Two-mode Excitation Research Articles

Conversion of waveguide modes in a periodic optical fiber

We investigate the distinctive features of the propagation and interaction of linearly polarized modes of an unperturbed step-index fiber, perturbed by a periodic distribution of the dielectric constant along the axis of the optical fiber. An analysis is made for unidirectional and opposite modes and two-mode excitation of the optical fiber.

Multi-Mode Control of Structures by Using Neural Networks with Marquardt Algorithms

This paper presents an integrated approach of identification, estimation and vibration control of smart structures by using neural networks with Marquardt adaptation algorithm. An identification neural network is first constructed to model the system dynamics of a composite laminate structure embedded with piezoelectric sensor and actuator. Based on the identification network, a neural controller is then developed to meet the vibration suppression performance specified by a reference model. In addition, an estimation neural network is also developed that enables the measurement of a single piezoelectric sensor to represent the state variables of displacement and velocity of the structure. The neural controller can then minimize the structure vibration by using a pair of piezoelectric sensors and actuators. Experimental verification shows that the vibration amplitude of the smart structure under two-mode harmonic excitation and random excitation can be reduced by 80% and 60%, respectively.

Theoretical investigation of the C60 infrared spectrum.

A semiempirical model of the infrared (IR) spectrum of the ${\mathrm{C}}_{60}$ molecule is proposed. The weak IR-active modes seen experimentally in a ${\mathrm{C}}_{60}$ crystalline sample are argued to be combination modes caused by anharmonicity. The origin of these two-mode excitations can be either mechanical (anharmonic interatomic forces) or electrical (nonlinear dipole-moment expansion in normal mode coordinates). It is shown that the electrical anharmonicity model exhibits basic features of the experimental spectrum while nonlinear dynamics would lead to a qualitatively different overall picture. \textcopyright{} 1996 The American Physical Society.

Density-fluctuation spectra of dilute mixtures ofHe3in superfluidHe4

The density-fluctuation spectra S/sub i/j(k,..omega..) (i,j = 3,4) are evaluated with a matrix dispersion-relation representation in terms of characteristic frequencies and complex self-energies by the incorporation of damping via decay into /sup 3/He quasiparticle-quasihole excitations and by the use of renormalized fluctuation frequencies. The hybridization of /sup 4/He rotons with /sup 3/He quasiparticle excitations is too small to induce a peak splitting in S/sub 44/(k,..omega..) at the level crossing while the effective phonon-roton exchange potential between /sup 3/He quasiparticles splits the spectrum S/sub 33/(k,..omega..) into two peaks. The energy shift of the main neutron scattering intensity with respect to pure /sup 4/He agrees semiquantitatively with experiments. It is caused by partly competing effects listed in the order of their importance: frequency changes induced by the altered structure of /sup 4/He in the mixture, coupling between /sup 4/He and /sup 3/He restoring forces, hybridization with /sup 3/He quasiparticle-quasihole excitations, and the contribution of S/sub 34/(k,..omega..). Decay into two-mode density excitations is investigated within a mode-coupling approximation.

Mode selection and intensity enhancement of x rays in crystals

Previously unreported intensity enhancement of x rays obtained by two-mode excitation is observed on the direct incident beam of synchrotron radiation. The excitation utilizes the coherent interaction of a six-beam Borrmann diffraction in a perfect silicon single crystal. The experiment provides a means of obtaining nearly divergenceless, monochromatic, spatially coherent, and intense x-ray sources for x-ray diffraction and imaging studies. Single mode excitation can be achieved by using a thick crystal.

Simultaneous two-mode ultrasonic excitation, by means of disoriented CdS layers

A technique for the generation of ultrasonic waves from highly disorientated CdS layers results in the simultaneous excitation of both the longitudinal and transverse acoustic modes in the substrate, so providing a means for the measurement of the two associated elastic constants. This technique therefore overcomes the difficulties encountered with the usual method, where it is necessary to work with two separate transducers, and further extends the range of applications for CdS films. Experiments have so far been performed on two single-crystal samples, Al2O3 and V3Si (an A15 compound).

Collective Excitations in the Classical One-Component Plasma at High Densities and Intermediate Wave Numbers

Density- and current-fluctuation spectra of a dense, classical one-component plasma are discussed within a simple model based on the mode-coupling theory of liquid dynamics. The appearance of a second high-frequency shear mode at intermediate wave numbers is explained in terms of a level repulsion effect between the bare transverse current mode and the two-mode excitation spectrum. The same interaction with the longitudinal current mode is responsible for a small additional high-frequency peak in the densityfluctuation spectrum.