Polarization Lidar for Shallow Water Depth Measurement

A bathymetric, polarization lidar system transmitting at 532 nanometers is developed for applications of shallow water depth measurement. The technique exploits polarization attributes of the probed water body to isolate surface and floor returns, enabling constant fraction detection schemes to determine depth. The minimum resolvable water depth is no longer dictated by the system's laser or detector pulse width and can achieve better than an order of magnitude improvement over current water depth determination techniques. In laboratory tests, a Nd:YAG microchip laser coupled with polarization optics, a single photomultiplier tube, a constant fraction discriminator and a time to digital converter are used to target various water depths. Measurement of 1 centimeter water depths with an uncertainty of ±3 millimeters are demonstrated using the technique. Additionally, a dual detection channel version of the lidar system is in development, permitting simultaneous measurement of co- and cross-polarized signals scattered from the target water body. This novel approach enables new approaches to designing laser bathymetry systems for shallow depth determination from remote platforms while not compromising deep water depth measurement, supporting comprehensive hydrodynamic studies.