Overlapping of resonances and stochasticity of electron trajectories in cyclotron masers

When an electromagnetic (EM) wave has a large amplitude and electrons have a large energy, the electron cyclotron frequency in the process of interaction with an EM wave can vary significantly. This can lead to overlapping of cyclotron resonances at different harmonics. It is shown that such an overlapping causes the stochasticity of electron orbits. Estimates show that this effect can be present in relativistic gyrodevices currently under development for driving future accelerators.     

Local vibration of an elastic plate and zero-group velocity Lamb modes

Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) at a nonzero value of the wave number. Using laser-based ultrasonic techniques, we experimentally investigate some fascinating properties of these ZGV modes: resonance and ringing effects, backward wave propagation, interference between backward and forward modes. Then, the conditions required for the existence of ZGV Lamb modes in isotropic plates are discussed. It is shown that these modes appear in a range of Poisson's ratio about the value for which the cutoff frequency curves of modes belonging to the same family intercept, i.e., for a bulk wave velocity ratio equal to a rational number. An interpretation of this phenomenon in terms of a strong repulsion between a pair of modes having a different parity in the vicinity of the cutoff frequencies is given. Experiments performed with materials of various Poisson's ratio demonstrate that the resonance spectrum of an unloaded elastic plate, locally excited by a laser pulse, is dominated by the ZGV Lamb modes.     

Characterization of mechanical properties of a hollow cylinder with zero group velocity Lamb modes

Hollow cylinders used in the industry must be regularly inspected. Elastic guided waves, similar to Lamb modes in a plate, can propagate in the axial direction or around the circumference. They are sensitive to geometrical and mechanical parameters of the cylindrical shell. The objective of this paper is to show that zero group velocity (ZGV) Lamb modes can be used to bring out anisotropy and to measure elastic constants of the material. This study provides experimental and numerical investigations on a Zirconium alloy tube extensively used by the nuclear industry in reactor core components. A non-contact method, based on laser ultrasound techniques and ZGV Lamb modes, demonstrates that the difference observed between axial and circumferential guided waves cannot be explained by an isotropic model. Then, a transverse isotropic model is used for the Zircaloy tube. Four of the five elastic constants are directly extracted from ZGV resonance frequencies. The last one is deduced from the measured dispersion spectra. With this complete set of constants, a good agreement is obtained between theoretical and experimental dispersion curves for both axially and circumferentially propagating guided waves.     

Ultrasonic guided wave parameters for detection of axial cracks in feeder pipes of PHWR nuclear power plants

The dispersion curves for the feeder pipes in PHWR nuclear power plants were determined. The wave modes used for the detection of notches in the feeder pipe were confirmed as F(m,2) and/or L(0,1) by an analysis of short time Fourier transformation (STFT). The axial notches in the straight pipe were not detectable, but an axial notch in a bent pipe was detected with the mode at the frequency of 500 kHz. Initial F(m,2) and/or L(0,1) modes contains a circumferential displacement and might be converted to certain complicated modes in the bent region, which is sensitive to the axial notch. The circumferential guided wave technique was also applied for quantitative evaluation of the axial notches. The waves generated by a rocking motion of the transducer along the circumferential direction were estimated as the circumferential guided waves after a review of the acquired data and the dispersion curves.     

Locking of the longitudinal modes of an SBS-laser oscillator by optical feedback

The use of a nonlinear mirror on the basis of the stimulated Brillouin scattering (SBS) as part of a resonator in a laser oscillator leads to the generation of bandwidth due to the shift in frequency that is connected with the reflection at the SBS-mirror. Another effect of the nonlinear SBS-reflection is a passive Q-switch, which leads to a generation of pulses with durations of approximately 100 ns. An almost perfect locking of the longitudinal modes is observed if an additional external resonator is applied and certain resonance criteria are obeyed. This locking of the longitudinal modes surfaces as a splitting of the Q-switch pulses into trains of pulses with subnanosecond duration. PACS 42.60.Fc; 42.60.Gd; 42.65.Es     

Exact stationary solution of a Fokker-Planck equation for multimode laser action including phase locking

We first derive the exact stationary solution of a Fokker-Planck equation where the complex drift coefficients are nonlinear functions of the variables, provided the drift and diffusion coefficients fulfill certain conditions. Then we apply the solution to

1. normal multimode action where no phase locking occurs at all.
2. phase locking in a laser with many axial modes having a narrow frequency spacing.

The atomic line which supports laser action may be homogeneously or inhomogeneously broadened. In case 1 the modes may be completely arbitrary, i.e. for instance running or standing waves. In case 2 we assume axial modes described by running waves. The treatment is valid in a region not too far below and not too far above laser threshold where the atomic variables adiabatically follow the motion of the lightfield variables. The drift coefficients are taken from the multimode Langevin equations ofHaken andSauermann. The diffusion coefficients are taken from a paper ofArzt et al. The only essential assumption is that the diffusion coefficients may be considered constant over the frequency range where modes participate in the laser process. If our results are specialized to single mode action we obtain Risken's solution.     

