Single Stage and Dual Stage Amplification for All-Optical Amplify and Forward Relays in FSO Links using DQPSK Signal

Abstract

Free-Space Optical (FSO) communication networks offer ultra-fast data transmission that exploits license-free spectrum and are also a good replacement of the traditional communication technologies. While multiple environmental factors including atmospheric conditions, electromagnetic disturbances, and physical obstacles causing significant degradation of signal quality and therefore data transmission reliability are highlighted, the question remains whether the current solutions and problems with respect to technology can be overcome. This thesis aims to address these challenges by examining the effectiveness of single and dual-stage amplification approaches for All-Optical Amplify and Forward (AOAF) relays in FSO links, with Differential Quadrature Phase Shift Quadrature Phase Keying (DQPSK) signal messages. The methodology consists of a structural comparison of some single-stage amplification applications with and without filters against different dual-stage amplification cases that contain one or two filters. The filter types used in this study are Bessel, Butterworth, Gaussian type, which are spectrum models. The results indicate similar performance for Bessel and Butterworth filters. The solution deals with signal degradation by strengthening the signal using amplification techniques that evenly overcome fading and multipath interference as well as reduce bit error rate to enhance reliability of such a link for more than 1km. Results from these experiments demonstrate that two-stage amplification method is the best option of all the options when it comes to reducing signal loss and ensuring signal transmission reliability, especially in high turbulence environments. These findings highlight the key role of a strategic amplification and filtering approach in optimal functioning of FSO systems aimed at achieving a better signal-to-noise ratio and stronger signal, which results in a minimal attenuation loss. In addition, this thesis explores on the fundamental limitations of FSO communication and gives realistic solutions to deal with the problems affecting the system performance.