Publications

Concept for Maritime Near-Surface Surveillance Using Water Raman Scattering

Shokair, Isaac R.; Johnson, Mark S.; Schmitt, Randal L.; Sickafoose, Shane S.

In this report we discuss a new maritime surveillance and detection concept based on Raman scattering of water molecules. Using a scanning lidar that detects Raman scattered photons from water, the absence or change of signal indicates the p resence of a non - water object. With sufficient spatial resolution a negative two dimensional imag e of the object can be generate d by the scanning lidar . Because Raman scatt er ing is an inelastic process with a relatively large wavelength shift for water , this concept completely avoids the problematic elastic sc attering for objects at or very close to the water surface . Elastic scattering makes it difficult to disc riminate between water and dark objects at or near the water surface especially when automated detection is required . It is also difficult to deal wit h elastic scattering from the bottom surface for shallow waters. The maximum detection depth for this concept is limited by the attenuation of the excitation and return Raman light in water. If excitation in the UV is used, fluorescence can be used for dis crimination between organic and non - organic objects. Range gating can be used for this concept for detection of objects below a specified depth. In this report we develop a lidar model for this concept to estimate the number of detected Raman photons fo r variable lidar parameters and depths in the presence of the solar background . We also report on the results of proof - of - concept measurements using the Sandia Ares lidar with excitation at 355 nm. The measurements show good agreement with the lidar mode l predictions. The detected number of photons for typical lidar parameter shows the concept is viable and applicable to a variety of day and nighttime detection scenarios. This concept has many potential applications including ne ar - surface mine detection, swimmer detection for security purposes, wide area search, as well as other civilian applications.