Modulational instability of optical beams in photovoltaic and photorefractive media due to two-photon photorefractive effect under open circuit condition

The modulational instability of broad optical beams in two-photon photorefractive (PR) photovoltaic materials under open circuit conditions has been investigated. Under linear stability framework, the one dimensional modulational instability growth rate has been estimated by considering the space charge field. Gain of the instability is shown to exist only when the photovoltaic fields orients in the same direction with respect to the optical c-axis of the medium. It is found that the behavior of the gain spectrum is different in low and high power regions. We have found by numerical simulations that the evolution of the soliton induced by the modulational instability at low photovoltaic field show the dynamical behaviors similar to those of the localized beam as the initial profile. However, it has been shown that increasing photovoltaic fields produce traveling, breathing, and mutually interacting solitons.     

Modulational instability for a self-attractive two-component Bose--Einstein condensate

By means of the multiple-scale expansion method, the coupled nonlinear Schr?dinger equations without an explicit external potential are obtained in two-dimensional geometry for a self-attractive Bose-Einstein condensate composed of different hyperfine states. The modulational instability of two-component condensate is investigated by using a simple technique. Based on the discussion about two typical cases, the explicit expression of the growth rate for a purely growing modulational instability and the optimum stable conditions are given and analysed analytically. The results show that the modulational instability of this two-dimensional system is quite different from that in a one-dimensional system.     

非线性单模光纤中的调制不稳定性

本文对非线性单模光纤中的调制不稳定性进行了详细研究,给出了调制不稳定性产生的条件与最大调制增长率等公式,得到了调制不稳定性既可在异常色散区又可在正常色散区产生的结论,确定了维持调制增长对光纤长度选取要求的范围。 关键词：     

Generation of surface soliton arrays

We discover that, at the edge of an optical lattice imprinted in a saturable nonlinear medium, one-dimensional surface solitons exist only within a band of light intensities and that they cease to exist when the lattice depth exceeds an upper threshold. We also reveal the generation of arrays of two-dimensional surface solitons mediated by the transverse modulational instability of one-dimensional solitons, a process that is found to exhibit specific features associated to properties of the optical lattice.     

Modulational instability effects in PSK modulated coherent fiber systems and their reduction by optical loss

We show using the propagating beam technique that if optical loss is neglected, the efficiency of PSK modulation in a coherent fiber system is severely compromised by modulational instability effects at sufficiently high signal powers. We note, however, that the intrinsic optical loss of a single mode fiber will to a large degree counteract the modulational instability and preserve the initial PSK pulse shape.     

Discrete modulational instability and bright localized spin wave modes in easy-axis weak ferromagnetic spin chains involving the next-nearest-neighbor coupling

We report a theoretical work on the properties of modulational instability and bright type nonlinear localized modes in one-dimensional easy-axis weak ferromagnetic spin lattices involving next-nearest-neighbor couplings.With a linear stability analysis, we calculate the growth rates of the modulational instability, and plot the instability regions.When the strength of the next-nearest-neighbor coupling is large enough, two new asymmetric modulational instability regions appear near the boundary of the first Brillouin zone.Furthermore, analytical forms of the bright nonlinear localized modes are constructed by means of a quasi-discreteness approach.The influence of the next-nearest-neighbor coupling on the Brillouin zone center mode and boundary mode are discussed.In particular, we discover a reversal phenomenon of the propagation direction of the Brillouin zone boundary mode.     

强激光在磁化等离子体中的调制不稳定性

采用洛伦兹变换推导出左旋椭圆偏振强激光在磁化等离子体中的非线性色散关系,根据Karpman方法推导出激光场包络的非线性控制方程,分析了在磁化等离子体中左旋椭圆偏振激光的调制不稳定性,得到了调制不稳定的时间增长率。分析结果表明,磁化等离子体中自调制不稳定的极大增长率较非磁化情况明显减小,且在激光等离子体临界面附近处调制不稳定性的时间增长率显著增大。     

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采用洛伦兹变换推导出左旋椭圆偏振强激光在磁化等离子体中的非线性色散关系,根据Karpman方法推导出激光场包络的非线性控制方程,分析了在磁化等离子体中左旋椭圆偏振激光的调制不稳定性,得到了调制不稳定的时间增长率。分析结果表明,磁化等离子体中自调制不稳定的极大增长率较非磁化情况明显减小,且在激光等离子体临界面附近处调制不稳定性的时间增长率显著增大。     

Relativistic effects on the modulational instability of electron plasma waves in quantum plasma

Relativistic effects on the linear and nonlinear properties of electron plasma waves are investigated using the one-dimensional quantum hydrodynamic (QHD) model for a two-component electron?Cion dense quantum plasma. Using standard perturbation technique, a nonlinear Schr?dinger equation (NLSE) containing both relativistic and quantum effects has been derived. This equation has been used to discuss the modulational instability of the wave. Through numerical calculations it is shown that relativistic effects significantly change the linear dispersion character of the wave. Unlike quantum effects, relativistic effects are shown to reduce the instability growth rate of electron plasma waves.     

