IDENTIFICATION OF MODAL PARAMETERS FROM MEASURED OUTPUT DATA USING VECTOR BACKWARD AUTOREGRESSIVE MODEL

In this paper, a modal identification system that is based on the vector backward autoregressive (VBAR) model has been developed for the identification of natural frequencies, damping ratios and mode shapes of structures from measured output data. The modal identification using forward autoregressive approach has some problems in discriminating the structure modes from spurious modes. On the contrary, the VBAR approach provides a determinate boundary for the separation of system modes from spurious modes, and an eigenvalue filter for the selection of physical modes is existent in the proposed method. For convenience of application, the backward state equation established from VBAR model is transformed into a forward state equation, which is termed as transformed VFAR model in this paper. In addition, the extraction of equivalent system matrix of state equation of motion for structures from the transformed VFAR model has been developed, and then the normal modes can be calculated from the identified equivalent system matrix. Two examples of modal identification are carried out to demonstrate the availability and effectiveness of the proposed backward approach: (1) Numerical modal identification for a three-degree-of-freedom dynamic system with noise level in 20% of r.m.s of measured output data; (2) experimental modal identification of a cantilever beam. Finally, to show the advantage of the proposed VBAR approach on the selection of physical modes, the modal identification by stochastic subspace method was performed. The results from both methods are compared.     

Counterdirectional mode coupling in ring resonators with QPM nonlinear crystals and effects on the characteristics of cw optical parametric oscillation

The effects of coupling between clockwise and counterclockwise modes in ring resonators due to back reflections from QPM gratings in nonlinear crystals are investigated. We demonstrate, using simulations, that the non-phase-matched back reflection due to typical imperfections of the QPM grating has a relevant magnitude and can give rise to the coupling. We developed an analytical model and evaluated parameters of a singly resonant OPO in the presence of this coupling, which indicates that the threshold pump power is doubled in the limit of strong coupling. We also evaluated the effect of the coupling with respect to resonant power enhancement for cavity-enhanced nonlinear frequency mixing applications, finding that the enhancement is reduced by up to a factor four. Experimental observations based on PPLN and orientation-patterned GaAs crystals and model predictions are in good agreement. We propose and demonstrate a method for active stabilization of ring resonators with mode coupling that relies on the dissimilarity of the coupled forward and backward modes.     

Coupling between counterpropagating cladding modes in fiber Bragg gratings

We present an experimental demonstration of energy transfer between counterpropagating cladding modes in a fiber Bragg grating (FBG). A strong FBG written in a standard photosensitive optical fiber is illuminated with a single cladding mode, and the power transferred between the forward propagating cladding mode and different backward propagating cladding modes is measured by using two auxiliary long period gratings. Resonances between cladding modes having 30 pm bandwidth and 8 dB rejection have been observed.     

少模光纤布拉格光栅折射率传感的分析与测量

理论分析和模拟计算了少模光纤布拉格光栅基模及高阶模的耦合与传输特性,得到在相同外部折射率变化情况下,少模光纤基模与高阶模耦合对应的布拉格波长变化,比正、反向基模之间耦合对应的布拉格波长变化显著增大.实验上制作了少模光纤布拉格光栅,测量了基模之间以及基模与高阶模之间对应的布拉格波长随外部折射率、温度变化的情况,得到与理论分析相符的结果.而对于温度变化对折射率测量结果干扰的问题,提出了通过计算布拉格波长差来克服温度影响的方法.这些结果为采用布拉格光纤光栅测量外部折射率变化提供了一种新的途径.     

Study of the competition between forward and backward stimulated Raman scattering in methane

Simultaneous forward and backward stimulated Raman scattering (SRS) in methane pumped by a single longitudinal mode Nd:YAG laser at 355 nm has been investigated both experimentally and numerically. For forward SRS, a maximum energy-conversion efficiency of 55% was obtained under 24 atm, while for backward SRS the corresponding figure was 58% under 4.7 atm. Under low pressures or at low pump energies, the conversion efficiency for backward SRS was found to be much higher than that for forward SRS, while the situation is quite the contrary under high pressures and at high pump energies. A quasi-two-dimensional numerical model is introduced to elucidate the competition between forward and backward SRS; we conclude that the high-pressure and high-pump-energy experimental results should be caused by the thermal defocusing effect in the SRS process. This is confirmed by the experimental fact that a decrease of the pulse-repetition rate increases the conversion efficiency for backward SRS but decreases that for forward SRS under high pressures. Our calculated results are in good agreement with the experimental results at a pulse-repetition rate of 2 Hz. PACS 42.55.Ye; 42.65.Dr; 42.65.Es     

