SiO2脊形条波导热极化引起的电光效应

通过理论模拟对具有上覆盖层的SiO₂脊形条波导结构进行了优化，在此基础上利用微电子工艺制作了SiO₂脊形波导Mach-Zehnder型电光调制器，并进行热极化引起的电光和非线性效应的研究.热极化过程大幅增强了样品的电光及非线性效应，二次电光系数由热极化前的1.56×10^-22(m/V)²提高到热极化后的8.50×10^-22(m/V)²，极化后得到了0.093pm/V的线性电光系数，对热极化的物理机理进行了理论分析. 关键词： 电光调制器 2光波导')" href="#">SiO₂光波导 电光效应 热极化     

Nonlinear Effects in the Cosmic Microwave Background

Major advances in the observation and theory of cosmic microwave background anisotropies have opened up a new era in cosmology. This has encouraged the hope that the fundamental parameters of cosmology will be determined to high accuracy in the near future. However, this optimism should not obscure the ongoing need for theoretical developments that go beyond the highly successful but simplified standard model. Such developments include improvements in observational modelling (e.g. foregrounds, non-Gaussian features), extensions and alternatives to the simplest inflationary paradigm (e.g. non-adiabatic effects, defects), and investigation of nonlinear effects. In addition to well known nonlinear effects such as the Rees–Sciama and Ostriker–Vishniac effects, further nonlinear effects have recently been identified. These include a Rees–Sciama-type tensor effect, time-delay effects of scalar and tensor lensing, nonlinear Thomson scattering effects and a nonlinear shear effect. Some of the nonlinear effects and their potential implications are discussed.     

Crosstalk in high-speed WDM produced by refractive index fluctuation nonlinear effect

With the development of the ultra-high speed and large capacity, the channel crosstalk cause by nonlinear effects become more apparent. This paper research the crosstalk produced by refractive index fluctuation nonlinear effects, establish the corresponding mathematical models and physical monitoring models. The experiment simulation results show when the input power is less than 50 mW, the channel crosstalk caused by the nonlinear effects is low and can be ignored; when input power greater than 50 mW, the channel crosstalk caused by the nonlinear effects is more high and affect transmission quality; and with the further increase in power, the channel crosstalk caused by SPM and XPM gradual increase, when the input power is greater than 300 mW, the crosstalk caused by SPM and XPM morn than by FWM.     

Ultrafast nonlinear absorption and reflection of ZnS/ZnSe periodic nanostructures

ZnS/ZnSe heterostructures under condition of ZnSe interband excitation by a 150 fs laser pulse exhibit strong narrow-band modification of absorption and wide-band modification of reflection. Mean decay time for nonlinear reflection in heterostructures ranges from 2 to 3 ps whereas in bare ZnSe monolayer it exceeds 5 ps. Possible physical processes responsible for nonlinear refraction in the transparency region include interplay of absorption driven nonlinear refraction via Kramers–Kronig relations and intrinsic dielectric properties of dense electron–hole plasma. For nonlinear absorption at ZnSe band edge, interplay of plasma screening effects and states filling effects are relevant.     

Nonlinear correction to the longitudinal structure function at small x

We computed the longitudinal proton structure function F_L, using the nonlinear Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (NLDGLAP) evolution equation approach at small x . For the gluon distribution, the nonlinear effects are related to the longitudinal structure function. As the very small-x behavior of the gluon distribution is obtained by solving the Gribov, Levin, Ryskin, Mueller and Qiu (GLR-MQ) evolution equation with the nonlinear shadowing term incorporated, we show that the strong rise that corresponds to the linear QCD evolution equations can be tamed by screening effects. Consequently, the obtained longitudinal structure function shows a tamed growth at small x . We computed the predictions for all details of the nonlinear longitudinal structure function in the kinematic range where it has been measured by the H1 Collaboration and made comparisons with the computation by Moch, Vermaseren and Vogt at the second order with input data from the MRST QCD fit.     

The effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system

We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings ∣K∣ are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant ∣K∣. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field B.     

Second Hugoniot relationship for solids

The term second Hugoniot is used to describe a locus of states directly attainable by shock compression beginning with an initial state that has itself been achieved by shock compression. For many materials the principal Hugoniot is linear in the shock velocity-particle velocity [U_s — u_p] representation. A second Hugoniot for such materials is not linear in this representation, but the nonlinear effects can often be neglected if the compressions are not too great. These results are based on the Mie-Grüneisen equation of state and are more or less independent of the explicit functional dependence of the Grüneisen parameter. A criterion to evaluate the magnitude of nonlinear effects is posed and Mg and Pb are treated as examples. Calculated second Hugoniots are compared with experimental results for regular reflection in Mg, Pb and U. For these materials nonlinear effects are not important from a practical standpoint for final volumetric compressions to roughly two-thirds of the initial volume. Some future experimental work is also suggested.     

