Abstract
The performance of free-space optical applications can be improved using beams of different wavelengths for the auxiliary actions of pointing/tracking or turbulence correction. Chromatic dispersion owing to the atmosphere is an issue for multiwavelength systems, and the dispersion of electromagnetic signals is typically predicted based on refractive conditions from standard atmospheric models. However, for long near-horizontal paths near the Earth’s surface, substantial refractive index gradients that are associated with features such as inverse temperature layers and ducts can be encountered. These features can significantly alter the ray trajectory, the chromatic divergence, and the angle of arrival of directional beams relative to standard atmosphere predictions. A ray tracing approach was implemented to examine the chromatic divergence and angle of arrival of the rays through various practical and extreme atmospheric conditions involving a temperature inversion layer. Over a distance of 150 km along the ground, a brief encounter with the layer can cause pairs of rays with wavelengths 532 and 1550 nm to diverge up to 4.5 times greater than their standard atmosphere predictions. For a single wavelength, a linear increase of angle of arrival with initial launch angle was found for the standard atmosphere, but this trend was significantly altered in the presence of an inversion layer. Extreme refractive conditions with a large inversion layer were simulated to produce optical ducting over long distances. Chromatic separation of rays as large as 280 m was observed when only one of the two wavelengths remained in the duct.
Highlights
The propagation of an optical signal through the clear atmosphere is affected mainly by atmospheric refraction and turbulence
We find that for standard atmosphere refraction conditions, the separation between the rays depends largely on the altitude of the source and is nearly independent of the launch angle
The presence of a temperature inversion layer introduces significantly higher gradients than the standard atmosphere and can cause much more ray separation than the standard atmosphere depending on the relative source position with respect to the inversion layer, launch angle, and wavelengths of the source
Summary
The propagation of an optical signal through the clear atmosphere is affected mainly by atmospheric refraction and turbulence. As light propagates through the Earth’s lower atmosphere in a horizontal or near-horizontal direction, refraction is responsible for some of the most interesting atmospheric optical phenomena such as mirages—the Fata Morgana and Novaya Zemlya effects—the green flash, and ducting These phenomena are usually associated with the formation of significant refractive index gradients close to the ground. Due to the wavelength dependency of the refractive index and its gradient, the amount of bending of the propagating signal is wavelength dependent This leads to chromatic dispersion where the ray paths for different wavelengths separate as they propagate. The atmosphere can generally be ignored for short propagation paths, but it becomes significant for applications such as laser communication or astronomical observation near the horizon, where long propagation paths (tens to hundreds of km) may be involved Chromatic corrections for these applications are commonly predicted based on the standard atmospheric models. Deviation from a linear relationship between the AOA and launch angle was observed as the rays encountered the inversion layer
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