ANALYSIS OF THE PERIODIC PRESSURE FLUCTUATIONS INDUCED BY FLOW OVER A CAVITY

Subsonic flows over Helmholtz resonators often cause strong periodic pressure fluctuations inside the resonators over a range of outer flow velocities. The flow-excitation mechanism is known to be governed by both the shedding of discrete vortices within the shear layer over the orifice and the acoustic response of the cavity. This self-sustained oscillation phenomenon is often analyzed by using a feedback loop model where the flow excitation and the acoustic response of the resonator are approximately modelled as a forward gain function and as a backward gain function respectively. In the present work, a similar approach was followed and a new forward gain function was derived based on the concept of “vortex sound” to model the flow excitation. The formulation combined this forward gain function with a backward gain function from previous work, within the framework of the feedback loop analysis. The approximate method allowed the frequency and the relative amplitude of the cavity pressure fluctuations to be predicted for a range of flow velocities. In addition, the extended Nyquist stability criterion was used to estimate the onset and the termination velocities of the first two modes of the shear layer flow oscillations. Experimental data were obtained using a rigid-walled cavity in a low-speed wind tunnel. The results showed that the model predictions were in reasonably good agreement with the experimental data.     

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.     

Fused angle-polished multi-points side-pumping coupler for monolithic fiber lasers and amplifiers

We have developed theoretical model to investigate characters of fused angle-polished side-pumping coupler for monolithic fiber lasers and amplifiers. In the simulation the interplay of different couplers is considered. Five pumping fibers with different polished angles are employed in the experiment. The results showed that, the backward coupling efficiency, forward and backward pumping leakage are lower than 4%, 8% and 10% respectively, while the forward coupling efficiency is about 100%, with the polished angle suitable for commercial fibers. The experimental results agree well with the simulations. Furthermore, we fabricate a two-points co-propagating fused angle-polished side-pumping coupler, with the coupling efficiency of 94.1% and 87.9%. These couplers are quite suitable for multi-points side-pumping high power monolithic fiber lasers and amplifiers.     

A cellular automata model for highway traffic

We present results relative to a simple cellular automata model without periodic boundary conditions for an highway with on-ramps. Simulations performed with this model reproduce experimental phenomena observed in traffic such as free flow, synchronized flow, congested flow, lane inversion, forward and backward propagating waves. On-ramps play the important role of nucleation points for the dynamic features of traffic. Received 4 February 2000 and Received in final form 5 May 2000     

Characteristics of guided waves in indefinite-medium waveguides

We have presented a discussion on the guided waves in the indefinite-medium waveguides. The characteristic equations describing TE and TM guided waves have been derived. Aiming mainly at the TE guided waves, we have analyzed the existence conditions for the guided and surface modes for four distinct cases, respectively. A number of exotic properties have been revealed in such waveguides, such as the coexistence of forward and backward modes, the absence of fundamental modes, the existence of surface modes and the relaxed requirements for waveguide core constitutive parameters. Numerical results have confirmed our analyses.     

Nonlinear guided waves and symmetry breaking in left-handed waveguides

We analyze nonlinear guided waves in a planar waveguide made of a left-handed material surrounded by a Kerr-like nonlinear dielectric, and predict that such a waveguide can support fast and slow symmetric and antisymmetric nonlinear modes. We study the symmetry breaking bifurcation and asymmetric modes in such a symmetric structure. The analysis of nonlinear dispersion properties of the guided waves shows that the modes can be either forward or backward.     

Energy Exchange at Degenerate Mixing of Four Fast TE 2 Modes of a Thin Left-Handed Film on a Nonlinear Substrate

Frequency-degenerate mixing of forward and backward propagating fast modes is considered in a planar optical waveguide based on a thin left-handed film and a right-handed substrate exhibiting the Kerr effect. Nonlinear coupled-mode equations are solved for the intensities of these modes in the neglect of the contributions of elementary processes that fail to satisfy the phase-matching condition. It is shown that the intramode self-action of four TE modes is accompanied by energy exchange between these modes. The efficiency of energy exchange varies periodically along the propagation direction of modes, thus leading to the inversion of the intensities of forward and backward propagating light fields on the interaction half-period.     

