Underwater Lidar: Remote Sensing in Strongly Scattering Media

A lidar backscattering signal from an opaque target object, passed through a 9-m water layer with scattering meshes on the laser beam path, has been detected (for the first time, to the best of our knowledge) when sensing by pulses with eye-safe radiation energy density (~1 μJ/cm²). The new principle of laser sensing makes it possible to measure the position of meshes on the lidar path, in contrast to conventional laser rangefinders, which measure the distance to only the first target. The lidar has been developed based on a pulsed diode-pumped Nd³⁺:YAG laser (532 nm, 3 ns, 2 µJ/pulse, pulse repetition rate 4 kHz) and gated single-photon avalanche photodiode (SPAD) with a gain up to ~10⁶, serving as a detector. The large gain of the detector and suppression of its noise by gating ensured a signal-to-noise ratio of ≈35 for the target signal, which provides an estimate of underwater sensing range up to 30 m, according to the 3σ detection criterion. Compact lidars based on diode lasers (~1 µJ/pulse) with a radiation wavelength (~450 nm) in the spectral range of minimum losses in water and the increase in the safety of manned and unmanned underwater vehicles at early detection of nets (invisible for sonars) by a lidar are discussed.