Two-Frequency Intensity Fluctuations In A Random Medium

The two-color correlation of scintillations was experimentally measured in a laboratory-generated random medium. The aperture averaging effect was examined by measuring the normalized spectral covari-ance and the correlation coefficient for He-Ne and Argon laser beams. Measurements were taken for several strengths of the refractive index inhomogeneities but saturation was not reached. Aperture averaging was significant and its effect on the variances and covariance became more prominent as the the strength of the refractive index inhomogeneities increased. The correlation coefficient was found to increase with the aper-ture diameter from 0.8 at the smallest aperture (0.1mm) up to almost unity at a detector diameter on the order of [Wavelength-Range] 1/2 . Also, the correlation coefficient decreased as the wavelength difference between the lasers increased.