Abstract

One method of inner scale measurement uses the irradiance variance of a diverging monochromatic wave (the irradiance being obtained through a small aperture at the receiver position) and the irradiance variance of a large-aperture $C_n^2$ scintillometer. The ratio of these two variances depends on the inner scale of turbulence l₀ but not on the refractive-index structure parameter $C_n^2$. Another method uses the bichromatic correlation of irradiances from waves having two different wavelengths, transmitted through a common small aperture (the irradiance being obtained through a small aperture at the receiver position) and the two corresponding monochromatic irradiance variances. The ratio of the bichromatic correlation to the geometric mean of the two monochromatic variances depends on l₀ but not on $C_n^2$. A third method is to obtain the ratio of the two monochromatic variances without using the bichromatic correlation. These methods are compared graphically using calculations based on an accurate atmospheric refractive-index spectrum. A scaling analysis is performed to determine the minimum number of parameters needed to describe the methods. It is suggested that systematic errors, rather than signal-to-noise limitations, determine the accuracy with which inner scale is measured. The effects of refractive-index dispersion between the two wavelengths must be taken into account. The results indicate that the bichromatic method has advantages when $l_0\lesssim 0.6\sqrt{L/k}$, whereas the method using small and large apertures has the advantage when $l_0\gtrsim 0.6\sqrt{L/k}$, where L is the propagation path length and k is the optical wave number.

© 1988 Optical Society of America

Optical-scintillation method of measuring turbulence inner scale

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Inner-scale dependence of scintillation variances measured in weak scintillation

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