| Free space optical communication (FSO) is a new communication way, whichcombines wireless communications and optical communications. It uses optical frequencyband as a carrier to achieve information transmission, which solves the problem of shortageof wireless spectrum in wireless communication. FSO communication has attractedconsiderable attentions increasingly from academic communities due to the advantages ofhigh transmission speed, high-capacity, high-bandwidth, high security, low cost and nospectrum resource constraints. Due to the advantages of high speed, fast deployment andlow cost, FSO system can be considered as an effective solution for the “last mile problem”.Thus it has attracted widely attentions from telecommunication operators. The research onFSO communication system has become one of the current wireless optical communicationhot topics.In FSO system, the laser signal is transmitted through atmosphere, so the atmosphericturbulence can have a tremendous impact on FSO communication systems, resulting inboth phase and amplitude fluctuations of the received signal, which seriously reduce theperformance of FSO system including the fiber coupling efficiency and the Signal to NoiseRatio (SNR). With the development of FSO communication technology, adaptive optics(AO) system has been applied to FSO communication systems to achieve the wave-frontdistortion correction and atmospheric compensation. Wave-front sensing technology is thecore of AO system, which can detect wave-front distortion and correct wave-frontdistortion, providing a solution for the influence of atmospheric turbulence in FSO system.Holographic modal wave-front sensor is a new wave-front sensing technology withthe adaptation of strong turbulence and scintillation environment, which reduces thecomputational complexity of traditional wave-front sensor and improves operatingbandwidth of AO system. Thus holographic modal wave-front sensing technology has abroad application space in the wave-front distortion detection and correction of FSOcommunication system. Holographic modal wave-front sensing coding is a key part of thedesign of wave-front sensors. Studying holographic modal wave-front sensing coding inFSO system has a great significance on improving the system performance.In this paper, we make a research on the principle of holographic modal wave-frontsensing technology and describe the system model and metrics of FSO systems. Based onanalyzing the traditional computer-generated hologram (CGH) coding theory and combining the characteristics of FSO systems, three kinds of off-axis correction coding ofCGH theory are applied to the holographic modal wave-front Sensing theory in FSOcommunication system. We use Matlab to simulate the computer-generated hologram withthe use of Burch coding, Huang coding and improved Lee coding, which are coded withsingle aberration mode. In addition, we use the residual aberrations, fiber couplingefficiency and signal to noise ratio (SNR) of the FSO system to evaluate the three kinds ofoff-axis coding in FSO system. Simulation results show that Burch coding has less residualaberration, better fiber coupling efficiency and better SNR than Huang coding andimproved Lee coding in FSO system.We make an extension on the basis of holographic modal wave-front sensing codingwith single aberration of FSO system. We use holographic multiplexing technology tosimulate the effect of the three kinds of off-axis coding on the performance of FSO systemwith multiple aberrations. And the wave-front aberrations are corrected based on thecorrection theory of AO system. Simulation results show that the performances of FSOsystem which uses Burch coding are better improved than those which use Huang codingand improved Lee coding. The fiber coupling efficiency, SNR and the intensity of the lightspots on the focal plane after correction are better improved with the three kinds of coding. |
PDF Full Text Request