Improvement of underwater ranging accuracy in turbid medium by using a spiral phase plate

Lidar has been widely used in underwater detection, survey, and seabed mapping. However, the performance of underwater lidar is deteriorated by scattering. Scattering not only contributes to attenuation but also worsens ranging accuracy or imaging contrast. Therefore, it was important to suppress scattering in underwater detection. We present investigations of applying a spiral phase plate (SPP) as a spatial filter to suppress scattering in an underwater lidar system. An SPP was inserted in the echo path to separate target-reflected signals from scattering clutters, because, in the echo, the target-reflected light maintained spatial coherence and was converted into an optical vortex ring after passing through the SPP, while the scattering clutters lost their spatial coherence and cannot be converted to the optical vortex but remained a centrally stronger distribution according to the Mie scattering law. A mask was produced by coating a glass sheet with opaque paint, leaving only a transparent ring for light on the optical vortex ring to pass through. Experimentally, the response of the light in the center of the vortex to the RF modulation frequency decreased with the increase of attenuation length, so it was mainly scattered light, while the light on the vortex was measured to maintain RF frequency modulation at high attenuation length, so it was dominated by signal light. The ranging results showed that the ranging error was significantly reduced in a turbid medium by blocking scattering clutters inside and outside the vortex. Moreover, a high-order SPP was more effective in reducing ranging errors. We reduced the ranging error from 30 cm to 6.6 cm with a 24-order SPP when the attenuation length was 15.