Laser Doppler Velocimetry for 3-D Seawater Velocity Measurement Using a Single Wavelength

In this article, we have experimentally demonstrated a laser Doppler velocimetry (3D-LDV) system capable of measuring 3-D flow velocities, employing a single emission wavelength and four photodetectors for capturing light scattered by particles in seawater. The optical measurement volume of the system is cylindrical and possesses dimensions that are significantly smaller than those of traditional acoustic Doppler systems—1.02 mm in diameter and 15.40 mm in length. This compact size renders the system particularly advantageous for applications demanding high spatial resolution, such as the observation of fine-scale turbulence. The performance of the 3D-LDV system was evaluated using a precision-controlled towing system in static seawater. It exhibited a measurement velocity range of 0.02–3.78 m/s, with a maximum relative error of 3.75%, a relative standard deviation of 1.49%, and an average directional angle deviation of 0.45° for angle changes within ±10°.