Nonlinear instability of charged liquid jets: Effect of interfacial charge relaxation

A linear and nonlinear study has been made of cylindrical interface, carrying a uniform surface charge in the presence of a finite rate of charge relaxation, is investigated by using multiple scales method. The linear stability flow is analyzed by deriving a dispersion relation for the growth waves, and solving it analytically and numerically to find marginal stability curves. We investigate the electric charge relaxation effects on the stability of the flow by considering various limiting cases. We also examine the effects of finite charge relaxation times in axisymmetric and nonaxisymmetric modes. In the nonlinear approach, it is shown that the evolution of the amplitude is governed by a Ginzburg–Landau equation. There is also obtained a nonlinear modified Schrödinger equation describing the evolution of wave packets for small charge relaxation time. Further, the classic Schrödinger equation is obtained when the influence of relaxation time charge is neglected. On the other hand, the complex amplitude of quasi-monochromatic standing waves near the cutoff wavenumber is governed by a similarly type of nonlinear Schrödinger equation in which the roles of time and space are interchanged. This equation makes it possible to estimate the nonlinear effect on the cutoff wavenumber. The nonlinear theory, when used to investigate the stability of charged liquid jet, appears accurately to predict a new unstable regions. The effects of the surface charge and charge relaxation on the stability are identified. The various stability criteria are discussed both analytically and numerically and the stability diagrams are obtained.     

Theoretical estimates of radial electric fields and plasma rotation induced in H-1NF by RF waves

The purpose of this study is to estimate, for planned experiments at the H-1NF heliac, nonlinear variations of the stationary radial electric field, and poloidal and toroidal plasma rotation, that result from ponderomotive forces exerted by large amplitude RF waves in the H-1NF. Similarly as in the previous studies of nonlinear transport effects induced by RF waves, the nonlinear ponderomotive force effects on the radial electric field, toroidal and poloidal plasma rotation become important for dissipated powers of the order of 1 MWm^–3 for RF waves with frequency of about 10 MHz. At these high RF powers, the nonlinear ponderomotive force effects might therefore result in important changes in plasma confinement and RF wave coupling in H-1NF.This work has been partially supported by the Australian National University, by the DIST Department of the Australian Government, by the Czech Grant Agency grants No. 202/96/1355 and 1350, and by the Queen Elisabeth II grant administered by G.G. Borg. The author is grateful to G.G. Borg and R.L. Dewar for many stimulating discussions.     

Modelling of nonlinear crack–wave interactions for damage detection based on ultrasound—A review

The past decades have been marked by a significant increase in research interest in nonlinearities in micro-cracked and cracked solids. As a result, a number of different nonlinear acoustic methods have been developed for damage detection. A general consensus is that – under favourable conditions – nonlinear effects exhibited by cracks are stronger than crack-induced linear phenomena. However, there is still limited understanding of physical mechanisms related to various nonlinearities. This problem remains essential for implementation of nonlinear acoustics for damage-detection applications. This paper reviews modelling approaches used for nonlinear crack–wave interactions. Various models of classical and nonclassical crack-induced elastic, thermo-elastic and dissipative nonlinearities have been discussed.     

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO<Subscript>2</Subscript> varistors

Ca,Ta-doped TiO₂ varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350 °C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 10⁵. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO₂ varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO₂ system are explained by analogy to a grain-boundary atomic defect model. Received: 24 October 2001 / Accepted: 8 January 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/826-49531, E-mail: wangwanyan@yahoo.com.cn     

弱非线性热声理论模型的建立及简化

本文指出非零的周期平均热声效应在本质上是非线性的,应保留到二阶精度。在小振幅条件下,利用摄动方法,以无限大平板流道为例,建立了二阶精度的弱非线性热声理论模型,并在不同条件下对模型做了进一步简化。这一理论为理解热声系统的工作机制以及设计优化热声系统提供了强有力的理论工具。     

Memory effects and heat transport in one-dimensional insulators

We study the dynamical correlation functions and heat conduction for the simplest model of quasi one-dimensional (1d) dielectric crystal i.e. a chain of classical particles coupled by quadratic and cubic intersite potential. Even in the weakly nonlinear regime, numerical simulation on long enough chains reveal sizeable deviations from the perturbative results in the form of a slower decay of correlations in equilibrium. Their origin can be traced back to the subtle nonlinear effects described by mode-coupling theories. Measures of thermal conductivity with nonequilibrium molecular-dynamics method confirm the relevance of such effects for low-dimensional lattices even at very low temperatures. Received 20 April 2000     

Multiple access interference rejection in OCDMA using a two-photon absorption based semiconductor device

An experimental demonstration of a two-channel OCDMA system with detection performed using standard linear detection or a TPA-based nonlinear detector is presented. These results show an improvement in the extinction ratio of the decoded signal by ∼5 dB using TPA detection. A simulation model of the TPA detector used during the experiments was created and used in a four-channel OCDMA system simulation using both linear and nonlinear detection methods. The simulation results show that error-free performance is achievable for a 4-user system using the nonlinear TPA detector while the OCDMA system employing linear detection is severely limited by the effects of noise generated by adjacent optical channels (multiple access interference).     

