EO signal propagation in a simulated underwater turbulence environment

Abstract

Underwater electro-optical, or EO, transmission is a function of medium properties and constituents within. While the majority of the research focus has been on the constituents, especially the particulate forms, recent research indicates that under certain conditions, the apparent signal degradation could also be caused by variations of the index of refraction associated with temperature and salinity microstructure in oceans and lakes. These would inherently affect optical signal transmission underwater, which is important to both civilian and military applications involving search and rescue, intelligence, surveillance and reconnaissance applications, as well as optical communications. To study the effect of optical turbulence and to mitigate its impacts, a controlled environment allowing various intensities of turbulent mixing is a critical asset. Numerical experiments as well as measurements have been carried out in such a simulated environment, in order to understand mixing setup time, development and dissipation rates. The domain is modeled after a large Rayleigh-Bénard convective tank with a length, width and depth dimension of 5, 0.5 and 0.5m, respectively. The convective mixing is realized by using heating and cooling plates at the bottom and top of the tank at given temperature differences. The computational fluid dynamics model is implemented with large eddy simulation approximation. Dissipation rates from model and measurements are compared and suggest fully developed turbulence has been achieved by this setup. Optical signal transmission under these conditions are also examined, through image degradation using image quality metric, and phase screen models from corresponding power spectrum. The integrated temperature variation along the transmission path is compared to generated phase screens, along with discussions on reducing uncertainties in estimation of key parameters.