Mitigation of atmospheric effects on terrestrial FSO communication systems by using high-speed beam tracking antenna

Abstract

When a free-space optical beam propagates through the atmosphere it experiences deterioration and deformation of its wave-front caused from small scale, randomly localized changes in the atmospheric index of refraction. This results in beam wander and scintillation effects which can reduce the link availability and may introduce burst errors. This paper outlines experimental work on a free-space optical (FSO) communication system which connects an optical beam directly to a single-mode fiber (SMF) without any optical-to-electrical (O-E) conversion. In order to effectively couple the 1550 nm transmitted optical beam to a SMF it is necessary to be able to track and control the beam angle-of-arrival (AOA) changes. To achieve this, we have developed an optical antenna which uses a fine positioning mirror (FPM) capable of performing high-speed beam tracking and steering thus reducing to a great extent the optical power fluctuations of the received beam coupled to the SMF. This optical power fluctuation is partly a result of beam angle-of-arrival fluctuations caused by atmospheric turbulence. In our experiments we have tried to measure and quantify the magnitude of atmospheric turbulence experienced by an optical beam propagating through the atmosphere. First we demonstrate the relation between the AOA fluctuations and the frequency characteristics of the scintillation effects for a free-space optical beam propagating through a turbulent atmosphere. We use this information to determine the optimum antenna FPM tracking speed for improved performance and error free transmission. The results in the improved fiber received power as well as continuous error free transmission are presented.