Experimental effects of finite transmitter apertures on scintillations

The use of large transmitter apertures has been theoretically predicted to result in a substantial reduction of laser-beam scintillations under certain conditions. Experiments have been conducted on transmitter-aperture effects through turbulence, including photographic studies for qualitative interpretation, and measurements of log-amplitude variance, covariance, probability distributions, and the spectra of scintillations. It was found that the predicted reduction imposes strict requirements on transmitter optical adjustments, and will not be realized in the presence of significant beam wander. In the case of either transmitter misadjustment or strong turbulence, the beam at the receiver plane consists of a proliferation of transmitter-diffraction-scale patches, with large attendant scintillations. The theory suggests a novel means of reducing scintillations arising from a thin layer of turbulence at the tropopause in earth-to-space systems. Comparisons of the beam-wave analyses with results from a recent theory of reciprocity suggest a limited range of validity for the analytical predictions.