An optical system for suppression of laser echo energy from the water surface on single-band bathymetric LiDAR

Abstract

When a bathymetric LiDAR (Laser Detection And Ranging) onboard unmanned ship is close to the water surface less than 50 cm, the laser echo energy from the water surface is much higher than that from the water bottom. This phenomenon causes a large dynamic range of echo signals that is output from the PMT (Photomultiplier Tube) detector, resulting in that the echo signal from the water bottom cannot correctly be detected in shallow water. For this reason, this paper proposes an optics system to suppress the amplitude of the echo signal from water surface through suppressing the laser echo energy. A two-channel receiving optical system, which receives echo signals from shallow water and deep water is designed and implemented, and then a polarized optical system is designed to embedded inside to suppress the laser echo energy from the water surface. The verifications are carried out through three sets of indoors experiments, and three sets of outdoors experiments. The experimental results demonstrate that the proposed system can effectively suppress 80.7% of the amplitude of the echo signal from the water surface when the voltage control of PMT detector is 0.40 V, which can largely improve the detection ability of the echo signals from water bottom in shallow water.