Technology Of Anti-Turbulence In Free Space Optical Communication Based On OAM-Hopping

Free-Space optical communication system has the advantages of high bandwidth,free spectrum,excellent security and immunity to electromagnetic interference.FSO can be used to quickly establish point-to-point high-speed communication links,which has a wide application prospect in emergency communication in disaster areas and rapid link expansion.However,due to the effects of atmospheric turbulence,the link stability of FSO system has been facing great challenges.Conventional anti-atmospheric turbulence technologies,such as diversity,aperture averaging and adaptive optics,are difficult to meet the development needs of modern wireless optical communication systems due to complex system structure and high cost.OAM（Orbital Angular Momentum）mode diversity which can obtain gain and improve performance from the dimension of spatial mode is studied more in recent years.But the parallel detection and analysis of multi-channel signals increases the complexity and computational power requirements of the system.A simple and effective anti-atmospheric turbulence technology based on OAM mode hopping was proposed in the paper.The OAM beam with rapidly changing topological charge replaces the traditional Gaussian beam as the carrier in the method,which can obtain the mode diversity gain like MIMO system.It is a scheme of instantaneous single-mode and macroscopical multi-mode transmission and obtains the mode gain while avoiding the difficulty of parallel detection of multi-channel signals.In this paper,the anti-atmospheric turbulence technology based on OAM-Hopping is fully analyzed and demonstrated,and its performance is preliminarily studied through numerical simulation and experiments.The main contents are as follows:1.The basic idea of OAM-Hopping and working principle of OAM mode hopping technology was expounded.Two implementation schemes of fast OAM-Hopping and slow OAM-Hopping was put forward and the differences between them were compared and analyzed to expound internal mechanism of OAM-Hopping to improve link stability.2.The transmission characteristics of OAM-Hopping was simulated with the random phase screen constructed with the power spectrum inversion method.The intensity fluctuation of OAM-Hopping is larger than that of Gaussian beam when the turbulence intensity is small(C_n²<5×10^-14).But when the turbulence intensity is large,OAM-Hopping has smaller intensity fluctuation and lower bit error rate.3.The OAM-Hopping demonstration system was designed and set up,in which the spatial light modulator and ultra-high speed optical switch was used as the mode hopping controller of slow OAM-Hopping and fast OAM-Hopping respectively.And the light intensity scintillation index under different turbulence intensity was measure.Experiments show that OAM-Hopping can effectively reduce the fluctuation of light intensity and the anti-turbulence performance of fast OAM-Hopping is better than that of slow OAM-Hopping.The anti-atmospheric turbulence technology based on OAM-Hopping proposed in this paper further improves the anti-turbulence performance of optical links without greatly increasing the complexity of the system.It is easy to be combined with the conventional FSO system and obtains the mode gain of vortex beam while free of the difficulty of parallel detection of multi-channel signals in mode diversity.It has the potential to dynamically adapt to different turbulence conditions and provides a new improvement idea for the practical development of FSO system.