Bragg spectroscopy of discrete axial quasiparticle modes in a cigar-shaped degenerate Bose gas

We propose an experiment in which long wavelength discrete axial quasiparticle modes can be imprinted in a 3D cigar-shaped Bose-Einstein condensate by using two-photon Bragg scattering experiments, similar to the experiment at the Weizmann Institute [J. Steinhauer et al., Phys. Rev. Lett. 90, 060404 (2003)] where short wavelength axial phonons with different number of radial modes have been observed. We provide values of the momentum, energy and time duration of the two-photon Bragg pulse and also the two-body interaction strength which are needed in the Bragg scattering experiments in order to observe the long wavelength discrete axial modes. These discrete axial modes can be observed when the system is dilute and the time duration of the Bragg pulse is long enough.     

Design and applications of discrete mode Fabry-Perot diode lasers

We describe how the multimode spectrum of a Fabry-Perot diode laser can be tailored using a non-periodic patterning of the cavity effective index. The cavity geometry is obtained from the solution of an inverse problem based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Experimental measurements are presented that demonstrate an all-optical memory element based on the injection locking bistability of a two-mode device. We also demonstrate passive harmonic mode-locking of a device designed to support a comb of six modes. Near-transform limited pulsed output with 2 ps pulse duration at 100 GHz repetition rate was obtained. Prospects for the extension of our approach to locking of larger numbers of modes over wider bandwidths are discussed. Similarities between the effective index profiles found in these devices and those of related devices and grating structures are also highlighted.     

Structural transitions and dynamical regimes for directional locking of vortices and colloids driven over periodic substrates

We examine collective dynamical locking effects for superconducting vortices and colloids interacting with square and triangular substrate arrays under a slowly rotated dc drive. A rich variety of lattice configurations associated with a series of steps in the velocity-force curves occur during the locking transitions. These include triangular, square, smectic, and disordered particle arrangements that can be identified using the structure factor. We show that the step widths vary with the ratio of the number of particles to the number of pinning sites. Unlike a static system, where matching effects occur at simple integer commensuration ratios, we find dynamical commensuration effects that arise when an integer number of particle chains flow between rows of pins. We identify two distinct types of locking as a function of substrate strength, distinguished by whether the particles flow along or between the pinning rows.     

Vibration and parametric excitation in asymmetric circular plates under moving loads

The natural frequencies and modes of transverse vibration of circular plates containing small imperfections are determined through a perturbation method. Incision of equally spaced, equal-size radial slots at the rim of the plate creates asymmetry in some, but not all, of the vibration modes, and it causes the repeated natural frequencies of these modes in the symmetric plate to split into two distinct values. These vibration modes are called the split modes, and those associated with the repeated natural frequencies are called the repeated modes. A relationship identifying the split and repeated modes for any configuration of slots is presented. The vibration of a plate containing any number of thin slots cut into it at the rim and with any number of rotating linear springs is analyzed. Parametric instability can be excited in the split modes of the plate by the springs rotating below critical speed, but it cannot be excited in the repeated modes. The response of the plate in forms such as traveling or standing waves at parametric resonance is discussed. The theoretical predictions of split and repeated vibration modes and of the excitation of parametric instability are confirmed by experiments.     

Theory of nonlinear excitation of surface polaritons in lossy media

The purpose of this paper is to study the nonlinear excitation of surface polaritons taking fully into account the damping of the active medium and the finite cross-section of the nonlinear polarization on the interface. This problem is solved using the guided wave calculation techniques where the EM field at the surface polariton frequency is expanded over a complete set of normal modes of the unperturbed interface. Using a “table method” we find that this set includes one guided mode, which is the surface polariton mode, and two classes of radiation modes. The expressions of all these modes are derived and interpreted physically. We then get the expression of the EM field excited at the surface polariton frequency inside and outside the pumped region and show that, in the general case, it is a mixture of all these normal modes. The end of the paper is mainly devoted to the study of the surface term occurring in the expression of the EM field at the surface polariton frequency: we point out the existence of a resonance phenomenon with two kinds of surface polariton modes: the “spatial” one and the “temporal” one. The corresponding dispersion curves, or resonance curves, are given and it is explained how each of them can be obtained experimentally.     

Guided wave dispersion curves for a bar with an arbitrary cross-section,a rod and rail example

Theoretical and experimental issues of acquiring dispersion curves for bars of arbitrary cross-section are discussed. Since a guided wave can propagate over long distances in a structure, guided waves have great potential for being applied to the rapid non-destructive evaluation of large structures such as rails in the railroad industry. Such fundamental data as phase velocity, group velocity, and wave structure for each guided wave mode is presented for structures with complicated cross-sectional geometries as rail. Phase velocity and group velocity dispersion curves are obtained for bars with an arbitrary cross-section using a semi-analytical finite element method. Since a large number of propagating modes with close phase velocities exist, dispersion curves consisting of only dominant modes are obtained by calculating the displacement at a received point for each mode. These theoretical dispersion curves agree in characteristic parts with the experimental dispersion curves obtained by a two-dimensional Fourier transform technique.     

