initiation thresholds of micro-TATB and 3D nanoporous TATB architectures show significantly decreases with the porosity increased.The decomposition reaction and thermal conductivity properties were further investigated to understand the initial response mechanism.High porosity provides more collapse sites to generate high temperature for formation of hot spots.The low thermal conductivity and decomposition temperature could enhance the formation and ignition of the hot spots,and initial decomposition reaction of TATB.The effect of the decomposition temperature is higher than that of the thermal conductivity on the shock initiation properties.The enhanced decomposition reaction could pro mote energy release and transfer process from the ignition to the combustio n.This work offe rs a new insight to understand the effects of microstructure on the shock initiation properties and the initial response mechanism of TATB.

New Soliton Solutions to the Initial Value Problem for the Two-Component Short Pulse Equation

We formulate a matrix Riemann-Hilbert problem to the initial value problem for the two-component system proposed by Matsuno. By solving the associated Riemann-Hilbert problem, we can get the soliton solutions of the two-component system. One and two soliton solutions are investigated in detail.

Grain refining in weld metal using short-pulsed laser ablation during CW laser welding of 2024-T3 aluminum alloy

The 2024 aluminum alloy is used extensively in the aircraft and aerospace industries because of its excellent mechanical properties.However,the weldability of 2024 aluminum alloy is generally low because it contains a high number of solutes,such as copper(Cu),magnesium(Mg),and manganese(Mn),causing solidification cracking.If high speed welding of 2024 aluminum alloy without the use of filler is achieved,the applicability of 2024 aluminum alloys will expand.Grain refining is one of the methods used to prevent solidification cracking in weld metal,although it has never been achieved for high-speed laser welding of 2024 aluminum alloy without filler.Here,we propose a short-pulsed,laser-induced,grain-refining method during continuous wave laser welding without filler.Bead-on-plate welding was performed on a 2024-T3 aluminum alloy at a welding speed of 1 m min−1 with a single mode fiber laser at a wavelength of 1070 nm and power of 1 kW.Areas in and around the molten pool were irradiated with nanosecond laser pulses at a wavelength of 1064 nm,pulse width of 10 ns,and pulse energy of 430 mJ.The grain-refinement effect was confirmed when laser pulses were irradiated on the molten pool.The grain-refinement region was formed in a semicircular shape along the solid–liquid interface.Results of the vertical section indicate that the grain-refinement region reached a depth of 1 mm along the solid–liquid interface.The Vickers hardness test results demonstrated that the hardness increased as a result of grain refinement and that the progress of solidification cracking was suppressed in the grain refinement region.

Skin Effect of Reversely Switched Dynistor in Short Pulse Discharge Application

The skin effect in the reversely switched dynistor （RSD） devices is investigated in this paper. Based on the plasma bipolar drift model of the RSD, the current density distributions on the chip are simulated with considering the skin effect. The results indicate that the current density on the border can be several hundred to a thousand A/cm2 higher than that in the center of the chip. The skin effect becomes more prominent as the voltage increases and the inductance decreases in the main circuit. The phenomenon that most of a certain group of chips break over on the border has proved the existence of the skin effect.

Comparative study of travelling-wave and numerical solutions for the coupled short pulse (CSP) equation

The Lie symmetry analysis is performed for the coupled short plus （CSP） equation. We derive the infinitesimals that admit the classical symmetry group. Five types arise depending on the nature of the Lie symmetry generator. In all types, we find reductions in terms of system of ordinary differential equations, and exact solutions of the CSP equation are derived, which are compared with numerical solutions using the classical fourth-order Runge-Kutta scheme.

Effect of Lorentz local field correction on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline (PNA) molecules

This paper investigates the effect of Lorentz local field correction （LFC） on the propagation of ultrashort laser pulses in a para-nitroaniline molecular medium under resonant and nonresonant conditions by solving numerically the full-wave Maxwell-Bloch equations beyond slowly-varying envelope approximation and rotating-wave approximation. The effect of the LFC is considerably obvious when pulses with large areas propagate in the dense molecular medium. In the case of resonance, the group velocity of the sub-pulses split from the incident pulse along propagation is severely decreased by the LFC, especially for the latest sub-pulse. However, in the case of nonresonance, the influence of the LFC on the temporal evolution of the pulse is less obvious and lacks homogeneity with an increase in incident pulse area, propagation distance and molecular density.

Exact Solutions to Short Pulse Equation

In this paper, dependent and independent variable transformations are introduced to solve the short pulse equation. It is shown that different kinds of solutions can be obtained to the short pulse equation.

Reciprocal transformations of the space-time shifted nonlocal short pulse equations

Reciprocal transformations of the space-time shifted nonlocal short pulse equations are elaborated.Covariance of dependent and independent variables involved in the reciprocal transformations is investigated.Exact solutions of the space-time shifted nonlocal short pulse equations are given in terms of double Wronskians.Realness of independent variables involved in the reciprocal transformations is verified.Dynamics of some obtained solutions are illustrated.

Optimization of Short-Pulse GPR Transmit Antenna

By analyzing the current distribution of Bow-Tie antenna used in short-pulse ground penetrating radar, the methods of antenna load and driving are presented in this paper to reduce strength of reflective wave both at antenna end and excitation point. The numerical simulation results show the strength of reflective wave is smaller than ?55 dB comparing with the driving wave when the methods are adopted. Key words short-pulse GPR - resistive loading - finite difference time domain method CLC number TN 820 Foundation item: Supported by the National Natural Science Foundation of China (49984001)Biography: LI Tai-quan (1961-), male, Ph. D candidate, research direction: antenna model and design

An experimental study of temperature and moisture content of wet porous materials under short-pulsed laser heating

The measurements of temperature and moisture content of a wet porous material were accomplished on the micro-seconds scale. The temperature wave was observed when the wet porous material was heated by short-pulsed laser with high power. It firstly revealed that the moisture content of wet porous material rapidly rises twice in one laser irradiation. The influences of laser parameters, the thickness and initial moisture content of the wet porous material on its temperature and moisture content were investigated.

