Development Of Modulational Instability Research Articles

Modulation instability of light beams and pulses propagating in absorbing media

The development of modulation instability of light beams and pulses propagating in an absorbing medium is analyzed. Based on the proposed approach, the domain of increasing perturbations is estimated more exactly. It is shown that, compared to the traditional approach, this domain can be enlarged by 20–50% in frequency and by 50% in the gain increment. For femtosecond pulses, the analysis of the modulation instability is carried out both on the basis of the well-known Schrodinger equation with the time derivative of nonlinear response of the medium and by using the original method of its transformation. It is shown that in the latter case, the nonlinearity dispersion increases the interval of frequencies for which the modulation instability is developed. This more adequate description of the development of the modulation instability in an absorbing medium allowed us to obtain more accurately the frequency intervals of its realization compared to the data available in the literature.

Amplification of spin oscillation noise by self-modulation in a traveling magnetostatic wave

A theoretical analysis is made of the formation and evolution of modulational instability of a traveling magnetostatic spin wave in a ferromagnetic film. Results of earlier experiments are analyzed and it is demonstrated that the experimental results can be explained using a model describing the evolution of the signal wave and initial noise allowing for their nonlinear interaction. The instability process is simulated numerically and the results are compared with calculations using a deterministic model. Mechanisms are discussed for the loss of spectral symmetry in the formation of modulation frequency satellites.

Modulation instability in pulsed surface-wave sustained discharges

Experimental results showing development of modulation instability and tendency to soliton formation in pulsed gas discharges sustained by surface waves are presented. The measurements are performed in argon discharges at gas pressure 13-1.3/spl middot/10/sup 3/ Pa sustained by pulsed microwave power at 2.45 GHz, The changes along the discharge length of the shape of the pulses of the total light emission, which is an indication for the plasma density, and of the wave electric field creating the discharge, are registrated. The cases when the modulation instability arises from the level of fluctuations in the plasma and when it is forced by the applied signal producing the discharge are demonstrated. The obtained results are discussed in terms of weak nonlinearity superimposed on the mechanism of strong ionization nonlinearity responsible for the discharge creation itself. A separation of a solitary-like wave front the leading edge of the pulse is also reported as an experimental result.

Wind effects on the nonlinear evolution of slowly varying gravity—capillary waves

A train of uniform two-dimensional gravity waves in deep water is known to be unstable to certain sideband disturbances. If the time of propagation is sufficiently long for the fourth-order terms to be important, the sidebands may grow at unequal rates, resulting in a downward shift of peak frequency. But this shift is only a temporary phase of a recurrent evolution process. Recent work by us (Hara & Mei 1991) has shown that wind and dissipation can help maintain this downshift at large time. In this paper we examine a similar two-dimensional problem for capillary–gravity waves. The basic flow in air and water is assumed to be steady, horizontally uniform and turbulent; the wave-induced flow in both media is assumed to be laminar. Evolution equations are deduced with wind and dissipation included in such a way that their influence is comparable to the asymmetric spectral evolution. After finding the initial growth rates of unstable sidebands, the nonlinear development of modulational instability is examined by integrating the evolution equations numerically. Computed results show that persistent downshift of frequency can happen for relatively long waves, but upshift occurs for very short waves.

Stimulated plasma resonances as an indicator of near-satellite plasma modification by powerful radio emission

The plasma resonances stimulated by topside sounder radioemission were additional means for near satellite plasma diagnostics from the very beginning of topside sounding experiments. The wide-band receiver data on-board IK-19 satellite, as well as the discovery of sounder accelerated particle effects have shown that those phenomena do not keep within weak turbulance frames. It was shown that considerable modification of the ambient plasma in the 100–200 m vicinity takes place: the strictional extrusion, the development of modulational instability, remarkable heating of electrons and ions. On the normalized stimulated resonance distribution which is represented, the different resonance regions are discussed as a function of Langmuire and gyrofrequency, the different mechanisms of resonance excitation and wave-particle interaction are identified.