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

Abstract

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