B.S.T.J. Briefs: On the Spectrum of Optical Waves Propagated throughthe Atmosphere

It is well known from our day-to-day experience that distant objects appear to shimmer, especially on still, hot days when relatively large temperature and humidity gradients may exist. It is true, however, that refractive index gradients are in the air to some degree at all times; that is, they may he present at night and also when the air is disturbed by the winds. It is not surprising therefore that the power received at some distance from an optical source fluctuates randomly and possesses a characteristic low-frequency power spectrum. The purpose of this note is to discuss typical low-frequency spectra resulting from propagation of 0.63-micron radiation over a 2.6-km path. A vertically polarized helium-neon maser 1 of power output 10 mw and a reflecting telescope of 9-cm diameter comprise the source. With all modes of the maser propagating, the beam spreads to an ill-defined and ever-changing disk of about 2o-cm diameter.* The receiver is a refracting telescope of 5-cm diameter with associated filters, polarizers, and attenuators which feed a photomultiplier; it is located in the dense central region of the transmitted beam. The beamwidth of the receiver is large compared with that of the transmitter. For measurement of the power spectrum, the output of the photomultiplier is taken to a wave analyzer whose bandwidth is 4 cps. A typical power spectrum is shown in Fig. 1, the measured points being indicated by open circles, f The abscissa, F = f -- / ,, , is sideband frequency, and the ordinate indicates relative received power.