Self-compression Of Laser Pulses Research Articles

Combined Effect of Spatio-Temporal Dynamics of Laser Pulse on Electron Acceleration in Relativistic Plasma

This article presents the spatial and temporal dynamics of high-power Gaussian laser pulse in relativistic plasma. At higher intensities, the self-focusing and self-compression of laser pulse take place due to the variation in the mass of plasma electrons moving with speed comparable to the speed of light. The two non-linear coupled differential equations for spatial and temporal pulsewidth parameters have been obtained using the moment theory approach. These equations have been solved numerically using the Runga-Kutta method. The variation in the spatial and temporal pulsewidth parameters with the distance of propagation is then used to calculate the energy gained by the electrons, which are trapped by the excited electron plasma wave during laser-plasma interaction. It is observed that spatio-temporal dynamics play an important role in electron acceleration in relativistic plasma.

Compression of laser pulses due to Raman amplification of plasma noises

The method of the laser pulse self-compression in the process of noise-seeded Raman scattering in a plasma created by a counterpropagating ionized pulse is considered. The sharpness of the ionization front ensures the formation of an output pulse close to the $\ensuremath{\pi}$-pulse solution with a sharp leading edge. The dependence of the gain increment on the transverse wave number leads to the formation of a smooth phase front of the output pulse. The energy efficiency in the output pulse reaches 28% of the total pump energy. A study of the influence of quasistatic inhomogeneities showed only a slight slowdown in the process, even in a plasma with 8% density modulation.

Self-compression of laser pulses in a discrete medium

Using the discrete nonlinear Schrödinger equation, we study the laser radiation self-action dynamics in a nanostructured waveguide system. It is shown that for a laser pulse energy exceeding the critical value, the nonlinear evolution of the wave field markedly differs from the corresponding process in a continuous medium. While the pulse propagates in a discrete medium, its length decreases to values comparable to the scale of the structure and the velocity decreases to zero. The system parameters are determined at which the wave field after the laser pulse termination begins to propagate in the opposite direction in a compressed form. The process of slowing down and stopping the pulse is accompanied by strong radiation losses, so that about a third of the energy of the initial field distribution remains in the final compressed state.

Stability of out-of-phase solitons and laser pulse self-compression in active multicore fibers

The out-of-phase soliton distribution of the wave field was found for a multicore fiber (MCF) from an even number of cores located in a ring. Its stability is proved both with respect to small wave field perturbations, including azimuthal ones and to small deformations of the MCF structure. As an example of using this soliton distribution, the problem of laser pulse compression in an active MCF is studied. The optimal fiber parameters, the minimum duration of the output pulse, and the compression length have been found, which are in good agreement with the results of numerical simulation. In order to achieve high energies in the output laser pulse, the requirements for MCF deformations are determined.

Multicore-fiber solitons and laser-pulse self-compression at light-bullet excitation in the central core of multicore fibers

The propagation of laser pulses in multi-core fibers (MCF) made of a central core and an even number of cores located in a ring around it is studied. Approximate quasi-soliton homogeneous solutions of the wave field in the considered MCF are found. The stability of the in-phase soliton distribution is shown analytically and numerically. At low energies, its wave field is distributed over all MCF cores and has a duration, which exceeds the duration of the NSE soliton with the same energy by many (five-six) times. On the contrary, almost all of the radiation at high energies is concentrated in the central core with a duration similar to the NSE soliton. The transition between the two types of distributions is very sharp and occurs at a critical energy, which is weakly dependent on the number of cores and on the coupling coefficient with the central core. The self-compression mechanism of laser pulses was proposed. It consists in injecting such MCF with a wave packet being similar to the found soliton and having an energy larger than the critical value. It is shown that the compression ratio weakly depends on the energy and the number of cores and is approximately equal to 6 times with an energy efficiency of almost 100\%. The use of longer laser pulses allows one to increase the compression ratio up to 30-40 times with an energy efficiency of more than 50\%. The obtained analytical estimates of the compression ratio and its efficiency are in good agreement with the results of numerical simulation.

Laser pulse compression up to few-cycle durations in multicore fiber.

A mechanism for self-compression of laser pulses in a multicore fiber on the basis of nonlinear combining of radiation from many cores, surrounding the central core in a ring and its trapping into the central core in a chirp-free and pedestal-free soliton-like pulse, is proposed. It is shown that the compression ratio is weakly dependent on the energy and the number of cores in the fiber and approximately equal to 6 with an almost 100% energy efficiency. The compression ratio can be increased to 40 with an efficiency of more than 50% by using additional compression of longer pulses with approximately the same energy. The possibility of a 38-fold compression of a laser pulse to a duration of 15fs with a pulse energy of 4nJ in the fiber with realistic parameters was demonstrated numerically.

