On the nonlinear propagation of few-cycle pulses in a uniaxial crystal

The soliton-like dynamics of few-cycle optical pulses propagating in a uniaxial crystal at an arbitrary angle to the optic axis is analytically studied. Solutions describing the one-dimensional dynamics of pulses representing coupled states of the ordinary and extraordinary field components are analyzed. The ordinary component can contain an arbitrary number (down to one period) of optical oscillations, whereas the extraordinary component is a unipolar video pulse. The effect of small-scale transverse perturbations on the propagation of such pulses is considered. Regions of values of parameters of the medium and the pulse in which the propagation is stable are found. The spectrum of the ordinary component is shown to substantially depend on the sign of the birefringence of the crystal as well as on the angle at which the pulse propagates with respect to the optic axis.     

Propagation of vector solitons in a quasi-resonant medium with stark deformation of quantum states

The nonlinear dynamics of a vector two-component optical pulse propagating in quasi-resonance conditions in a medium of nonsymmetric quantum objects is investigated for Stark splitting of quantum energy levels by an external electric field. We consider the case when the ordinary component of the optical pulse induces ?? transitions, while the extraordinary component induces the ?? transition and shifts the frequencies of the allowed transitions due to the dynamic Stark effect. It is found that under Zakharov-Benney resonance conditions, the propagation of the optical pulse is accompanied by generation of an electromagnetic pulse in the terahertz band and is described by the vector generalization of the nonlinear Yajima-Oikawa system. It is shown that this system (as well as its formal generalization with an arbitrary number of optical components) is integrable by the inverse scattering transformation method. The corresponding Darboux transformations are found for obtaining multisoliton solutions. The influence of transverse effects on the propagation of vector solitons is investigated. The conditions under which transverse dynamics leads to self-focusing (defocusing) of solitons are determined.     

Longitudinal—transverse dynamics of two-component pulses in uniaxial crystals

Based on the Ritz—Whitham variational principle, a set of equations is derived that describe the longitudinal-transverse dynamics of parameters of anelectromagnetic pulse whose ordinary component is a quasi-monochromatic pulse and whose extraordinary component is a videopulse. The critical parameters that determine the pulse stability relative to self-focusing are determined.     

Soliton regimes of ultrashort pulse propagation through an array of asymmetric quantum objects

Nonlinear regimes of two-component ultrashort pulse propagation through an array of anisotropic quantum systems are analyzed theoretically under conditions when the slowly varying envelope approximation is inapplicable. It is shown that if the spectral width of the pulse is much larger than the quantum transition frequency, then the dynamics of its ordinary component can be described by a modified sine-Gordon equation that belongs to the class of integrable models but has never been used in physical applications. It is shown that the extraordinary pulse component responsible for the dynamic Stark shift of the transition frequency is coupled to its ordinary component by an algebraic relation. The soliton solutions to the derived equation are classified. Depending on the pulse duration, the solutions have the form of 2π pulses (kinks and antikinks) or steady traveling 0π pulses having no counterparts in isotropic models (neutral kinks). Interactions between the medium and solitons of different types and soliton collisions are analyzed.     

Pulse dynamics in a three-component system: Stability and bifurcations

In this article, we analyze the stability and the associated bifurcations of several types of pulse solutions in a singularly perturbed three-component reaction-diffusion equation that has its origin as a model for gas discharge dynamics. Due to the richness and complexity of the dynamics generated by this model, it has in recent years become a paradigm model for the study of pulse interactions. A mathematical analysis of pulse interactions is based on detailed information on the existence and stability of isolated pulse solutions. The existence of these isolated pulse solutions is established in previous work. Here, the pulse solutions are studied by an Evans function associated to the linearized stability problem. Evans functions for stability problems in singularly perturbed reaction-diffusion models can be decomposed into a fast and a slow component, and their zeroes can be determined explicitly by the NLEP method. In the context of the present model, we have extended the NLEP method so that it can be applied to multi-pulse and multi-front solutions of singularly perturbed reaction-diffusion equations with more than one slow component. The brunt of this article is devoted to the analysis of the stability characteristics and the bifurcations of the pulse solutions. Our methods enable us to obtain explicit, analytical information on the various types of bifurcations, such as saddle-node bifurcations, Hopf bifurcations in which breathing pulse solutions are created, and bifurcations into travelling pulse solutions, which can be both subcritical and supercritical.     

Nonlinear regimes of ultrashort pulse propagation in an array of anisotropic tunneling states

Dynamics of an ultrashort electromagnetic pulse in a system with an array of anisotropic tunneling states spanned by the pulse spectrum are analyzed. A system of nonlinear wave equations is derived for the ordinary and extraordinary components of the pulse propagating at an arbitrary angle to the anisotropy axis. Different regimes of ultrashort pulse propagation parallel and perpendicular to the anisotropy axis are examined. Ultrashort-pulse propagation regimes analogous to self-induced transparency and extraordinary transparency are identified. The properties of rational soliton-like pulses having no quasi-monochromatic analogs are analyzed. A longitudinal electric field component is generated in each regime, whereas off-resonance quasi-monochromatic pulses propagating under similar conditions (parallel and perpendicular to the anisotropy axis) have no longitudinal components. Stability of the solutions obtained and the effect of diffraction on ultrashort pulse dynamics are analyzed. The values of pulse parameters for which defocusing dominates over self-focusing are calculated.     

