Vortex modulation for spatial separation of coherent and incoherent light in lidar-based underwater detection technology

Abstract

We developed the prototype of a vortex lidar system that incorporates a vortex beam generator and a spatial filter in the signal receiver unit. The design and optimization of the key parameters of the prototype were determined through preliminary experiments and the signal-to-noise ratio algorithm. Coherent light and incoherent light can be spatially separated by using vortex modulation and spatial filtering. Both vortex lidar and reference lidar were tested in experiments in which a water tank was used for signal detection. Experimental results show that the return signals acquired with such a vortex lidar from a hard target are weaker in intensity compared to the reference signal (single scattered light + multiple scattered light + background light). Nevertheless, this vortex lidar technique allows for an improved differentiation of the vortex signal (single-scattered light) from background light and multiple-scattered light. The attenuation length (AL), defined as the distance where light intensity decreases to 1/e of its initial value, is used to quantify the target detection range. The maximum detection range for a hard target increases from 10 AL (for the reference lidar) to 14 AL (for the vortex lidar). Furthermore, the target signal intensity measured by the vortex lidar decreases exponentially (according to the Beer–Lambert law) with the distance between the instrument and the underwater target. This result indicates that the vortex signal is dominated by single scattering.