Using A Structured Beam to Enhance the Accuracy of Time-of-Flight Underwater Optical Ranging in Single-Beam Measurements

Underwater optical ranging is of increasing interest for high-resolution measurements over several meters, with timeof- flight (ToF) being one key approach. Besides the temporal information used in this approach, utilizing the spatial degree of freedom might further enhance ToF ranging performance. In this work, we experimentally demonstrate enhanced underwater ToF ranging performance by spatially structuring a 520-nm light beam carrying a temporal ToF waveform. The transmitted beam, composed of two Bessel-Gaussian (BG) modes, features a distance-dependent rotation of the transverse intensity profile with multiple rotation periods over the measurement range. Upon reflection from the object, the beam is captured by a ToF camera for simultaneously acquiring the temporal phase delay and spatial intensity profile for ToF-based ranging and structured-beam-based ranging, respectively. The ToF result provides coarse ranging to identify the correct rotation period, while the rotation-angle measurement of the structured beam enables fine distance retrieval within that period. Experimental results show that the combined approach reduces the mean absolute error (MAE) from 65.4 mm to 3.5 mm over 0-0.8 m under scattering water with attenuation coefficient γ = 2.3 m-1, compared to the ToF-only approach using a Gaussian beam. Multiple independent distance measurements further show a reduction in standard deviation (SD) from >30 mm to <4 mm, indicating improved stability of the combined approach under scattering conditions.