磁通交流响应的锁模特性研究

本数值研究无序磁通点阵的交流响应．我们在交流驱动力叠加的速度-直流驱动力曲线上发现了锁模台阶．台阶宽度随交流驱动力振幅呈Bessel函数振荡．台阶速度线性依赖于交流驱动力频率．此外，我们还首次考察了温度对锁模特性的影响．发现：台阶速度不随温度的变化而改变，直至台阶消失；热涨落将抹平钉扎势，因此破坏微观速度的调制，另外热涨落还破坏磁通的相干运动，从而导致台阶宽度随温度呈指数衰减．     

Rectification and phase locking of graphite

Rectification phenomena and the phase locking in a two-dimensional overdamped Frenkel–Kontorova model with a graphite periodic substrate were studied. The presence of dc and ac forces in the longitudinal direction causes the appearance of dynamicalmode locking and the steps in the response function of the system. On the other hand, the presence of an ac force in the transverse direction causes the appearance of rectification, even though there is no net dc force in the transverse direction. It is found that whereas the longitudinal velocity increases in a series of steps, rectification in the transverse direction can occur only between two neighbor steps. The amplitude and phase of the external ac driving force affect the depinning force, rectification of the system and particles trajectories.     

Collective Directional Transport in Symmetric Periodic Potentials by Breaking the Coupling Symmetry

Collective unidirectional motion of an asymmetrically coupled array of oscillators in symmetric periodic potentials is studied. A directed current is observed when the drift coupling is presented, while no external biased force is applied. Negative directed current is found when varying system parameters. An addition of a periodic rocking force may enhance the efficiency of directed transport. Resonant steps of the current are found and interpreted as the mode locking between the array and the ac force. Noise-assisted transport is observed, and an optimal noise intensity can give rise to a most efficient transport. The directed transport thus can be optimized and furthermore controlled by suitably adjusting the parameters of the system.     

Injection locking and phase control of spin transfer nano-oscillators

We have directly measured phase locking of spin transfer oscillators to an injected ac current. The oscillators lock to signals up to several hundred megahertz away from their natural oscillation frequencies, depending on the relative strength of the input. As the dc current varies over the locking range, time-domain measurements show that the phase of the spin transfer oscillations varies over a range of approximately +/-90 degrees relative to the input. This is in good agreement with general theoretical analysis of injection locking of nonlinear oscillators.     

Directed Current Induced by an Symmetrically ac Force Coexisting with a Time-Delayed Feedback

We study the transport of overdamped Brownian particles in a symmetrically periodic potential in the presence of an asymmetrically ac driving force and a time-delayed feedback.It is found that for low frequencies,the average velocity can be negative by changing the driving amplitude,for high frequencies,there exists an optimized driving amplitude at which the average velocity takes its maximum value.Additionally,there is a threshold value of driving amplitude below which no directed transport can be obtained for high frequencies.For the large value of the delay time,the average velocity is independent of the delay time.     

Irrational mode locking in quasiperiodic systems

A model for ac-driven systems, based on the Tang-Wiesenfeld-Bak-Coppersmith-Littlewood automaton for an elastic medium, exhibits mode-locked steps with frequencies that are irrational multiples of the drive frequency, when the pinning is spatially quasiperiodic. Detailed numerical evidence is presented for the large-system-size convergence of such a mode-locked step. The irrational mode locking is stable to small thermal noise and weak disorder. Continuous-time models with irrational mode locking and possible experimental realizations are discussed.     

Mode locking of argon ion laser by use of cylindrical ultrasonic waves

The phenomenon of light diffraction by a cylindrical ultrasonic wave has been investigated. The diffraction was applied in a new type of acousto-optic modulator used for laser mode locking. The paper describes the modulator and presents results on mode locking of argon ion laser. The mode locking was obtained by means of a cylindrical acoustic wave launched in water at the frequencies of ultrasound 33-34 MHz. A standing wave regime of operation of the modulator resulted in a reliable generation of laser pulses with the duration 720 ps.     

