D. Sette

Interferometric study of phase fluctuations of a laser beam through the turbulent atmosphere.

Phase fluctuations of a laser beam propagating through a turbulent atmosphere are studied as a function of the distance in a cross section of the beam. An interferometer is described that allows simultaneous measurements of phase fluctuations for pairs of rays that travel in the beam at different distances between each other. Experiments with this method have been performed in an urban center at a distance of 3.5 km and 0.5 km from the source (a helium-neon laser). The results of these measurements are described, and the behavior of the mean square fluctuation of the phase difference is studied. The values of the structure constant C(n) are calculated.

Correlation Measurements on Partially Coherent Beams by Means of an Integration Technique

A method based on the measurement of the time-integrated irradiance obtained in an interference experiment is described. This method is of particular utility in cases in which amplitude and phase or both are stochastic fluctuating variables. When only phase fluctuations are present or when amplitude and phase are stochastic independent variables, simple relations can be derived from which the parameters of the distribution can be derived. In these cases, if a gaussian-distribution law for the phase fluctuations is assumed, the mean-square value of the phase fluctuations is directly measured. The method has been applied to the study of phase fluctuations of a laser beam, caused by atmospheric turbulence, with results that agree with the findings of other techniques. The anisotropy of turbulence can also be evaluated.

Atmospheric Turbulence Effects on the Phase of Laser Beams

Measurements of the phase structure function of a laser beam propagated through the turbulent atmosphere along two different paths are reported. The first path was chosen in an urban zone, the second one was along a flat and quiet region. A comparison of the results with the theoretical predictions of Tatarsky and Lutomirsky and Yura is made. A discussion of the range of validity of the theoretical treatments is presented.

On the Possibility of Measuring Optical Visibility by Using a Ruby Laser

The atmospheric attenuation of a ruby laser beam is studied. The possibility of using the laser back scattered radiation to probe optical visibility is investigated. Some experimental measurement support ing this possibility is reported.

Optical Processing of the Phase Correlation Induced by a Turbulent Medium in a Laser Beam

A general method which allows precise measurements of the frequency spectrum of a phase modulated wave down to a few cycles per second is presented. The momenta of the distribution of the instantaneous frequency (time derivative of the phase) can also be easily obtained. An experiment is described in which the study of the phase correlation induced by a laboratory generated thermal turbulence in a laser beam is described.

Atmospheric extinction measurements of a He–Ne laser

The extinction of a He-Ne laser in a 300-m long urban path has been measured and correlated to temperature and relative humidity variations. The results are contrasted with the scattering coefficient theoretically calculated for a model of aerosols. The agreement seems to indicate that the values of the extinction are strongly correlated with relative humidity in inhomogeneous urban paths.