Modulation of Laser Beams by Atmospheric Turbulence - Depth of Modulation

From the point of view of communications, one of the serious effects of the atmosphere on propagation of laser beams is the fluctuations in * Part of this paper was presented at the 1966 International Quantum Electronics Conference, April 12-15, 1966, Phoenix, Arizona. 623 624 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1967 the received signal caused by variations in the dielectric constant of the air. An important measure of the fluctuations is their power spectrum. Hogg 1 first measured these spectra and obtained an exponential distribution with a baseband width of the order of a few hundred cycles. Hogg used a multimode 6328 A laser and a range of 2.6 km. The receiver, 5 cm in diameter, was located in the center of the received beam, which was about 25 cm in diameter. Hogg observed that an increase in the angular beamwidth of the source caused an increase in spectral width. Hinchman and Buck 2 measured the low-frequency fluctuations in a 6328 A laser beam at distances of 9 and 90 miles. Their receiver, 3 inches in diameter, collected a very small fraction of the total beam power. They observed a very large depth of modulation. The spectral density of the fluctuations was found to decrease with increasing frequency up to 50 Hz, the highest frequency measured. Subramanian and Collinson 3 propagated a single-mode, diffractionlimited 6328 A beam and examined the dependence of the spectrum on a variety of parameters. The transmitted beam diameter was changed from 1 to 38 mm, beam divergence was adjusted by focusing the telescope employed, ranges of 120 and 360 meters were used, and the receiver aperture was varied from much smaller to much larger than the received beam size.