Underwater laser beam profiles after transmission through a wavy sea surface


  • Tulldahl Michael

Publish date: 2001-01-01

Report number: FOI-R--0316--SE

Pages: 31

Written in: English


The effects of surface waves on laser beam transmission through the sea surface are experimentally examined. The purpose is to obtain experimental data for validation of laser pulse propagation models. Such data require simultaneous measurements of the time and space variability of the air-sea interface. Two significant consequences of transmission through the sea surface are investigated: beam widths at different depths averaged over several surface wave periods, and surface wave focusing or defocusing quantified by the irradiance fluctuations as a fractional fluctuation (standard deviation divided by the mean). The irradiance fractional fluctuations show a near-surface maximum and decay with depth. A submerged screen, filmed by an underwater video camera, is used to measure the downwelling irradiance profiles. The laser beam diameter at the water surface is 42 cm, and irradiance profiles are measured from 1.2 m to 3 m depth. All measurements are performed with the laser beam directed vertically downwards. Transmission for off-nadir angles is not tested in this experiment and no polarization effects are measured. The measurements are made in calm winds, 1 m/s to 3 m/s. The surface waves are measured with a video camera, and the dominant seasurface wavelength is about 0.25 m.