Dynamics of pulse propagation in phase-shifted fibre Bragg gratings: a numerical investigation

Fibre Bragg gratings (FBGs) are an integral component of optical fibre based communication systems for signal processing. Uniform FBGs and phase-shifted FBGs find major applications as add-drop filters in dense wavelength division multiplexing (DWDM) systems where these are used to filter out channels and to shape the signal spectrum. We investigate numerically the spatiotemporal deformation of pulses as they propagate through the gratings. In particular, the effect on the pulse dynamics of phase shift or defect present in the gratings, often introduced intentionally in a grating to generate a narrow transmission spike within the stopband, is studied in time domain. This study is of significance since the deformation in the pulse in time limits the maximum bit rate achievable, and hence the pulse duration and shape need to be analysed and controlled accurately. A time domain numerical tool based on the Transmission Line Modelling (TLM) method is developed to study the pulse dynamics. The results obtained give useful information about the signal dynamics in gratings with defects, including the signal oscillations trapped in the defect, to be considered in applications such as while designing spectral filters.     

一种间接测量惯性管阻抗的方法

惯性管是脉冲管制冷机的一个重要部件,需要保证其在脉冲管热端提供合适的阻抗幅值和相位。如何准确确定惯性管阻抗一直是个难点。本文利用直线压缩机背腔内压力波动计算体积流率,从而间接得到惯性管入口体积流率,进而得到惯性管阻抗特性,并与DeltaEC计算做了详细对比,为进一步优化脉冲管制冷机打下基础。本文首先介绍了惯性管的热声计算模型及其湍流修正方法,并详细介绍利用压缩机了测量体积流率的方法。实验中重点考察了频率、惯性管入口压力波动、气库体积大小对阻抗的影响,并对实验结果进行了分析。实验结果表明,阻抗幅值及相角跟湍流模型计算结果总体而言相当吻合,但在某些区域还需进一步修正。     

On the effect of transverse perturbations on the propagation of soliton-like two-component pulses in a uniaxial crystal

The combined approach including the method of analytic continuation of dispersion parameters to the complex plane and an averaged variational principle of the Ritz-Whitham type is used for studying the propagation of a soliton-like two-component pulse in a uniaxial crystal in the direction perpendicular to its optic axis. The ordinary component of the soliton is a pulse with an arbitrary number of light oscillations (down to one period), and the extraordinary component is a video pulse. Such a pulse is shown to be stable against small transverse perturbations, including small-scale ones.     

Polarization dynamics of unidirectional optical pulse evolution in a laser amplifier

Polarization dynamics of optical pulses in an isotropic two-level medium is analyzed by solving an integrable system of evolution equations without using the slowly varying envelope approximation. The analysis is focused on the regime of unidirectional pulse generation in an initially inverted medium. Qualitative difference in polarization dynamics is revealed between few-cycle and quasi-monochromatic pulse propagation     

Effect of the field of a nonlinear electromagnetic wave on the shape of a soliton in a semiconductor superlattice

The effect of the electric component of the field of a high-frequency (HF) nonlinear electromagnetic (EM) wave on the propagation of a solitary EM wave (soliton) in a quantum semiconductor superlattice is studied. It is noted that in the collisionless approximation, the solution of the modified sine-Gordon equation corresponding to the amplification of an EM pulse that, with allowance made for interminiband electron transitions, transforms into a dissipative soliton is possible. The effect of electron collisions with irregularities of the crystal lattice on the soliton dynamics under the action of the field of a HF nonlinear wave is considered. The condition for an increase in the traveling time of the solitary wave is found.     

Nonlinear propagation dynamics of an ultrashort pulse in a hollow waveguide

We present a theoretical investigation of the self-focusing dynamics of femtosecond pulses in a hollow waveguide. We show that transverse effects play an important role in these dynamics, even for pulses that are significantly below the critical power for self-focusing in free space, and that excitation of higher-order modes of the waveguide results in the spreading of the pulse in time. Inclusion of self-steepening and space-time focusing in our model is necessary for properly capturing the pulse dynamics.     

Time resolved ADMR applied to the triplet state of the primary donor of bacterial photosynthetic reaction centers

The population and depopulation kinetics of the triplet sublevels of reaction centers ofRhodobacter (Rb.) sphaeroides strains R26 and GA were determined at 9 K by time resolved absorption detected magnetic resonance (ADMR) using the RF pulse method. The comparison of the saturation and pulse methods of time resolved ADMR proofs the latter to be superior. The kinetic behavior of the 2|E| signal was investigated for the first time. It exhibits an unexpected slow decaying component with a rate comparable to thez-level decay. Using a simple kinetic 4-level model for the triplet dynamics we conclude that this slow component as well as the sign of the ADMR signal of the 2|E| transition can be explained by a selective spin lattice relaxation channel connecting the triplet sublevelsx andz.     

