This paper is based on the premise that an object in water can be detected using a lidar (Light Detection and Ranging) that images the absence of water within a volume. The lidar does this by detecting the Raman scattering from water, which has a characteristic frequency shift from the transmitted light. Where there is no Raman signal, there is something other than water. The authors estimate that a modest lidar operating at 355 nm will produce a Raman return that is detectable in the presence of solar background light. With neither an imaging lidar nor an ocean, the authors were still able to make measurements to demonstrate the principle. A non-scanning lidar with a spectrally-resolved receiver illuminated a water-filled pipe containing a target. The target, which blocked a portion of the beam, was placed at different positions along the pipe, so the total volume of water illuminated by the laser was varied. The authors then showed that the total return in the portion of the spectrum corresponding to the Raman return varied in accordance with theoretical expectations.
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