Forward and Backward Bellman Equations Improve the Efficiency of the EM Algorithm for DEC-POMDP

Decentralized partially observable Markov decision process (DEC-POMDP) models sequential decision making problems by a team of agents. Since the planning of DEC-POMDP can be interpreted as the maximum likelihood estimation for the latent variable model, DEC-POMDP can be solved by the EM algorithm. However, in EM for DEC-POMDP, the forward–backward algorithm needs to be calculated up to the infinite horizon, which impairs the computational efficiency. In this paper, we propose the Bellman EM algorithm (BEM) and the modified Bellman EM algorithm (MBEM) by introducing the forward and backward Bellman equations into EM. BEM can be more efficient than EM because BEM calculates the forward and backward Bellman equations instead of the forward–backward algorithm up to the infinite horizon. However, BEM cannot always be more efficient than EM when the size of problems is large because BEM calculates an inverse matrix. We circumvent this shortcoming in MBEM by calculating the forward and backward Bellman equations without the inverse matrix. Our numerical experiments demonstrate that the convergence of MBEM is faster than that of EM.     

400 AMeV 12C诱发乳胶核反应重电离粒子前后关联研究

对400 AMeV 12C诱发乳胶核反应重电离粒子的前后关联进行了研究。重电离粒子来源于靶核碎片,分为灰径迹粒子和黑径迹粒子两种。实验结果很好地验证了核-核碰撞几何模型。灰径迹粒子的发射在前、后半球是各向异性的,而黑径迹粒子的发射基本是一致的。向前、向后发射的灰径迹粒子、黑径迹粒子平均多重数对重电离粒子数n_h呈线性依赖,黑径迹粒子、灰径迹粒子的向前发射随n_h的增加而增加的幅度分别大于其向后发射随n_h的增加而增加的幅度,但核内级联效应受靶核大小的影响,向后半球内发射的灰径迹粒子平均多重数随n_h的增加有饱和现象,这些多重数关联特性基本上可以利用基于碰撞几何的旁观体-反应体模型及级联蒸发模型来解释。The forward-backward multiplicity correlation of heavily ionized particles produced in 12C-emulsion interactions at 400 AMeV is investigated. The heavy ionized particles, come from the target fragments, are divided into grey track particle and black track particle. The experimental results can be well explained by the geometry model of the nucleus-nucleus collisions. The emission of grey track particles in forward and backward hemisphere is not isotropic, but the emission of black track particle is almost isotropic. The averaged multiplicity of grey track particles and black track particles in forward and backward hemisphere linearly depend on the number of heavily ionized particle n_h, the correlation strength in forward hemisphere is greater than that in backward hemisphere, but the dependence of grey track particle in backward hemisphere on the number of heavily ionized particle n_h shows the saturation because of the intranuclar cascade effect is influenced by target size. The characteristics of multiplicity correlations can be well explained by the participant-spectator model based on the colliding geometrical picture and the cascade evaporation model of high energy nucleus-nucleus collisions.     

Anomalous dispersion and group velocity properties in subwavelength uniaxial metamaterial waveguide with metal cladding

We investigate propagation behavior of electromagnetic waves in a slab uniaxial metamaterial waveguide with metal cladding and discuss the dispersion relation, group velocity and energy flux distribution of transverse electric guided modes in two kinds of uniaxial metamaterial waveguides with different orientations of optical axis in detail. The numerical results show that there exist anomalous dispersion properties such as backward modes existing in low-frequency range, and the group velocity can be negative or approach zero. If the orientation of optical axis is selected properly, each guided mode has two dispersion curves—one is normal dispersion and another belongs to anomalous dispersion and shows a transition from a backward mode to a forward one with the angular frequency increasing. In addition, the condition of the existence of cutoff frequency is clarified. These properties may have potential applications in optical information storage, optical communication, integrated optics and nanophotonic devices.     

