Demonstration of Combined Coarse-and-Fine Underwater Ranging Under Turbidity Using Structured Beams With Varying Longitudinal Wavenumbers

Optical ranging systems using temporal information of the light may encounter challenges in turbid underwater environments. Alternatively, one ranging approach leveraging the transverse spatial properties of a light beam has been demonstrated with relatively stable performance under scattering conditions. This scheme utilizes the range-dependent angular rotation of a spatially structured beam comprising 2 Bessel-Gaussian (BG) modes for underwater ranging. In this paper, we enhance the measurement range and resolution of the structured-beam-based ranging by combining the coarse and fine approaches. The longitudinal wavenumber difference of BG modes is tuned to generate structured beams with varying rotation rates. Each rotation rate corresponds to a different measurement range and resolution. We experimentally demonstrate a 1-m combined coarse-and-fine underwater ranging system. The coarse beam (1.0-period rotation) and fine beam (4.5-period rotation) are alternated in each measurement step. Our results show a ∼4X enhanced measurement range and accuracy compared to the fine-only or coarse-only approach, respectively. An average ranging error of 6.2 mm is achieved over 1-m turbid water with an attenuation coefficient of 2.0 m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>. We also simulated a 10-m underwater ranging system with varying scattering strengths. Under an attenuation coefficient of 0.4 m-1, compared with the coarse-only approach, the simulated ranging error is reduced from 11.5 cm to 1.6 cm using the combined coarse-and-fine approach.