A wideband noise-like transmitter approach for underwater lidar using diode lasers and passive fiber optic processors

A new wideband noise-like transmitter approach is presented for high resolution underwater lidar sensing. The transmitter approach is based on small-footprint, low-cost components, using low coherence time laser diodes and passive fiber processors to generate wideband noise-like intensity modulation signals in the blue-green optical spectrum. Prototype transmitters are demonstrated using both blue and green laser diodes with passive fiber interferometer structures. Laboratory water tank experiments using a two-diode 516/518 nm prototype transmitter show centimeter range error and 30 cm range resolution while detecting a submerged gray target in up to ten attenuation lengths of turbid water. Experimentally observed challenges for target rangefinding are discussed, including shot noise, backscatter returns, and self-clutter. Strategies are proposed to mitigate these challenges and enhance performance when operating at long standoff distances in turbid waters.