Simulation, Modeling, and Design of Underwater Optical Communication Systems

Abstract

Underwater free-space optical communications has the potential to provide high speed, low latency communications for undersea vehicles and sensors. This thesis describes the design and vali- dation of aMonte Carlo numerical simulation tool for underwater optical communications systems. The simulation tool can also be used more generally for other systems that require calculations of the underwater light-field. The program, named Photonator, was validated experimentally in a laboratory tank where the absorption and scattering was controlled by the addition ofMaalox to vary the water conditions from open ocean to turbid harbor water. These resultswere also compared with custom blue/green light emitting diode and laser transmitters and receivers that allowed the wavelength and field-of-view (FOV) to be controlled. An emphasis was placed on understanding the requirements of point-to-point underwater com- munication links. Results are presented for on and off-axis received power for a series of receiver apertures and fields-of-view. Also presented are the scattering histograms at the receiver and the temporal bandwidth of each communication link. A two-termexponential power loss model is de- veloped and compared with the simulated outputs to agreement within 30% over twelve orders of magnitude power loss. This type of power loss model is useful in constructing link budgets which are more accurate than the usual Beer’s law assumption in water environments where scattering is appreciable. Several results are presented that are of interest to the underwater optical systems designer: 1. The simulations and experiments showthat the power gain from FOV and aperture changes of an optical system are independent in highly turbid waters. 2. A power-law relationship between FOV and received power is shown for turbid water environ- ments for fields-of-view up to 45 degrees. 3. A systematic series of simulations showhowthe scattering orders at the receiver evolve as water quality is varied which provides a physical underpinning to understanding temporal dispersion of underwater pulses. 4. A systematic series of simulations shows howthe temporal bandwidth of underwater optical communication systems varies strongly with the receiver field of view, but weakly with aperture size.