Ranging through turbid underwater using structured optical beams

We demonstrate optical ranging through turbid underwater medium using a structured beam. This beam consists of two Bessel modes, each carrying a pair of orbital angular momentum order and longitudinal wavenumber. As a result, the beam has a “petal-like” intensity profile with different rotation angles at different distances. The object’s distance (z) is retrieved by measuring the rotation angle of the petal-like profile of the back-reflected beam. We demonstrate ⪅ 20-mm ranging errors through scattering with extinction coefficient γ up to 9.4 m-1 from z = 0 to 0.4 m. We further experimentally demonstrate the enhancement of ranging accuracy using multiple (⪆2) Bessel modes. With the number of modes increasing from two to eight, the average error decreases from approximately 16 mm to approximately 3 mm for a Υ of 5 m-1. Moreover, we simulate both coarse- and fine-ranging by using two different structured beams. One beam has a slower rotating petal-like profile, leading to a 4X larger dynamic range for coarse ranging. A second beam has a faster rotating profile, resulting in higher accuracy for fine ranging. In our simulation, ⪅ 7-mm errors over a 2-m dynamic range are achieved under 𝛾 = 4 m-1 .