Influence of laser frequency chirp on deuteron energy from laser-driven deuterated methane cluster expansion

The simulations of three-dimensional particle dynamics are carried out to investigate the Coulomb explosion dynamics of deuterated methane clusters under the irradiation of an ultrashort intense laser pulse. The final kinetic energy of deuterons produced from the cluster explosion is calculated as a function of the pulse width, the laser intensity and the pulse chirp. It is found that the deuteron energy obtained in an intense laser pulse with negative chirp is higher than that with positive chirp, which agrees qualitatively with the experimental results reported by Fukuda et al. [Y. Fukuda et al., Phys. Rev. A 67, 061201 (2003)].     

On nonlinear propagation of extremely short pulses in optically uniaxial media

Nonlinear wave equations describing the propagation of optical pulses of duration up to a period of electromagnetic oscillations in transparent media with uniaxial optical anisotropy are derived on the basis of a quantum-mechanical model of material response. The electron and electron-vibrational nonlinearities, electron and ion dispersion, and diffraction are taken into account. It is shown that the inclusion of the electron response alone leads to a system of two constitutive equations for the ordinary and extraordinary polarization components. When a pulse propagates across the optical axis, this system is reduced to an inhomogeneous model of the Henon-Heiles type and, hence, generalizes the Lorentz classical electron model. In order to take into account stimulated Raman scattering (SRS) processes, an anisotropic analog of the Bloembergen-Shen quantum-mechanical model taking into account the population dynamics of SRS sublevels is obtained. The generation of an extraordinary wave video pulse with the help of the high-frequency ordinary component in the Zakharov-Benney resonance mode is investigated. Some analytic soliton-like solutions in the form of propagating bound states of ordinary and extraordinary video pulses corresponding to different birefringence modes are considered and their stability to self-focusing is analyzed.     

Dynamics of optical pulses in a dispersive chiral medium

The dynamics of frequency-modulated pulses in an isotropic nonreciprocal chiral medium is studied. It is shown that the discrepancy between the group velocities and carrier frequencies of the circular components of the pulse causes it to split up both along the fiber and in the spectral domain. The strong dependence of the material parameters on the pulse frequency may lead to the asymmetric dynamics of the electric and magnetic fields of the pulse in different frequency intervals. For both components of the wave field, envelopes moving with a supraluminal speed may arise.     

激光脉冲和电子束对撞准确度的测量方法

超强激光脉冲与相对论电子束相互对撞是当前主要的强场量子电动力学（QED）实验手段。如何测量超强激光脉冲和电子束对撞的准确度,进而实现微米精度的准确对撞,是目前限制实验发展的重要因素。利用蒙特卡罗数值模拟方法,系统研究了超强激光脉冲和相对论电子束相互对撞过程,重点关注了电子和辐射光子动力学信息与激光脉冲和电子束对撞偏移量之间的对应关系。研究发现:辐射光子的空间分布信息,可以有效反映出激光脉冲和电子束的对撞偏移量。基于该研究结果,实验中可利用光子空间分布的信息,实现对激光脉冲和电子束对撞准确度的调节,从而有望促进强场QED实验技术的发展。     

On the soliton-like dynamics of ultrashort acoustic pulses in a paramagnetic crystal

A system of nonlinear equations describing the dynamics of the longitudinal and transverse components of an acoustic pulse in a cubic crystal containing paramagnetic impurities has been obtained. On the basis of this system, the dynamics of a two-component ultrashort acoustic pulse propagating under the Zakharov-Benney resonance conditions has been analytically investigated.     

Spatio-temporal dynamics of multicolor spatial Kerr solitons

We present numerical and experimental results showing the rich and complex spatio-temporal dynamics of multicolor spatial vector soliton generated by stimulated Raman scattering in a Kerr planar waveguide. By taking into account their temporal dimension, we found the precise domain of existence of multicolor soliton whose components are mutually guided and spatio-temporally superimposed. In addition, we observe the Raman Stokes component in the trailing edge of the pump pulse and show that it is associated with the slow-light process induced by the sharp resonance of the Raman gain medium.     

Experimental investigation and numerical simulation of the spatio-temporal dynamics of nanosecond pulses in Q-switched Nd:YAG lasers

This paper reports on an experimental investigation and numerical simulation of the spatio-temporal dynamics of the pulse formation in flashlamp-pumped Q-switched Nd:YAG lasers. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD camera. The measurements are performed for two lasers with different cavity configurations. A laser with an optically stable resonator and an internal mode aperture generated pulses with a spatial intensity distribution which is Gaussian at all times during the 10-ns-long pulse. During the pulse evolution the value of the beam-quality factor M²remains below 1.3. In a laser with a positive-branch unstable resonator the laser pulse also starts with a Gaussian intensity distribution, but becomes rapidly non-Gaussian. The corresponding M²values increase from about 1 at the beginning of the formation of the pulse to more than 12 in the tail of the pulse. The measurements are compared with the results of a numerical simulation which takes the laser amplification, the properties of the laser cavity, and the diffraction of the beam in the laser cavity into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results.PACS 42.60.Jf