超常介质中部分相干光束传输的调制不稳定性研究

超常介质与常规光学介质的一个最重要的区别是前者具有色散磁导率,其折射率呈现出可正可负,这样调制不稳定性出现新特点。借助于Wigner变换,从一般的非线性薛定谔方程出发,得到了部分相干光满足Wigner-Moyal的方程,获得了色散关系和增益谱,揭示了部分相干度不仅可抑制调制不稳定性,而且还可以缩小发生不稳定性的频率范围。     

On Modulational Interaction of the Lower-Hybrid Drift Oscillations

The general nonlinear equation of the third order in field strength for the lower-hybrid drift waves in inhomogeneous plasma is obtained on the basis of kinetic theory. This equation enables us to describe strong turbulence effects (modulational instability, soliton-like solutions, etc.) as well as weak turbulence effects (decays, scattering). The investigation of the modulational instability of the lower-hybrid drift waves is carried out. It is demonstrated that the development of the lower-hybrid drift wave modulational instability is possible only when the wavevector of the modulational perturbations is less or of the order of the wavevector of the pump wave. The condition on the wave vectors, when the nonlinear response defining the character of the modulational instability is determined by the inhomogeneity effects, is obtained.     

Photo-induced dynamics of Bose-Einstein condensates in optical superlattice

The photo-induced dynamics of cold atoms in a one-dimensional optical superlattice is observed. Steady state distribution of the probability amplitudes and the site population in a one-dimensional optical superlattice is found. It is shown that this solution of the equations, which describes the temporal behavior of a Bose-Einstein condensate in a superlattice, is unstable at the sufficiently high level of boson density. The expression for the increment of modulational instability is obtained on the basis of the linear stability analysis. The numerical examples of non-stationary solutions for boson density in a superlattice for the general model are discussed as applied to both the attraction and repulsion potentials of boson interaction.     

Modulational Instability and Energy Localization in a Chain of Interacting Frenkel Excitons

The modulational instability in one-dimensional molecule chain of interacting Frenkel excitons is investigated. The formation of localized modes via modulational instability is predicted and the previous numerical and analytical results are explained. The Peierls-Nabarro (PN) potential barrier for the localized modes is also discussed.     

Modulational instability in one-dimensional saturable waveguide arrays: Comparison with Kerr nonlinearity

Discrete modulational instability within the first band of uniform one-dimensional waveguide arrays possessing a saturable self-defocusing nonlinearity is investigated in detail within the coupled mode approach. Explicit analytical results for both the threshold and the maximal gain of instability are compared with the corresponding data from waveguide arrays exhibiting Kerr nonlinearity. We find that saturation bounds the interval of existence of discrete modulational instability, stabilizes the frequency region of perturbations around ±π/2 and decreases both gain and critical spatial frequency of perturbations.     

Incoherent modulation instability in instantaneous nonlinear Kerr media

We demonstrate theoretically and experimentally in an optical fiber system that partially temporally incoherent light exhibits modulational instability during its propagation in an instantaneous response nonlinear medium. We show that the modulation frequency and gain are substantially increased with respect to the corresponding values of coherent modulational instability.     

Modulational instability of a strip beam in a bulk type I quadratic medium

The evolution of a strip (one-dimensional) fundamental beam with propagation distance owing to spatial modulational instabilities was analyzed in a quadratic medium near type I phase matching. We obtained the gain coefficient for the modulational instability and showed that the wave evolves into a clean periodic sequence of solitary waves and does not reproduce the incident beam.     

Influence of intrapulse raman scattering on the modulational instability in optical fibres

An analysis of the intrapulse Raman scattering (IRS) influence on the modulational instability in optical fibres is presented. The extrema of the spatial growth rates are analytically determined in two characteristic regions of instability. These results may help the analysis of femtosecond pulse propagation.     

Modulational instability of incoherently coupled beams in azobenzene-containing polymer with photoisomerization nonlinearity

The modulational instability of two incoherently coupled beams in azobenzene-containing polymer with photoisomerization nonlinearity is investigated analytically and numerically. Our results show that as a precursor to spatial optical soliton formation, modulational instability can be adjusted and controlled by the wavelength combinations of the signal and background beams. We also discuss the dependences of strength of modulational instability on intensities of two signal beams and background beam. These findings make it possible to predict the formation of incoherently coupled soliton pairs in azobenzene-containing polymer.     

Modulational instability of nematic phase

We numerically observe the effect of homogeneous magnetic field on the modulationally stable case of polar phase in F = 2 spinor Bose-Einstein condensates (BECs). Also we investigate the modulational instability of uniaxial and biaxial (BN) states of polar phase. Our observations show that the magnetic field triggers the modulational instability and demonstrate that irrespective of the magnetic field effect the uniaxial and biaxial nematic phases show modulational instability.     

具有饱和非线性的非克尔光纤的调制不稳定性

理论上分析了当存在高阶色散时具有饱和非线性的非克尔光纤的调制不稳定性.三阶色散对调制不稳定性没有影响,四阶色散效应导致在正常和反常色散区出现了新的调制不稳定区.并且对比了不同他和参数的增益谱,在新的调制不稳定区,发现同样有一个临界调制频率对应两个入射功率的现象.