Strategies for limiting interference and interception of free space optical communications

Abstract

Free space optical systems provide an attractive solution to communication needs that require inexpensive, easily deployable links capable of high data rate transmissions. A major challenge of free space optical communication is ensuring the integrity and confidentiality of the transmitted information. In an optical wireless network with close-in users, communication between two users could interfere with another link in the network. Such systems are also susceptible to eavesdropping, especially inside the main lobe of the transmitted beam. In this thesis, we propose a method of controlling the direction of energy propagation from an optical transmitter to maximize the power received by the remote terminal of a link while limiting the power received in a broadly defined region within the main lobe of the transmission. We consider specifically an optical transmitter comprised of an array of apertures with controllable amplitudes and phases, and we approximate the intended suppression region with a finite number of points. We assume the total transmitted power is held fixed. Via iterative numerical methods, we solve a nonlinear optimization problem for the weight vector that maximizes the intensity at the receiver while limiting the intensity at the specified suppression points to below some fraction of the intensity at the receiver. For a linear aperture array, we show that without overly limiting the power to the intended receiver, it is possible to suppress the signal intensity in a 1 beamwidth region located 0.2 beamwidths from the intended user down to one tenth of the intensity at the intended receiver. For a two dimensional array, we show that we can similarly suppress a !!!!! beamwidth region as close as 0.2333 beamwidths to the intended receiver. We further show that by increasing the number of suppression points used to approximate the suppression region, we can suppress a region much closer to the receiver, but at the cost of significantly lowering the intensity at the receiver. We also observe a tradeoff between the size of the suppression region and our ability to limit the signal intensity throughout the entire region. We show that our ability to successfully suppress power to the required region is limited by both our available transmit power and our uncertainty of the position of the eavesdropper or network user.