Statistics of turbulence-induced laser irradiance fluctuations: impact of receiver aperture size

Probability density functions of Gaussian laser beam irradiance measured after propagation over 7km atmospheric path in comparison with various theoretical models are presented. The initial laser beam diameter at the e' intensity level was about 6 mm, the receiving aperture size was 14.4 cm. The experimental observations were performed in a wide range of turbulence strengths. The cases of weak, moderate, and strong intensity fluctuation regimes have been analyzed. Different receiving aperture radiuses were considered. The chi-square metric was used to estimate the agreement between the experimental and different theoretical statistics. The fractional gamma distribution has shown the best results for probability density distributions of apertures with sizes about 1 cm and 4 cm under strong turbulence conditions. The aperture averaging effect results in excluding near-zero irradiance values, which are typically observed on-axis at strong turbulence and high value of scintillation index which qualitatively transforms the observed statistics, so that experimental probability density functions can be well approximated by the fractional gamma distribution. With the increase of the aperture size, a further transformation of the statistics was observed. The statistics of experimental data for moderate and weak fluctuation regimes approached the lognormal and gamma distributions.