Security enhancement in free-space optics using acousto-optic deflectors

In this paper, we propose a novel physical layer design for beam transmission that enhances the data security in a terrestrial free-space optical (FSO) communication system. The optical transmitter sends successive packets through different beam paths between the transmitter apertures and the receiver apertures using acousto-optic deflectors (AODs) with synthetic holographic gratings. Increasing the radius and intensity of the beam at the optical link raises the probability of eavesdropping; however, these parameters are sensitive to atmospheric turbulence. More precisely, as atmospheric turbulence increases, the radius of the beam increases while the intensity of the beam decreases. Thus, if the intensity of the beam is adjusted based on the strong turbulence in order to receive sufficient intensity at the receiver, then the intensity of the beam increases when atmospheric turbulence decreases, which causes the link security to be reduced. We formulate the beam settings at the transmitter where the AOD´s parameters are varied, which cause the Gaussian beam changes to the secondary Gaussian Schell-model (GSM) beam with different parameters. For such a situation, it is demonstrated, analytically supported by simulation results, that the radius of the beam at different beam paths can be controlled by changing the AOD´s parameters in different atmospheric turbulence. In addition, the radius of the adjusted beam in the secured FSO transceiver is compared with that of the ideal Gaussian and the ideal GSM beams in various turbulence conditions.