Wavefront Curvature Effects Research Articles

Four-dimensional rough surface scattering realizations for multilayered fluid-elastic media

Broadband synthetic seismograms of scattering from two dimensionally rough surfaces in a three dimensional, but horizontally stratified media are presented. These results are obtained using the elastic perturbation theory of Kuperman and Schmidt [J. Acoust. Soc. Am. 86, 1511–1522 (1989)] under the Born approximation, for scattering scenarios where rough surfaces of finite extent, which are different on each interface, are insonified by an acoustic point source. These results, while constrained to the single scattering physics of perturbation theory, graphically illustrate the conversion of acoustic energy into trapped Scholte interface modes on elastic half-spaces, and Von Schmidt, antisymmetric, horizontally polarized shear (SH) and Love waves in elastic layers, as well as into a continuous scattered spectrum in the water column. Examples include a small compact scatterer attached to an elastic plate insonified by an acoustic point source, where forward scattering into the flexural mode, backscattering into the quasilongitudinal mode and out-of-plane scattering into the SH mode may be observed. Point source scattering from a larger scattering region is also illustrated, where the effects of wave front curvature become important. The scattering region, which is a stochastic realization of an isotropic power law surface, excites forward-scattered flexural waves in a broad beam, but it is continuously observed that the off axis response is dominated by shear waves. Other scattering scenarios are also presented where the three-dimensional aspects of scattering continue to be important, even for plane wave incidence. [Work supported by ONR.]

Nonlinear interaction of crossing FM waves with a gas

The new nonlinear phenomenon of the appearance of the modulation of the intensity of the frequency-modulated travelling wave saturated absorption medium is investigated. This effect enables one to estimate the shifts of the stabilized laser frequency caused by the wavefront curvature effect.

Self-bending and asymmetric spatial self-phase modulation effects in nematic liquid-crystal films

Laser-induced beam bending and the far-field asymmetric diffraction ring pattern due to the self-bending and asymmetric spatial self-phase modulation (SPM) effects in nematic liquid-crystal films are investigated. The measured far-field patterns are in good agreement with numerical calculations based on the diffraction theory, taking into account the two dimensionality of the problem, the nonlocal effect, and the wave-front curvature effect. The individual contributions of the aforementioned effects to the far-field pattern are analyzed for the first time. It is shown that the outermost diffraction peak of the far-field pattern is not the strongest one and that a large fraction of the transmitted intensity remains in the central region (within ±6 mrad of the beam axis). Merging of some of the diffraction peaks in this region should also occur. Although the transmitted beam does deflect, the deflection angle does not increase monotonically with the incident laser intensity, as is the case for a purely Kerr-like medium. In the experiments discussed here, higher-order effects are present. As an example, it is shown that equally pronounced third- and fifth-order nonlinear optical SPM effects are responsible for an observed five-ring asymmetric ring pattern.

Effect of wavefront curvature within the cavity on the shift of the saturated-absorption resonance

The shift of a nonlinear resonance due to wavefront curvature is shown to be proportional to the difference of the intensities of the counterpropagating waves with in a cavity. The depends on the direction of observation shift is calculated to be, i.e. from the left or to the right of the resonator.

Spherical-wave scattering by a finite-thickness solid plate of infinite lateral extent, with some implications for panel measurements

The solution to the problem of the interaction of a spherical wave with a homogeneous and isotropic solid plate of infinite lateral extent, but finite thickness, is considered theoretically. Both the source and the plate are immersed in an infinite, inviscid fluid. Appropriate boundary conditions are imposed on the full three-dimensional elasticity equations. The solution is evaluated numerically for a variety of materials for a 1-kHz incident spherical wave and for a 5-kHz incident spherical wave. For the numerical study, the fluid medium is taken to be water. Under certain conditions, ‘‘overpressures’’ are predicted for both the reflected and transmitted fields (i.e., the amplitude of the reflected pressure and/or the transmitted pressure can exceed the maximum value of the amplitude of the incident pressure on the plate surface). These overpressures are consistent with the law of conservation of energy in the sense that, for a plate composed of lossless material, the total incident power is found to be equal to the sum of the total reflected power plus the total transmitted power. An important conclusion of this research is that the practice of attempting to reduce the influence of edge diffraction in panel tests by using samples of increasingly larger lateral extent may result in measurements that are substantially corrupted by wave-front curvature effects, particularly if the sample panel includes a steel backing plate.

