Oceanic in Situ Fraunhofer-Line Characteristics (Fraunhofer-Line Underwater eXperiment: FLUX)
Final rept. FY89-FY91,
NAVAL COMMAND CONTROL AND OCEAN SURVEILLANCE CENTER RDT AND E DIV SAN DIEGO CA
Pagination or Media Count:
Daytime performance of underwater optical communications in the upper ocean is limited by solar background. To enhance performance, numerous researchers have proposed using signal wavelengths within the reduced solar background provided by Fraunhofer lines. The perfor- mance improvement references 1 and 2 gained by operating in a Fraunhofer line can be significant and, at some wavelengths, approaches an order of magnitude in signal-to-noise ratio. However, recent work in optical oceanography suggests the presence of an additional light field references 3-7 in the upper ocean. The effect of an increased light field could significantly reduce potentially eliminate the absorption depth of Fraunhofer lines and reduce their solar-rejection benefits to optical communications. For a number of years, optical oceanographers have measured abnormally low values for oceanic diffuse attenuation coefficients K and in certain cases, these values have been lower references 8-12 than those for molecular water. These abnormally low values of K indicate the presence of more light at depth than for the case of molecular water. The increased light level has been suggested to result from internal-radiant emission caused by an inelastic Raman scattering process. This action produces a wavelength-shifted band of light references 3-7 with a mean frequency shift of 3357 cm. In other words, Raman scattering causes solar irradiance from shorter wavelengths to be wavelength-shifted to longer wavelengths, thus increasing ambient light at the longer wavelengths. The result of this process should be more apparent in the green spectral region reference 13 due to the transmission characteristics of seawater.