Development of a high current short pulse electron gun

Dielectric wall accelerator(DWA), towards high gradient acceleration field(30 MeV/m–100 MeV/m), is under development at Institute of Modern Physics. A prototype was designed and constructed to prove the principle. This needs a short pulse high current electron source to match the acceleration field generated by the Blumlein-type pulse forming lines(PFLs). In this paper, we report the design and test of a new type short pulse high current electron gun based on principle of vacuum arc discharge. Electron beams of 100 mA with pulse width of 10 ns were obtained.

Central Opioid Peptide-containing Neurons Mediates Therapeutic Effect of Short-pulse Gastric Electrical Stimulation on Dyspepsia-like Symptoms in Dogs

This study investigated whether the curative effect of short-pulse gastric electrical stimulation （GES） on the vasopressin-induced dyspeptic symptoms was mediated by central opioid peptide-producing neurons. Five female beagle dogs implanted with 1 pair of electrodes in gastric serosa were used in a two-experiment study. In experiment one, the brain was scanned by positron emission tomography in 3 dogs with and without short-pulse GES, and the radioactivity in nuclei of solitary tract （NST） and hypothalamus was detected. Experiment two was composed of 4 sessions. In session one, the dogs were injected with vasopressin in the absence of short-pulse GES. With session two, the short-pulse GES was simultaneously given via the electrodes with the injection of vasopressin. In sessions three and four, naloxone and naloxone methiodide was administered respectively in the presence of short-pulse GES. Motion sickness-like symptoms were scored and compared among the different sessions. The results showed that the short-pulse GES significantly increased the radioactivity in NST and hypothalamic nuclei （P〈0.05, vs control）. The short-pulse GES could ameliorate the vasopressin-induced motion sickness-like symptoms in dogs. Naloxone, but not naloxone methiodide could attenuate the curative effects of short-pulse GES. It is concluded that NST and hypothalamic nuclei may participate in the mediation of the curative effects of short-pulse GES on dyspepsia-like symptoms. Central opioid peptide-containing neurons presumably mediate the therapeutic effect on dyspeptic symptoms of short-pulse GES.

Short-pulse high-power microwave breakdown at high pressures

The fluid model is proposed to investigate the gas breakdown driven by a short-pulse（such as a Gaussian pulse） highpower microwave at high pressures.However,the fluid model requires specification of the electron energy distribution function（EEDF）;the common assumption of a Maxwellian EEDF can result in the inaccurate breakdown prediction when the electrons are not in equilibrium.We confirm that the influence of the incident pulse shape on the EEDF is tiny at high pressures by using the particle-in-cell Monte Carlo collision（PIC-MCC） model.As a result,the EEDF for a rectangular microwave pulse directly derived from the Boltzmann equation solver Bolsig＋ is introduced into the fluid model for predicting the breakdown threshold of the non-rectangular pulse over a wide range of pressures,and the obtained results are very well matched with those of the PIC-MCC simulations.The time evolution of a non-rectangular pulse breakdown in gas,obtained by the fluid model with the EEDF from Bolsig＋,is presented and analyzed at different pressures.In addition,the effect of the incident pulse shape on the gas breakdown is discussed.

Dynamics of cooperative emissions in a cascade three-level molecular system driven by an ultrashort laser pulse

This paper investigates the dynamics of cooperative emissions in a cascade three-level system driven by an ultra, short laser pulse by solving numerically the full-wave Maxwell-Bloch equations. The 4, 4＇-bis（dimethylamino） stilbene molecule is used as the model molecule because of its strong two-photon absorption property. The two-colour cooperative emissions are studied as functions of molecular number density and dephasing rate of the dipole coherence. The propagation effects on the evolution of the cooperative radiations are also taken into account. The cooperative radiations are enhanced for large number density of the molecule, while the fast dephasing of the dipole coherence reduces the intensity of the cooperative radiations and delays the emission times or even inhibits the formation of the emissions. The delay time of the radiation decreases with the increase of the molecular number density and the propagation distance.

Relative energy zero ratio-based approach for identifying pulse-like ground motions

Pulse-like ground motions are capable of inflicting significant damage to structures. Efficient classification of pulse-like ground motion is of great importance when performing the seismic assessment in near-fault regions. In this study, a new method for identifying the velocity pulses is proposed, based on different trends of two parameters: the short-time energy and the short-time zero crossing rate of a ground motion record. A new pulse indicator, the relative energy zero ratio(REZR), is defined to qualitatively identify pulse-like features. The threshold for pulse-like ground motions is derived and compared with two other identification methods through statistical analysis. The proposed procedure not only shows good accuracy and efficiency when identifying pulse-like ground motions but also exhibits good performance for classifying records with high-frequency noise and discontinuous pulses. The REZR method does not require a waveform formula to express and fit the potential velocity pulses;it is a purely signal-based classification method. Finally, the proposed procedure is used to evaluate the contribution of pulse-like motions to the total input energy of a seismic record, which dramatically increases the seismic damage potential.

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation （HHG） of neon gas with 5-fs and 25-fs driving laser pulses. It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHC spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs; whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs. The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse. The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.