Radio frequency modulation on an optical carrier for target detection enhancement in sea-water

We have experimentally assessed the efficiency of a new underwater-target d etection scheme, called modulated lidar. The technique used is based on a r emarkable physical property of the propagating medium; sea-water has a low- pass transfer function in backscattering configuration. As the target retur n is less frequency-dependent, the use of a radio frequency modulated laser source along with a narrow band filtering at the detection drastically red uces the backscattering clutter, but does not affect the target return; the modulation frequency of the source is substantially above the cut-off freq uency of the propagating medium. This technique allowed us to greatly impro ve underwater-target contrast. We associated with this frequency detection a temporal localization of the underwater target by using a pulse-modulated laser source.