Researches And Experiments On The Electrical Part Of Free Space Optical Communication System

Free space optical system (FSO) is a new-style wireless communication technology that transmits data,voice and video by laser. FSO system provides high bandwidth with low cost, high secrecy, convenient deployment and flexibility. So it has wide applying field, especially in short-haul broadband applications, and good economic benefit. The purpose of this project is designing a low cost laser communicating system with high bandwidth which is consist of optical system, transmittal and receiver module and pointing,acquiring and tracking system. And the main assignment of this dissertation is to design the electrical part of the FSO system based on analyse of the key components.An FSO communication system is made up of two communicaton stations.Each of them includes optical antenna, transceiver module, and pointing, acquisition and tracking (PAT) subsystem. The transmittal station chooses the newest style of vertical cavity surface emitting laser (VCSEL) and the driving chip to complete optical/electric transformation. The designed circuit can inspect and adjust the laser power so that VCSEL can keep unvarying power output. In the other station, the APD is chosen as the optical receiver. And the control circuit based on the analysis of the relation between signal-to-noise ratio (SNR) and optimal multiplication gain of APD and the relation among the temperature of APD,and reverse voltage and the gain is designed.In order to mitigate the effect of building base-motion,the pointing,acquiring and tracking system is designed to assure the communication security during working period. In the PAT subsystem, the position error signal is obtained by PSD and forces the FSM to change the direction of laser transmitting, in order to ensuring communication continuity.In addition,some academic analyse on the FSO system is done in this dissertation, and some method to indicate FSO performance are introduced, including the power budget function and two curves: link margin versus range and attenuation versus range. With the calculated power budget of this system and some experiments,the optical system's feasibility is approved.