Spatial and temporal properties of parametric fluorescence around degeneracy in a type I LBO crystal

We present a theoretical and experimental study of the spatio-temporal properties of the spontaneous parametric emission generated in a type I LBO crystal around degeneracy. The number of quasi-phase-matched modes is shown to be equal to the number of spatio-temporal degrees of freedom of the image that can be parametrically amplified. From this number, we demonstrate the possibility of predicting the total number of photons generated by parametric fluorescence. Correlation is observed between spatial intensity fluctuations corresponding to pairs of signal-idler modes. Received 18 February 1999 and Received in final form 9 June 1999     

Two-dimensional array of room-temperature nanophotonic logic gates using InAs quantum dots in mesa structures

We present a detailed study of the dynamics of light in passive nonlinear resonators with shallow and deep intracavity periodic modulation of the refractive index in both longitudinal and transverse directions of the resonator. We investigate solutions localized in the transverse direction (so-called Bloch cavity solitons) by means of envelope equations for underlying linear Bloch modes and solving Maxwell’s equations directly. Using a round-trip model for forward and backward propagating waves we review different types of Bloch cavity solitons supported by both focusing (at normal diffraction) and defocussing (at anomalous diffraction) nonlinearities in a cavity with a weak-contrast modulation of the refractive index. Moreover, we identify Bloch cavity solitons in a Kerr-nonlinear all-photonic crystal resonator solving Maxwell’s equations directly. In order to analyze the properties of Bloch cavity solitons and to obtain analytical access we develop a modified mean-field model and prove its validity. In particular, we demonstrate a substantial narrowing of Bloch cavity solitons near the zero-diffraction regime. Adjusting the quality factor and resonance frequencies of the resonator optimal Bloch cavity solitons in terms of width and pump energy are identified.     

Transmission characteristics of wave modes in a rectangular waveguide filled with anisotropic metamaterial

In this paper, transmission characteristics of wave modes in a rectangular waveguide filled with lossless anisotropic metamaterial are theoretically investigated. The wave equation and dispersion relations for TE and TM modes in the waveguide are obtained and analyzed. It is shown that the negative constitutive parameters of the filling anisotropic metamaterial can be used to control the wave magnitude and the phase velocity direction in the waveguide over the entire frequency domain, both below and above the cutoff frequency. Particularly, not only backward waves, but also forward waves can propagate below the cutoff frequency in the waveguide. Furthermore, a typical example is calculated to demonstrate transmission characteristics of waves inside the waveguide. Numerical results are obtained in the paper and compared with theoretical predictions: a good agreement is found.     

Strong electromagnetic field localization near the surface of hemicylindrical particles

We theoretically study the forward and backward plane wave illuminations of hemicylindrical dielectric particles with different radii and refractive indices. Near field effects providing strong field enhancement and narrow beam width are obtained from hemicylindrical particles, which can be associated with whispering gallery modes (WGMs) and photonic nano-jets (PNJs). It should be noted that existence of PNJ and WGM effects depending on forward/backward light illuminations is studied for the first time in all-dielectric hemicylindrical particles. Compared to dielectric cylinders, hemicylindrical aided PNJs show relatively small influences of WGM regime, when exists, to PNJ behavior. Under the backward excitation, on the other hand, typical nano-jet is produced. The designed hemicylindrical structure could be implemented in high-resolution optical imaging and sensing applications.     

Wave Instabilities of a Collisionless Plasma in Fluid Approximation

Wave properties and instabilities in a magnetized, anisotropic, collisionless, rarefied hot plasma in fluid approx‐imation are studied, using the 16‐moments set of the transport equations obtained from the Vlasov equations. These equations differ from the CGL‐MHD fluid model (single fluid equations by Chew, Goldberger, and Low [5,9]) by including two anisotropic heat flux evolution equations, where the fluxes invalidate the double polytropic CGL laws. We derived the general dispersion relation for linear compressible wave modes. Besides the classic incompressible fire hose modes there appear four types of compressible wave modes: two fast and slow mirror modes – strongly modified compared to the CGL model – and two thermal modes. In the presence of initial heat fluxes along the magnetic field the wave properties become different for the waves running forward and backward with respect to the magnetic field. The well known discrepancies between the results of the CGL‐MHD fluid model and the kinetic theory are now removed: i) The mirror slow mode instability criterion is now the same as that in the kinetic theory. ii) Similarly, in kinetic studies there appear two kinds of fire hose instabilities ‐ incompressible and compressible ones. These two instabilities can arise for the same plasma parameters, and the instability of the new compressible oblique fire hose modes can become dominant. The compressible fire hose instability is the result of the resonance coupling of three retrograde modes ‐ two thermal modes and a fast mirror mode. The results can be applied to the theory of solar and stellar coronal and wind models (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)