Optical ranging through turbid underwater media using spatially structured beams

The use of optics for underwater ranging has gained growing attention because of its potential for higher accuracy over acoustic methods. Typical ranging systems using the temporal/frequency information of light waves, however, may face limitations such as signal distortion induced by high turbidity or the need for high-bandwidth devices. Alternatively, a recently proposed ranging scheme utilizes spatially structured beams for distance retrieval. In this paper, we review the advances of the underwater ranging technique based on spatially structured beams. Such beams comprise a combination of Bessel–Gaussian (BG) modes with different orbital angular momentum orders and longitudinal wavenumbers, exhibiting a two-petal-like transverse spatial intensity profile that rotates along the beam propagation. The distance of the object can be retrieved by measuring the rotation angle of the petals. This approach has been shown to provide relatively accurate ranging measurements under scattering environments and only requires a continuous-wave laser without high-bandwidth devices. We begin by introducing the property of the BG beam, the effects of scattering on optical beams propagating in underwater media, and the ranging principle. Next, we present the basic work of using range (z)-dependent angular rotation of a structured beam’s intensity profile for ranging. Finally, we cover several extensions in enhancing this approach via tailored beam parameters, including increasing the petal signature, combining the coarse and fine ranging, and mitigating the petal attenuation from power loss.