Angular Spectrum Measurements of an Underwater Optical Communication Channel

This thesis describes an investigation of certain aspects of laser light beam propagation in a simulated ocean environment. The specific parameter of interest here is the angular spectrum, or angular distribution, of the received optical power after it traverses the underwater medium. Knowledge of the angular spectrum width is required by the communication engineer in order to specify the field of view of a receiver system for underwater optical communication. In the course of this work, results from existing atmospheric scattering theory were modified to apply to the underwater channel. This modified theory was used to predict angular spectrum widths for varying underwater conditions. An experiment was set up to measure angular spectra under these conditions. The experiment consisted basically of a HeNe laser (X = 6328 A) transmitting through chemically prepared water toward a submerged narrow-field-of-view radiometer. The radiance at the receiver was measured by scanning the radiometer in the azimuth direction while the elevation angle was held constant. The angular spectrum width was computed as the angle at which the measured radiance was down to 50% of its peak value. The theoretical and experimental values were then compared. The major conclusion of this thesis is that the theory accurately predicts the angular spectrum of the multiply-scattered component of the received power. The theory is deficient, however, in that it assumes that the multiply scattered light is the dominant component. It is shown in this thesis that there is a significant amount of unscattered and singlescattered light under the conditions simulated. Furthermore, this nonmultiply-scattered component is strong enough to completely swamp out the multiply-scattered component under certain conditions, and tends to make the angular spectrum narrower than predicted by the theory. Thesis Supervisor: Harold E. Edgerton Institute Professor, Emeritus