Effects of the atmosphere on laser beam propagation

An experimental study of horizontal laser beam propagation over paths up to 145 km long was made in which beam diameter and shape, intensity fluctuations, and optical phase distortion were measured. It was found that (1) received beam diameter decreases (on-axis power density increases) with increasing transmitter aperture to a limit reached at an aperture of about 11 cm, (2) beam diameter varies as the 1.2 power of path length, (3) the amplitude of intensity fluctuations decreases with increasing receiver aperture and is nearly independent of path length for paths longer than 0.55 km, (4) the fluctuation spectrum shows a decrease in spectral power with increasing frequency, this negative slope becoming steeper with increasing receiver aperture and remaining constant with path length, and (5) rms fluctuation in phase path length was observed to be 0.25 micro over a 48.8-m path. In addition, it is shown that geometrical optics based on the effects of large scale atmospheric irregularities does not adequately account for signal intensity fluctuations. These results may be helpful in predicting the capability of specific communications systems and in understanding better the nature of atmospheric turbulence.