The interaction of high- and low-frequency waves in a free electronlaser

When a free electron laser (FEL) interaction occurs inside a waveguide, there are two possible operating frequencies at the two intersections of the beam line with the waveguide dispersion curve. Using a one-dimensional model valid for a low-gain oscillator, the mode competition between the high- and low-frequency waves is studied. It is found that the low-frequency wave can be nonlinearly suppressed, even when its linear growth rate is positive. When the frequency ratio is two, phase locking occurs and the nonlinear gain of the low-frequency wave can exceed its linear growth rate     

光纤激光经过模清洁器后的强度噪声分析

对1560 nm单频光纤激光光源通过一个光学模清洁器后的强度噪声进行了分析研究.实验观察到模清洁器虽然对于激光的高频强度噪声有明显的抑制作用,但强度噪声特性随频率呈一定周期变化,且在低频处模清洁器对强度噪声有放大作用.本文认为这是由于模清洁器具有类似于光纤延迟线的时延效应,将激光源的部分相位噪声转化为强度噪声.通过理论分析,由相位噪声转化的相对强度噪声幅度与激光相干时间、模清洁器的平均延时参数以及分析频率相关.分析结果与实验测量结果符合良好.此外,通过在光路中加入声光调制器进行反馈调制,激光的线宽从26 kHz压窄至16 kHz,模清洁器的锁定质量明显提高,经过模清洁器后的激光强度噪声有所减小,与理论相符.该结果进一步证实了由相位噪声转化而来的强度噪声与模清洁器的锁定质量无直接关系.通过该研究,完善了光学模清洁器对激光的噪声抑制模型.     

Critical behavior of a passively mode-locked laser: rational harmonic mode locking

The critical behavior of passive mode locking has been demonstrated in a figure-eight fiber laser that performs rational harmonic mode locking (RHML). On both the repetition rate and the pulse amplitude distribution, the observed pulse trains near the threshold exhibit the same regulations as the rational harmonic mode-locked ones. The theory also shows that there should be a middle status of RHML before achieving normal mode locking. It is important to note that the results provide what we believe to be the first confirmed attempt to address a fundamental question: how does a laser become mode locking with an increase of pump power?     

Charge shuttle as a nanomechanical rectifier

We consider the charge shuttle proposed by Gorelik et al. driven by a time-dependent voltage bias. In the case of asymmetric setup, the system behaves as a rectifier. For pure ac drive, the rectified current shows a rather rich frequency dependent response characterized by frequency locking at fractional values of the external frequency. Because of the nonlinear dynamics of the shuttle, rectification is present also for very low frequencies. These effects could be useful to unveil the internal dynamics of nanomechanical devices.     

A parametric study of negative feedback Nd:YAG laser

The output pulse parameters of a mode-locked Nd:YAG laser with a passive negative feedback element were studied experimentally. The pulse evolution within the train was experimentally recorded using a modified second harmonic generation autocorrelator. By comparing the laser operation with and without an acousto-optic modulator, we found that with the later there is a significant increase in the mode locking probability and the pulse acquires a temporally Gaussian symmetric pulse shape. Further with the active modulator there is a relaxation in alignment tolerances and increase in the range of permissible dye concentrations for stable mode locking. It was also observed that the pulse width of the negative feedback laser depends on the saturation intensity of the mode locking dye and reduces for a dye with higher saturation intensity. The pulse width was also found to reduce linearly as the initial transmission of the dye is reduced.     

Theory of coupled mode oscillation in a laser with axial magnetic field

The nonlinear laser theory without noise is applied to calculate amplitudes, frequencies and beat frequencies of two circularly polarized modes in a laser with axial magnetic field. The resonator is allowed to have different damping constants for waves polarized inx- andy-direction. For small beat frequencies frequency locking occurs, and the modes combine to a linearly or elliptically polarized mode. The plane of polarization rotates with increasing magnetic field up to ± π/4.     

