Remote Sensing of Sound Speed in the Ocean via Brillouin Scattering
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abstract
A Brillouin LIDAR approach to range-resolved, remote measurements of sound speed (and temperature) in the ocean is described. Two greatly simplified versions can also provide (1) for very accurate measurements of the particulate scattering function p() at =180, and (2) for detection and identification of submerged objects. For sound speed (temperature), realistic objectives are an accuracy of 0.2 m/s (0.1C) over a range of the order of 100 m in clear ocean with a range resolution of approximately 1 m. Our approach provides high-resolution spectroscopic capabilities even in very severe vibration environments; it is based on the use of edge filters to provide a high-resolution determination of the Brillouin frequency shifts. The simplest edge filters are molecular iodine absorption lines; they have been used for the laboratory data to be presented. But, even more promising are excited-state Faraday anomalous dispersion optical filters that are nearing fruition. Our transmitter is a commercial, injection seeded, frequency-doubled Nd:YAG laser that we have modified in two ways. First, we changed its operating temperature to obtain lasing at a frequency consistent with our choice of iodine absorption lines. Second, we implemented the Ramp and Fire technique we had developed so that the laser operates in a single longitudinal mode even when there are severe environmental disturbances. Test results clearly demonstrate the efficacy of this new concept. 2012 SPIE.
