The presence of clouds of ice particles in the uplink and downlink path of an illumination beam can severely impede the performance of an active imaging system. Depending on the optical depth of the cloud, i.e., its density and depth, the beam can be completely scattered and extinguished, or the beam can pass through the cloud with some fraction attenuated, scattered, and depolarized. In particular, subvisual cirrus clouds, i.e., high, thin cirrus clouds that cannot be observed from the ground, can affect the properties and alignment of both uplink and downlink beams. This paper discusses the potential for active imaging in the presence of cirrus clouds. We document field data results from an active imaging experiment conducted several years ago, which the authors believe to show the effects of cirrus clouds on an active imaging system. To verify these conclusions, we include the results of a simulation of the interaction of a coherent illumination scheme with a cirrus cloud.

A new technique for modeling image transfer through cirrus clouds is presented. The technique uses a ray trace to model beam propagation through a three-dimensional volume of polydisperse, hexagonal ice crystals. Beyond the cloud, the technique makes use of standard Huygens-Fresnel propagation methods. At the air-cloud interface, each wave front is resolved into a ray distribution for input to the ray trace software. Similarly, a wave front is reconstructed from the output ray distribution at the cloud-air interface. Simulation output from the ray trace program is presented and the modulation transfer function for stars imaged through cirrus clouds of varying depths is discussed.

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The effect of atmospheric phase perturbations on the diffractive and coherent properties of the uplink and downlink paths of an active imaging illumination beam has been studied in some detail. Similarly, the scattering and depolarization induced by water and ice cloud particles in the path of coherent laser illumination is currently an area of much productive research. In contrast, the effect of cloud particles on the diffractive properties of a laser illumination beam has not received as much attention due primarily to the daunting mathematics of the physical model. This paper seeks to address some of the mathematical issues associated with modeling the interaction of a coherent illumination beam with a cloud of ice particles. The simulation constructs a three-dimensional model of a cirrus cloud consisting of randomly oriented hexagonal ice crystals in the shape of plates, columns, and bullet rosettes. The size, shape, and vertical distribution of the crystals are modeled after measured particle concentrations and distributions. An illumination pattern, in the form of a grid of rays, is traced through the cloud, and the properties of the exiting wavefronts are analyzed.