Self-compression of laser pulses in an active system of weakly-coupled light guides

We propose a method of laser pulse self-compression in an array of equidistant, identical, weakly-coupled active light guides. It is shown that laser pulse self-focusing in such one-dimensional arrays decreases the compression length significantly (by more than 15 times) in comparison with the case of laser pulse self-compression in a single active light guide. Analytical estimates for the minimal pulse duration and the length of self-compression of laser pulses in a system of weakly-coupled active light guides are derived, and they agree well with the numerical simulation results.

Laser pulse self-compression in an active fibre with a finite gain bandwidth under conditions of a nonstationary nonlinear response

We study the influence of a nonstationary nonlinear response of a medium on self-compression of soliton-like laser pulses in active fibres with a finite gain bandwidth. Based on the variational approach, we qualitatively analyse the self-action of the wave packet in the system under consideration in order to classify the main evolution regimes and to determine the minimum achievable laser pulse duration during self-compression. The existence of stable soliton-type structures is shown in the framework of the parabolic approximation of the gain profile (in the approximation of the Gnizburg – Landau equation). An analysis of the self-action of laser pulses in the framework of the nonlinear Schrödinger equation with a sign-constant gain profile demonstrate a qualitative change in the dynamics of the wave field in the case of a nonsta­tionary nonlinear response that shifts the laser pulse spectrum from the amplification region and stops the pulse compression. Expressions for a minimum duration of a soliton-like laser pulse are obtained as a function of the problem parameters, which are in good agreement with the results of numerical simulation.

Self-compression of soliton-like laser pulses in the process of self-focusing

We study the possibility of efficient self-compression of femtosecond laser pulses in nonlinear media with anomalous dispersion of group velocity during the self-focusing of wave packets with a power several times greater than the critical self-focusing power. The results of qualitative analysis of the evolution of three-dimensional wave packets with the quasi-soliton field distribution are confirmed by the computer simulation. The simulation proves that the considered regime of compression of high-power laser pulses with initial durations of about ten optical cycles is stable relative to filamentation instability, due to the influence of the nonlinear dispersion. We demonstrate the possibility of self-compression of laser pulses at a multi-millijoule energy level and up to one optical cycle with an energy efficiency of more than 50%.

Self-action of Bessel wave packets in a system of coupled light guides and formation of light bullets

The self-action of two-dimensional and three-dimensional Bessel wave packets in a system of coupled light guides is considered using the discrete nonlinear Schrodinger equation. The features of the self-action of such wave fields are related to their initial strong spatial inhomogeneity. The numerical simulation shows that for the field amplitude exceeding a critical value, the development of an instability typical of a medium with the cubic nonlinearity is observed. Various regimes are studied: the self-channeling of a wave beam in one light guide at powers not strongly exceeding a critical value, the formation of the “kaleidoscopic” picture of a wave packet during the propagation of higher-power radiation along a stratified medium, the formation of light bullets during competition between self-focusing and modulation instabilities in the case of three-dimensional wave packets, etc. In the problem of laser pulse shortening, the situation is considered when the wave-field stratification in the transverse direction dominates. This process is accompanied by the self-compression of laser pulses in well enough separated light guides. The efficiency of conversion of the initial Bessel field distribution to two flying parallel light bullets is about 50%.

Extreme self-compression of laser pulses in the self-focusing mode resistant to transverse instability

We show that a self-focusing mode for intense laser pulses comprising less than about ten optical cycles is resistant to the transverse modulational instability. For such pulses, a method of pulse self-compression based on adiabatic decrease of the duration of a solitonlike wave packet and transverse cumulating of energy during the self-focusing in Kerr-like media with anomalous dispersion is proposed. This method can be used for generation of high-quality, high-energy, few-cycle pulses down to a single-cycle duration.

Power dependent filamentation of a femtosecond laser pulse in air by focusing with an axicon

In the present work, femtosecond laser filament generation by focusing the laser pulse with an axicon in air is studied at different input laser powers both experimentally and numerically. It is found that the length of the filament increases almost linearly with the input laser power. Moreover, the laser intensity inside the filament starts to saturate at a power much higher than the critical power of self-focusing for a Gaussian beam. We have also observed the laser pulse self-compression during nonlinear propagation. The shortest pulse duration could be obtained at the center of the effective focal region produced by the axicon.

Optimization of the regime of shortening of relativistically strong laser pulses in the process of excitation of a plasma wake wave

A detailed theoretical investigation is performed for the self-compression of relativistically strong laser pulses under plasma wake wave excitation conditions. An analytical expression is obtained for the characteristic length of laser pulse self-compression in the one-dimensional case. A good agreement between the theoretical and numerical calculation of the dependence of the wave-packet compression length on both amplitude and plasma concentration is demonstrated. The influence of the wave beam self-focusing radiation process on the rate of shortening of the laser pulse duration is considered.