On the role of flexoeffect in synchronization of electroconvective roll oscillations in nematics

We describe the dynamics of zigzag oscillations in a system of convective rolls in a nematic liquid crystal above the electroconvection threshold under the action of an ac voltage with a biased position of the mean value. It is found that an increase in the contribution from the constant component leads to a substantial increase in the spatiotemporal ordering of zigzag rolls and their synchronization with the homogeneous twist mode. The results confirm the flexoelectric mechanism of locking.     

Polyhedra formation and transient cone ejection of a resonant microdrop forced by an ac electric field

New deformation or fission phenomena are reported for microdrops driven by an ac electric field at their resonant frequencies. The Maxwell forces that pull out the vertices from a drop can be enhanced when the ac frequency is comparable to both the drop resonant frequency and the inverse charge relaxation time of the diffuse layer. The selected polyhedra possess symmetries that ensure a global force balance of the Maxwell forces and a linear dimension consistent with a sphere whose nth harmonic (n is up to six in the observation) coincides with the applied ac frequency. At high voltages, the resonant focusing of charges by the vibration modes produces evenly distributed and transient Taylor cones that can eject charged nanodrops.     

Additive-pulse mode locking of a diode-pumped Nd:KGd(WO4)2 laser

Self-starting additive-pulse mode locking of a diode-pumped Nd:KGd(WO₄)₂ laser is demonstrated for the first time. An output power of 0.85 W is achieved with 2.3-ps pulses at a repetition frequency of 76.5 MHz. The shortest pulse generated had a duration of 1.9 ps. Received: 2 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +44-141/5482891, E-mail: a.major@phys.strath.ac.uk     

超晶格电流振荡的分岔图和Poincaré映射

基于超晶格输运的分立漂移模型,数值模拟了掺杂弱耦合GaAs AlAs超晶格在外加交流偏压下电场畴的输运行为.以黄金分割比固定交流驱动频率,模拟并分析不同交流幅度下电流的准周期、锁频及混沌现象.     

Injection locked multi-section gain-coupled dual mode DFB laser for terahertz generation

A dual mode multi-section gain-coupled distributed feedback laser with tunable mode spacing is subharmonically injection locked at 0.315 THz. The injected signal consists of an optical comb with harmonics 35 GHz apart and a bandwidth of approximately 1.9 THz. The optical comb is a result of strong four-wave mixing in a highly-nonlinear dispersion-shifted fiber. In order to observe locking of the multi-section laser, the output is optically downconverted to RF frequencies using the same optical comb. The locked multi-section DFB laser is a coherent and tunable optical source suitable for continuous-wave terahertz generation systems.     

Mode locking of spin waves excited by direct currents in microwave nano-oscillators

A spin-wave theory is presented which explains the frequency pulling and mode locking observed when two closely spaced spin-transfer nanometer-scale oscillators with slightly different frequencies are separately driven in the same magnetic thin film by spin-polarized carriers at high direct-current densities. The theory confirms recent experimental evidence that the origin of the phenomena lies in the nonlinear interaction between two overlapping spin waves excited in the magnetic nanostructure.     

Kerr lens effects in a folded-cavity four-mirror linear resonator

The Kerr-lens model locking of a folded-cavity four-mirror linear Ti: sapphire laser is simulated by applying the ABCD ray-tracing technique for a Gaussian beam. The intensity dependent Kerr-lens ABCD matrix of the Brewster-plate shaped active medium is derived. A split-step parabolic refractive index model is used. the astigmatism compensation condition is derived from the ABCD ray-tracing. Simulations are carried out for a symmetric resonator and an asymmetric resonator design. Hard-aperture loss mode locking and soft-aperture gain mode locking are discussed. The optimum design parameters (slit position for hard-aperture loss mode locking, pump beam focusing for soft-aperture gain mode locking, gain cavity spacing, gain medium position) for Kerr-lens mode locking are determined.