Search

The applicability limits of the small complex phase perturbation (Rytov) theory for laser light propagation in soft biological tissues have been tackled. We have found that for typical coherent opt...

We compare experimental results for s- and p-polarized light scattered specularly and diffusely from two-dimensional randomly rough metal surfaces with the predictions of three perturbation theories and the Kirchhoff approximation. The wavelength of the incident field is m. We conclude that the results of phase perturbation theory are closest to all the experimental results in both s- and p-polarization for the reflectivities as well as for the contribution to the mean differential reflection coefficient from the diffuse component of the scattered light for in-plane, co-polarized scattering.

Sea surface motions can produce different measured Doppler shifts with respect to instrumental configurations (incidence angle, electromagnetic wavelength, polarization). Under Gaussian statistics for the sea surface elevation and in the general framework of asymptotic theories for ocean surface electromagnetic wave scattering, Doppler shifts can be predicted. The small-slope, Kirchhoff, local curvature and resonant curvature approximations are compared in the backscatter configuration. Predicted Doppler shifts for Kirchhoff and small-slope approximations in co-polarized configuration are insensitive to the polarization state. On the other hand, the local and resonant curvature solutions, through a phase perturbation formalism, yield to significant differences between co-polarized predicted Doppler shifts. Comparisons with data are shown to confirm the polarization and wind speed sensitivities.

In this survey, we fully review almost all potential ionospheric effects on the performance of space-based radar systems (SBRs), which operate in the ambient ionosphere environment; in particular, we review the use of space-based synthetic aperture radar systems (SARs) for imaging. There are two families of effects involved. One is the effects of the background ionosphere (non-turbulent ionosphere), such as dispersion, group delay, refraction, Faraday rotation, and phase shift. The other is the effects due to ionospheric irregularities, such as refractive index fluctuation, phase perturbation, angle-of-arrival fluctuation, pulse broadening, clutter, and amplitude scintillation. These effects adversely affect SAR imaging in several respects, such as by causing image shift in the range, and degradations of the range resolution, azimuthal resolution, and/or the resolution in height (elevation). We also review ionospheric irregularity characteristics and descriptions, propagation channel statistics, ...

We study the scattering of a scalar plane wave from a two-dimensional, randomly rough surface, on which the Dirichlet boundary condition is satisfied. The scattering amplitude is obtained in the form of the Fourier transform of an exponential, in which the exponent is written as an expansion in powers of the surface profile function. It is shown that the latter expansion can be written in such a way that the corresponding scattering matrix is manifestly reciprocal. Numerical results for the reflectivity, and for the contribution to the mean differential reflection coefficient from the incoherent component of the scattered field, are obtained and compared with the predictions of small-amplitude perturbation theory and the Kirchhoff approximation. As the wavelength of the incident wave is varied continuously the results of the phase-perturbation theory change continuously from those of the small-amplitude perturbation theory to those of the Kirchhoff approximation.

A novel phase-perturbation method as well as an experimental system were established to study the deviation of the laser diode (LD) far-field intensity distribution from the ideal Gaussian. An analytical expression of the LD far-field intensity distribution and a numerical method based on the Monte Carlo calculation were developed to fit the measured results. The loss of the LD to the single-mode fiber (SMF) coupling system caused by this kind of deviation of the LD far-field intensity distribution was quantitatively analyzed.

The phase perturbation technique is numerically examined in the case of scalar waves scattered by one-dimensional normally distributed random rough surfaces, on which Dirichlet boundary conditions hold. Particular attention is devoted to surfaces for which the Rayleigh parameter has intermediate or large values. It is shown that the considered method is not limited to either small roughness or gentle undulations. It is demonstrated that the phase perturbation technique smoothly interpolates between the two classical approximations, namely the perturbation approach and the Kirchhoff method. The cases in which the phase perturbation theory converges to physical optics are characterized by large values of the Lynch parameter. A conclusion is drawn that the phase perturbation technique is amenable to surfaces whose roughness spectra are wide. Asymptotic expansions, simplifying the evaluation of the phase perturbation backscattering cross-section in the high roughness limit are derived.

There is a need for a solution to the problem of wave scattering from rough surfaces that is accurate when both the classical field perturbation and Kirchhoff approximations are not. In previous work it was shown numerically that the phase perturbation reflection and backscattering coefficients reduce to those of field perturbation theory and the Kirchhoff approximation in the appropriate limits; however, for cases when the classical solutions did not apply, the accuracy of the phase perturbation results could not be determined. In this paper we examine the validity of the phase perturbation technique for a region in parameter space when the two classical solutions fail. Numerical results for the bistatic scattering cross section are compared with exact numerical results for one-dimensional surfaces having a Gaussian roughness spectrum and satisfying a Dirichlet boundary condition. It is found that in the region considered the phase perturbation results agree with the exact results over all scattered angl...

Moisture measurements have been made from the phase-perturbation response of a microwave quarter-wave coaxial cavity by a wct paper sheet. In one method the phase (φ) response is measured directly as a function of the moisture content (md) at a fixed frequency (f0) equal to the resonant frequency of the moisture detector. This single-frequency approach has two essentially linear response regions, at 0–10 per cent and 40–100 per cent, and a non-linear response from 10 to 40 per cent. The accuracies in the 0–10 per cent and 40–100 per cent ranges are respectively ±0.3 per cent and ±11/2 per cent. These results were determined from measurements on several samples. In a second method the phase-frequency slope dφ/df is shown to be a function of the moisture content (md) and the base frequency (f). This method has interesting wide linear response ranges (e.g. 0–90 per cent) depending on the choice of the base frequency. In a third method the phase-frequency slope (dφ/df) is used to optimize the base fre...