Laboratory development of a LIDAR for measurement of sound velocity in the ocean using Brillouin scattering

There have been several studies of the potential accuracy of LIDAR measurements of sound velocity in the ocean by measuring the spectral shift of the backscattered Brillouin lines, However, due to technical limitations, such systems have not previously been experimentally demonstrated. Measurement of the Brillouin shift as a function of depth in the ocean requires a stabilized, narrow linewidth, pulsed laser, and a high-resolution spectroscopic technique. We have used a scanning Fabry-Perot to obtain the first frequency resolved measurements of Brillouin scattering in water using a pulsed laser; these results will be presented here. But for the practical application of measuring Brillouin shifts as a function of depth in the ocean a non-scanning spectroscopic technique is required to measure the small frequency shifts; the edge technique meets this requirement. Using it in conjunction with the edges of absorption lines in the molecular spectra of I-2 and/or Br-2, avoids the limitations associated with use of a Fabry-Perot etalon; specifically, its small solid angle of acceptance and its vulnerability in noisy environments. This new approach will be briefly described.