Reduction of Scattering Clutters in an Underwater Lidar System by Using an Optical Vortex

An optical vortex was applied in an underwater lidar system to improve ranging accuracy. A self-developed intensity modulated continuous wave 532 nm laser beam was used as the light source. The detector was a photomultiplier tube (PMT). The output signal from the PMT was correlated with a reference signal to retrieve the distance of the target. A spiral phase plate (SPP) was inserted before the detector to convert the spatially coherent target reflection onto an optical vortex ring. A mask with a transparent ring that matched the vortex was used to block the scattering light distributed inside and outside the ring. Experimental results showed that the ranging error was significantly reduced in a turbid medium by the method, in addition, a high-order SPP worked better. With a 24-order SPP, we obtained a ranging error reduction from 20 cm to 2.73 cm when the attenuation length was 9.