The Coupling And Beam Control Technology In Free Space Optical Communication

With advantages of broader bandwidth,convenient to install and no frequency spectrum licence,free space optical(FSO)communication has got intensive research in recent years.Both the fiber-coupling and beam-control technology play an important role in FSO communication system.In this dissertation,a profound research of the two technologies mentioned above has been carried out with main results as follows:(1)Under the ideal condition,a simplified formula is derivated to calculate the coupling efficiency of plane wave and Gauss beam coupling into a single mode fiber by a lens.It is proved that the coupling efficiency has an extreme value while the relative aperture of the lens increases.(2)Under the weak atmospheric turbulence condition,we use the Mutual Coherence Function(MCF)for plane wave under Von Karman spectrum to derivate the formula which is to calculate the coupling efficiency,and use Cross Coherence Function(CCF)to derivate the formula which is calculate the relative variance of the coupling power scintillation under the same spectrum when beams coupled into a single-mode fiber.And those formulas are separately derivated when the beams are convergenced by a lens,a lens-array and a Maksutov-Cassegrain telescope.The performance of the array and large aperture coupling technology is also proved experimentally in this dissertation.(3)We have proved that the simulated annealing algorithm and particle swarm optimization algorithm can be used to the automatic alignment system of the fiber-coupling.And the influence of relevant parameters of the algorithm on the coupling efficiency is also analyzed.(4)It is proved that the self-coupling phenomenon exists when the array Gauss beam propagates in turbulence.And Off-axis emission technology is proposed to solve the attenuation problem caused by the second mirror of the Maksutov-Cassegrain.In this way,the transmission efficiency is lifted.(5)A transceiver antenna based on Maksutov-Cassegrain is designed and emited with single mode fiber and received with multimode fiber.The work of this dissertation provides a way to realize the compatibility of FSO system and optical fiber communication system,and also lays a theoretical and experimental foundation for the application of lens-array and large-aperture coupling technology in atmospheric turbulence.