Weakly relativistic and ponderomotive effects on self-focusing and self-compression of laser pulses in near critical plasmas

The spatiotemporal dynamics of high power laser pulses in near critical plasmas are studied taking in to account the effects of relativistic and ponderomotive nonlinearities. First, within one-dimensional analysis, the effects of initial parameters such as laser intensity, plasma density, and plasma electron temperature on the self-compression mechanism are discussed. The results illustrate that the ponderomotive nonlinearity obstructs the relativistic self-compression above a certain intensity value. Moreover, the results indicate the existence of the turning point temperature in which the compression process has its strongest strength. Next, the three-dimensional analysis of laser pulse propagation is investigated by coupling the self-focusing equation with the self-compression one. It is shown that in contrast to the case in which the only relativistic nonlinearity is considered, in the presence of ponderomotive nonlinearity, the self-compression mechanism obstructs the self-focusing and leads to an increase of the laser spot size.

Hose instability of relativistically strong femtosecond laser pulses with few field oscillations in a plasma under the excitation of a plasma wake wave

A detailed theoretical analysis is carried out of the hose instability of relativistically strong laser pulses propagating in a plasma, whose duration is less than the period of a plasma wake wave. An analytic expression is obtained for the displacement of the mass center of a wave pulse, and the effect of this instability on the modification of the spectrum of laser radiation is analyzed for a wide range of initial parameters. It is shown that the development of instability is characterized by a power-law (rather than exponential) time dependence along the propagation path and does not deteriorate the self-compression of laser pulses.

High-energy laser-pulse self-compression in short gas-filled fibers

We examine the spatiotemporal compression of energetic femtosecond laser pulses within short gas-filled fibers. The study is undertaken using an advanced nonlinear pulse propagation model based on a multimode generalized nonlinear Schrodinger equation that has been modified to include plasma effects. Plasma defocusing and linear propagation effects are shown to be the dominant processes within a highly dynamical mechanism that enables 100-fs pulses to be compressed into the few-cycle regime after <50 mm of propagation. Once the mechanism has been introduced, parameter spaces are explored and compressor designs suitable for performing high-field experiments in situ are presented. We finish by showing how these designs may be extended to novel wavelengths and driving pulses delivered by state-of-the-art high-repetition-rate lasers.

Enhanced High Harmonic Generation from Multiply Ionized Argon above 500 eV through Laser Pulse Self-Compression

By combining laser pulse self-compression and high harmonic generation within a single waveguide, we demonstrate high harmonic emission from multiply charged ions for the first time. This approach enhances the laser intensity and counteracts ionization-induced defocusing, extending the cutoff photon energy in argon above 500 eV for the first time, with higher spectral intensity and cutoff energy than He for the same input laser parameters. This Letter demonstrates a pathway for extending high harmonic emission to very high photon energies using large, multiply charged, ions with high ionization potentials.

Ionization self-compression of intense femtosecond pulses propagating through gas-filled dielectric capillaries

A mechanism of the ionization-induced self-compression of femtosecond laser pulses propagating in a gas-filled hollow dielectric capillary has been investigated both experimentally and theoretically. In particular, the double self-compression of a laser pulse from 76 to 40 fs has been experimentally demonstrated. A theoretical model that explains the mechanism of such a self-compression and provides a good agreement with the experimental data has been developed. The model also predicts that a laser pulse shorter than 10 fs can be generated in the optimal regime with an energy efficiency exceeding the efficiency of self-compression on a filament widely discussed at present.

Self-compression and splitting of laser pulse in plasmas

We study the self-compression and splitting of a circularly polarized laser pulse propagating in plasmas with density window from 1/4 critical to slightly below critical density by solving the nonlinear Schrdinger equation numerically. It is demonstrated by the numerical calculation that the effective self-compression of laser pulse can be achieved in even shorter distance by increasing both the background plasma density and intensity of the laser pulse, or decreasing the width of pulse. The full-width at half maximum of the compressed pulse can reach 1/35 of the initial one's or even smaller. It has been found that this kind of self-compression occurs in the process of formation of a high-order soliton when a laser pulse propagates in a plasma, so that we can obtain greats compression ratio than in thin plasmas. We also obtained the splitting of a high-order soliton formed after self-compression of a laser pulse propagating in plasmas from the result of the numerical calculation in this situation. The phenomenon of self-compression and splitting is also observed by using one-dimensional particle-in-cell simulations and the result was consistent with the numerical calculation.

Self-compression of laser pulses in plasma.

We study self-compression of weakly relativistically intense laser pulses in subcritical plasmas using one- (1D) and three-dimensional (3D) direct particle-in-cell (PIC) simulations. The self-compression works in the density window from 1/4 critical to slightly below critical density, where the Raman instability is prohibited. An analytical model is developed to describe the self-compression. The model admits pulsing Gaussian solutions and a long-lived running soliton solution. The 1D PIC results agree well with the analytical model, and compressions by an order of magnitude are observed. In the 3D geometry, the longitudinal self-compression competes with the transverse self-focusing/filamentation. To damp the filamentation we use a periodic plasma-vacuum structure. The 3D PIC simulations suggest that a 30 fs long laser pulse is efficiently compressed to 5 fs.