B.S.T.J. Briefs: Modulation of Laser Beams by Atmospheric Turbulence

M o d u l a t i o n of Laser B e a m s by Atmospheric T u r b u l e n c e By M. SUBRAMANIAN and J. A. COLLINSON (Manuscript received January 11, 1965) When laser beams are propagated through the air, they are modulated with a noise-like spectrum 12 having a baseband width the order of hundreds of cycles and a nearly exponential frequency distribution.1 Hogg1 used a 2.6-km path; Hinchman and Buck2 used paths of 9 and 90 miles. In each case the optics and range were such that the receiver collected a small fraction of the total beam. Since atmospheric refraction causes twinkling and tearing of the beam, one would expect amplitude modulation of the signal received under these conditions even for constant intensity of the total beam. We report here that the shape of the noise spectrum is unchanged when all of the detectable beam is received. Moreover, the spectrum is unaffected by changes in the diameter or geometrical divergence of the transmitted beam, by whether the receiver is in the near or far field of the transmitter, or by a threefold change in transmission distance. The general spectrum characteristics appear to be determined by atmospheric conditions. Wo have transmitted a horizontally polarized 6328-A laser beam over a 120-meter path 8 meters above black-top pavement. The beam was detected through a 3A-wide interference filter by an RCA 7265 photomultiplier tube. The frequency spectrum of the signal was analyzed and displayed on a CRT by a Singer Metrics TA-2 spectrum analyzer.