چكيده به لاتين
In recent years, significant advances have been made in the laser production with different characteristics and specifications. These developments have been done in order to use the laser for specific applications and also to optimize them. The possibility of laser beam usage in various applications of wireless communication in one side and ever-increasing human need to the more bandwidth for transferring any kinds of data in the other side, lead to the vast research in this field which uses laser beam as a data carrier in a wide range of strategic applications. Free space optical communication, satellite communication, Lidars, remote sensing, imaging, underwater communication, marine exploration and research, targeting and navigation are examples of these important applications.
Most of these applications are based on the laser beam propagation in media with random refractive indexes (i.e. Optical turbulence) such as the atmosphere, the sea and ocean waters. In addition to the atmospheric transmission channel which has provided the appropriate medium for the operation of a wide range of different applications and has been studied for decades, in recent years, due to the existence of human in the seas and oceans for exploratory, commercial, and military reasons, the use of secure underwater communication channel is also increasing dramatically. In all of these applications, for precise design and characterization, along with the well-behaved absorption and scattering phenomena, different effects of the challenging optical turbulence phenomenon such as beam spreading, reduction of spectral degrees of coherence and polarization, beam wandering, and scintillation have to be considered in calculations, simulations, and experimental investigations.
Therefore, due to the weakening and destructive role of optical turbulence phenomenon and its most important effect, namely the scintillation effect, on the efficiency and the quality of underwater wireless optical links and in order to exploitation of the potential benefits of such systems, this thesis is focused on theoretical and experimental study of optical turbulence effects on the underwater wireless optical communication link's parameters. Achieving these aims, the two basic steps have been considered. In the first step, based on the extended Huygens-Fresnel method, firstly, the effects of optical turbulence on the propagation behavior of statistical characteristics of single and array laser beams with different profiles (Gaussian and Flat-topped) such as intensity distribution, beam width, spectral degrees of polarization and coherence have been analytically investigated. Then, with numerical simulation of scintillation phenomenon, its influence on the underwater wireless optical communication (UWOC) link quality parameters such as the Bit Error Rate (BER) and the Signal to Noise Ratio (SNR) have been studied. In the second step, with the help of the controllable experimental setup and simulation of an underwater turbulent environment (in which turbulence induced by temperature gradient), the scintillation phenomenon and its effects on different parameters of UWOC links have been studied under various conditions. All the simulation results of this thesis have been shown by graphs and they have been analyzed in detail.
It is hoped that the results of analytical calculations and experimental simulations which are done in this thesis have significant influences on localizing this progressive technology and in the performance improvement of underwater laser communication links in different situations.
