Ranging accuracy improvement by using a spiral phase plate in a time-of-flight underwater lidar system

We report on an investigation of spatial coherence filtering using a spiral phase plate (SPP) to reduce the amount of scattered light collected in an underwater lidar. This approach exploits the difference in spatial coherence between target-reflected light and scattered light as a means of separating and filtering out the multiple-scattered clutters. An underwater target was illuminated by a Gaussian beam, and both the target-reflected light and scattered light passed through an SPP. The spatially coherent target-reflected component formed a ring-shaped vortex beam, while the spatially incoherent scattered component was unaffected and remained a centrally stronger distribution obeying the Mie scattering law. A mask was placed behind the SPP which allowed only the light on the vortex ring to pass through. Therefore, an all-optical spatial coherence filter composed of an SPP and a mask provides an effective way to reject undesired scattered light without affecting the target-reflected light. A numerical simulation based on Zemax software was carried out and showed that the approach reduces the power of received scattered light without changing the power of received target-reflected light. Experimental results showed that the approach reduced the temporal broadening and excessive delay of the returned pulse caused by forward scattering, therefore reducing the ranging error. Moreover, using a vortex beam with a bigger topo-logical charge improved the ranging accuracy more obviously, especially in turbid water. When the attenuation length was 12.5, by using a 40th-order SPP, the ranging error was reduced from 18 cm to 5.3 cm with a diffuse target.