A compact automatic wavemeter for use with tunable infrared diode lasers

A laser wavemeter based on a fringe-counting Michelson interferometer is described and its use with infrared tunable diode laser (TDL) sources is demonstrated. The wavemeter features compact vacuum-tight construction, the use of a vernier technique to ensure integral fringe counts, and the use of an inexpensive home computer circuit board for direct automatic read-out of the TDL wavenumber or wavelength. The design emphasizes compactness and ease of use at some expense in accuracy, which is limited mainly by TDL alignment and wavefront curvature effects. For routine use in the 5 to 10 μm region, the wavemeter is reliable to about 0.03 cm−1 in unfavorable cases, and a factor of ten better ordinarily. With extra care in alignment of the TDL beam, or with a single “calibration” in a given region, accuracies of 0.001 cm−1 may be obtained (better than 1 part in 106).

Lower-hybrid wave resonance cone detection via CO2-laser scattering

Lower-hybrid waves are studied in the Princeton ACT-I steady-state toroidal plasma device using a radially scanning CO2 laser scattering system with both amplitude and phase sensitive detection techniques. Clearly defined resonance cones launched from external electrostatic antennas are seen to disappear as the plasma density is raised. Scaling of a lower-hybrid wave (LHW) laser signal with radio-frequency (rf) power in the presence of resonance cones shows nonlinearities associated with rf-induced changes in the effective laser scattering volume. Absolute fluctuation level estimates suggest that this occurs when eΦ/Te≥1. Wave-front curvature effects can cause a complete loss of resonance cone laser signals, even though probes indicate that cones are still present. Measurements of the wave k⊥ spectrum in the plasma show direct evidence for electron Landau filtering of the original wave k∥ spectrum launched from the antenna at the plasma edge, and strong dependence on antenna phasing. Finally, frequency shifts and loss of the resonance cone signal are associated with high levels of plasma density edge turbulence.

The effect of wavefront curvature on the coherence properties of laser light scattered by target centres in uniform motion

Moving speckle phenomena produced in the presence of wavefront curvature when laser light is scattered by an assembly of uniformly moving target centres are investigated. The first- and second-order space-time correlation functions of the scattered radiation are calculated and it is shown that the conditions for cross-spectral purity are not satisfied. It is pointed out that the effects offer a means of measurement of the velocity of the scatterers.

The accuracy of acoustic reconstructions of lightning channels

The effects of wave front curvature, wind, and temperature on the determination of the direction of propagation of thunder at an array of microphones is examined, and it is shown that the direction of propagation can be measured at the array with an error of less than ±1° under the following conditions: the wind velocity at the array should be measured with an accuracy of ±1 m/s, the temperature at the array with an accuracy of ±2°C, and the range to the source with an accuracy of ±20%. The positions of the source points on the lightning channel, which represent the total lightning event at that location, are mapped by acoustic ray tracing techniques; this procedure requires information on the temperature and wind velocity as a function of altitude; without this information and in adverse conditions, errors of 10% for high-altitude sources and 25% for sources near the horizon will occur. By using supportive data on wind and temperature profiles all error estimates can be reduced to 5%, which is more than sufficient to do comparative studies of lightning channel structures inside clouds with the macrophysical properties of the clouds.

Path Length Dependence of Optical Homodyne Signal Strength

A theoretical analysis has been performed in order to determine optical homodyne signal strength as a function of path length difference between coherent spherical reference and signal waves. Signal strength is found to decrease with increasing path length mismatch. Experimental verification of the results are also given, along with methods for minimizing the detrimental effect of wavefront curvature.