Observations of ferromagnetic resonance modes on FeCo-based nanocrystalline alloys

Uniform and symmetric resonance modes (known as Aharoni’s exchange resonance modes) are derived from micromagnetic equilibrium condition in the linear approximation. To investigate the uniform and symmetric resonance modes in ferromagnetic nanoscale grains, the microwave permeability of FeCo-based nanocrystalline alloy particles/paraffin composites was measured and calculated in the range 0.5-18 GHz. The measured dynamic permeability curves exhibit a broad resonance band at 4-6 GHz; some curves also exhibit a narrow resonance band at 13 GHz. The former behavior is in qualitative agreement with the uniform mode, and the latter is attributed to the first eigenvalue mode of the symmetric resonance modes excited in nanocrystalline monodomain grains in FeCo-based alloys. The difference value (Δω₁₁) between the uniform resonance frequency and the first frequency eigenvalue of the symmetric resonance modes shows good agreement with experiment.     

基于包层模谐振的三包层石英特种光纤温度传感特性研究

提出了一种基于包层模谐振的光纤温度传感器. 它是通过将三包层石英特种光纤(TCQSF)两端分别与普通单模光纤(SMF)电弧熔接构成的SMF-TCQSF-SMF结构. 根据耦合模理论, 首先将TCQSF等效为三个同轴波导, 按各波导模场的分布特点标量计算其传输模式的色散曲线, 并深入研究其耦合长度与传输谱线之间的关系; 其次根据光纤的热光效应及热膨胀效应, 分析计算该传感器的温度灵敏度; 最后选取耦合长度为一个拍长时的传感器进行温度传感实验. 实验结果表明, 在35-95 ℃的温度变化范围内, 其温度灵敏度为73.74 pm/℃, 与理论计算结果一致. 因此, 该传感器具有结构简单、制备容易、灵敏度高、包层模激发可控等优点, 可用于工业生产、生物医学等温度传感领域.     

Resonant modes and inter-well coupling in photonic double quantum well structures with single-negative materials

We investigate the resonance tunneling modes of photonic double quantum well made of two photonic crystals with two different single-negative materials. It is found that these resonance modes split pairs, due to a coupling between two photonic wells. It is observed that when two photonic quantum wells are far away from each other, resonance modes appear as a single peak. And the quality factors of the transmittance resonance peaks can be greatly improved by increasing the period number of the outer barrier. The effects of the losses coming from ENG and MNG materials on the resonance modes are also specifically explored.     

Rotational stabilization of resistive wall modes by the shear alfvén resonance

It is found that resistive wall modes with a toroidal number n = 1 in tokamaks can be stabilized by plasma rotation at a low Mach number, with the rotation frequency being lower than the ion bounce frequency but larger than the ion and electron precession drift frequencies. The stabilization is the result of the shear-Alfvén resonance, since the thermal resonance effect is negligible in this rotation frequency range. This indicates that tokamaks can operate at normalized pressure values beyond the no-wall stability limit even for low values of plasma rotation, such as those expected in fusion reactor scale devices.     

Superradiance resonance cavity outside rapidly rotating black holes

We discuss the late-time behavior of a dynamically perturbed Kerr black hole. We present analytic results for near-extreme Kerr black holes that show that the large number of virtually undamped quasinormal modes that exist for nonzero values of the azimuthal eigenvalue m combine in such a way that the field oscillates with an amplitude that decays as 1/t at late times. This prediction is verified using numerical time evolutions of the Teukolsky equation. We argue that the observed behavior can be understood in terms of the presence of a "superradiance resonance cavity" immediately outside the black hole, and discuss whether it may be relevant for astrophysical black holes.     

Experimental modulation bandwidth beyond the relaxation oscillation frequency in a monolithic twin-ridge laterally coupled diode laser based on lateral mode locking

Monolithic twin-ridge laterally coupled diode lasers emitting at 1.3microm are presented that have a small-signal modulation bandwidth beyond the relaxation oscillation frequency of a single ridge. Spectra and spectrally resolved far fields are presented for three bias conditions: only one ridge lasing, both ridges lasing just above threshold, and both ridges lasing at biases well above threshold. In the first two cases the spectrum has single-peaked longitudinal modes, whereas the third cases shows splitting to in-phase and out-of-phase modes. The splitting frequency of the optical spectrum is measured to be 7.7 GHz. Small-signal modulation measurements reveal a strong resonance at 7.7 GHz, demonstrating an effect of lateral mode locking. As a result of this effect, the twin-ridge laser can be made to have a -3-dB bandwidth beyond that associated with its relaxation oscillation frequency.     

Theoretical Research on Sensitivity for Resonance Wavelength of Long Period Fiber Grating

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