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The small-angle approximation for the radiative transfer theory is based on the assumption that the scattered wave is confined within a small angle in the forward direction. This assumption is generally valid for the turbulence case in which the turbulence size is much greater than the wavelength and the index of refraction is close to 1. However, the validity of this assumption for a discrete random medium with an index of refraction much greater than the surrounding medium is not well known. In spite of this, the small-angle approximation has sometimes been used for discrete random media because of its mathematical simplicity.

In recent years there has been increasing interest in the use of polarization for imaging objects in a cluttered environment. Examples are optical imaging through clouds, optical detection of objects in a biological medium, and microwave detection of objects in clutter. We extend previous studies of continuous-wave scattering to pulse-polarization scattering in discrete scatterers. We solve the time-dependent vector radiative transfer equation for a plane-parallel medium by using Mie scattering and the discrete ordinates method. The time-dependent degree of polarization and cross-polarization discrimination are calculated and verify the advantages of circular over linear polarization in maintaining greater copolarized components rather than cross-polarized components.

There is an increasing interest in free space optics (FSO) communication systems among data service providers. The cost of installing a new fiber network in an urban environment is often very expensive, and a less expensive method is needed. A high-speed data link, for example, can be designed using either optical or millimeter-wave (MMW) point-to-point communication systems. The optical system is often the preferred method because of its simplicity and security. The data rate for short distance FSO systems in an ideal condition can exceed one Gbit/sec. However, both MMW and optical links are susceptible to adverse weather conditions. The traditional method to estimate the effects of an atmospheric channel is to calculate the attenuation characteristics based on the size distribution and concentration of particles along the signal path length. From the signal attenuation rate, the bit-error rate is usually estimated. This approach, however, does not consider the propagation characteristics of modulated signals. We give a numerical method to calculate the waveform of an intensity modulated optical signal through a layer of fog and cloud. Our approach takes into account the multiple scattering effects and reveals a substantial amount of waveform distortion.

Fog is a highly dispersive medium at optical wavelengths, and the received pulse waveform may suffer significant distortion. Thus it is desirable to have the impulse response of the propagation channel to recover data transmitted through fog. The fog particle density and the particle size distribution both strongly influence the channel impulse response, yet it is difficult to estimate these parameters. We present a method using a dual-wavelength free-space optical system for estimating the average particle diameter and the particle number density and for approximating the particle distribution function. These parameters serve as inputs to estimate the atmospheric channel impulse response using simulation based on the modified vector radiative transfer theory. The estimated channel response is used to design a minimum mean-square-error equalization filter to improve the bit error rate by correcting distortion in the received signal waveform due to intersymbol interference and additive white Gaussian noise.

Many random media such as clouds and fog are often inhomogeneously distributed and often have a layered structure. This paper presents the effects of such inhomogeneities on the image transmission in terms of the modulation transfer function (MTF). We derive an expression of the MTF for a layered inhomogeneous medium using the small-angle approximation which is valid for size parameters of >10 and optical distances of

Recently, the backscattering enhancement by densely distributed particles of a size comparable to the wavelength was reported. It has been explained as the constructive interference of two waves traveling in opposite directions. This enhancement was observed only in densely distributed particles, and its existence in sparsely distributed media has not been verified yet. In this paper we present the experimental evidence of backscattering enhancement by sparsely distributed very large particles. Experiments are conducted using 45-microm latex particles which are approximately 100 times the wavelength. Both copolarized and crosspolarized components are measured for different particle concentrations. Unlike for small particles, backscattering enhancement is most noticeable when the particle concentration is low. The angular width of the peak is comparable to the ratio (wavelength)/(particle size) and is independent of the optical distance.