Spatiotemporal structure of a laser beam at a path length of 144 km: comparative analysis of spatial and temporal spectra.

We present an analysis of spatial and temporal spectra of the observations of scintillations in a laser beam (532 nm, ∼200 mW power) traveling along a 144 km path at an altitude of 2-2.4 km above sea level, just above the atmospheric boundary layer, between the islands of La Palma and Tenerife. The observations were performed during nighttime on July 18 and 21, 2011, by means of a telescope with an aperture diameter of 1 m. Strong scintillations were observed. We compared the temporal and spatial spectra of the scintillations. For the temporal spectra, we performed the correction for the aliasing effect due to the low frame rate. The 2D spatial spectra of the scintillations in the observation plane were found to be close to isotropic. This allowed for transforming them into 1D spectra. We found a good agreement between the temporal and 1D spatial spectra. This corroborates the applicability of the Taylor hypothesis of frozen turbulence for finite-size laser beams and strong scintillations.

Spatiotemporal structure of a laser beam over 144 km in a Canary Islands experiment.

We analyzed the observations of scintillations in a laser beam (532 nm, ~200 mW power) traveling along a 144 km path at an altitude of 2.2-2.4 km above sea level, just above the atmospheric boundary layer, between the islands of La Palma and Tenerife. The observations were performed during nighttime on 18 July 2011, by means of a telescope with an aperture diameter of 1 m. Strong scintillations were observed. The estimates of spatial spectra and correlation functions indicated that the observed intensity fields possess, statistically, a locally isotropic structure, which agrees with the idea of a locally isotropic turbulence. The estimates of spatial autospectra and autocorrelation functions of the intensity field indicated that the characteristic scale of the internal structure of the observed clusters is 6.5-8 mm, while the characteristic size of the clusters is 4-5 cm. The major contribution to the observed scintillations comes from the inhomogeneities of the intensity field with scales from 1-2 cm up to 10-12 cm. The analysis of the cross-spectra indicated that the hypothesis of frozen turbulence introduced by Taylor can be used for the description of spatiotemporal structure of intensity fluctuations of laser beams traveling through long paths in the atmosphere.