Enhanced underwater ranging using an optical vortex

An optical vortex is used to enhance the ranging accuracy of an underwater pulsed laser ranging system. An experiment is conducted whereby an underwater object is illuminated by a pulsed Gaussian beam, and both the object-reflected and scattered light are passed through a diffractive spiral phase plate prior to being imaged at the receiver. An optical vortex is formed from the spatially coherent non-scattered component of the return, providing an effective way to discriminate the desired objected reflected light from the spatially incoherent scatter. Experimental results show that the optical vortex permits a spatially coherent ballistic target return to be more easily discriminated from spatially incoherent forward scattered light up to eight attenuation lengths. The results suggest new optical sensing techniques for underwater imaging or lidar.