Simulation of three-dimensional nonlinear fields of ultrasound therapeutic arrays

A novel numerical model was developed to simulate three-dimensional nonlinear fields generated by high intensity focused ultrasound (HIFU) arrays. The model is based on the solution to the Westervelt equation; the developed algorithm makes it possible to model nonlinear pressure fields of periodic waves in the presence of shock fronts localized near the focus. The role of nonlinear effects in a focused beam of a two-dimensional array was investigated in a numerical experiment in water. The array consisting of 256 elements and intensity range on the array elements of up to 10 W/cm² was considered. The results of simulations have shown that for characteristic intensity outputs of modern HIFU arrays, nonlinear effects play an important role and shock fronts develop in the pressure waveforms at the focus.     

Nonlinear diffraction in spontaneous three-wave and coherent four-wave light scattering by polaritons

Spontaneous three-wave and coherent four-wave scattering by polaritons in crystalline media with periodically modulated nonlinear quadratic susceptibility have been studied both theoretically and experimentally. Phase matching conditions and an expression for the scattering spectral line shape with due account of cascade processes in nonlinear diffraction for coherent Stokes polariton light scattering have been derived. Measurements of the light intensity distribution due to the three-and four-wave scattering in a LiNbO₃:Nd:Mg crystal with a periodic domain structure are in good agreement with theoretical results. The prospects for using the effects of nonlinear diffraction in spectroscopic studies of media with periodic distributions of nonlinear optical parameters, specifically, in precision measurements of the IR refractive index dispersion and determination of the period and profile of the quadratic susceptibility distribution are discussed. Zh. éksp. Teor. Fiz. 112, 2001–2015 (December 1997)     

Forced vibrations of a torsional oscillator with Coulomb friction under a periodically varying normal load

The forced vibration response of a single degree of freedom torsional system with Coulomb friction, under a periodically varying normal load, is studied. First, an enhanced multi-term harmonic balance method is developed to calculate nonlinear responses directly in frequency domain; this should be in general applicable to periodically varying nonlinear systems. Second, a pulse width modulated normal load is approximated by a truncated Fourier series with a reasonable number of harmonic components and utilized for case studies. Finally, the effects of duty ratio on nonlinear frequency responses are examined.     

Optimum design of adiabatic weakly guiding nonlinear optical fibre tapers

We investigated the design features of adiabatic, weakly guiding optical fibre tapers for nonlinear device applications. In general, where the fibre taper core and cladding are of different material nonlinearities, the nonlinear effects that can be achieved depend critically on the taper parameters. For optical switches a suitable choice of taper parameters minimizes the power × length product for nonlinear operation. The maximum effective utilization of the nonlinear cladding material was found to be 21.3%. Also, nonlinear effects can be totally suppressed with an appropriate choice of design, enabling nonlinear effects that are undesirable in some applications to be avoided.     

Internal resonance of axially moving laminated circular cylindrical shells

The nonlinear vibrations of a thin, elastic, laminated composite circular cylindrical shell, moving in axial direction and having an internal resonance, are investigated in this study. Nonlinearities due to large-amplitude shell motion are considered by using Donnell’s nonlinear shallow-shell theory, with consideration of the effect of viscous structure damping. Differently from conventional Donnell’s nonlinear shallow-shell equations, an improved nonlinear model without employing Airy stress function is developed to study the nonlinear dynamics of thin shells. The system is discretized by Galerkin’s method while a model involving four degrees of freedom, allowing for the traveling wave response of the shell, is adopted. The method of harmonic balance is applied to study the nonlinear dynamic responses of the multi-degrees-of-freedom system. When the structure is excited close to a resonant frequency, very intricate frequency–response curves are obtained, which show strong modal interactions and one-to-one-to-one-to-one internal resonance phenomenon. The effects of different parameters on the complex dynamic response are investigated in this study. The stability of steady-state solutions is also analyzed in detail.     

光子晶体光纤中超连续谱产生的理论与实验研究

研究了光子晶体光纤中超连续激光光源的产生机理.利用非线性偏振旋转技术产生的中心波长为1 556.0 nm的飞秒光脉冲作为泵浦光源,在69 m长的高非线性光子晶体光纤中,得到了20 dB带宽约为140 nm的超连续谱;采用实验和数值模拟方法,研究了不同泵浦功率下超连续谱形成的过程.结果表明,在不同的泵浦功率下,超连续谱的形成机理不同,在各种非线性效应的共同作用下,泵浦光脉冲的峰值功率越高,得到超连续谱的带宽越宽,实验与数值模拟结果一致.另外,要想获得平坦的宽带超连续谱,必须选择合适的光纤长度.