Underwater irradiance fluctuations of a laser beam after transmission through a wavy sea surface

摘要

The effects of surface waves on laser beam transmission through the sea surface are experimentally examined. The purpose is to obtain experimental data for comparison with laser propagation models. Simultaneous measurements of the time and space variability of the air-sea interface are performed. A submerged screen, filmed by an underwater video camera, is used to measure the downwelling irradiance profiles. The measurements are made in calm winds, in a sheltered harbor environment. Calibrated values of downwelling irradiance are obtained by reference measurements in laboratory. Two significant consequences of transmission through the sea surface are investigated: beam width at different depths averaged over several surface wave periods, and surface wave focusing or defocusing quantified by the irradiance fractional fluctuations (standard deviation divided by the mean). We compare the irradiance fractional fluctuations from our experiment with published data from underwater sunlight measurements and from laser beam simulations. The irradiance fractional fluctuations show a near-surface maximum and decay with depth. Low wind speed is expected to increase the fractional fluctuations caused by wave focusing effects. Our measurements qualitatively agree with the compared data, but exhibit larger fractional fluctuations.