Experimental Study On Fiber Array Partially Coherent Optical Communication System

Because of the advantages of large capacity,high bandwidth,good security and unrestricted frequency band,wireless laser communication has attracted more and more attention.However,the phenomenon of scintillation,beam wander and beam propagation will be made when laser propagates in turbulent atmosphere,which can make the bit error rate higher to reduce the communication performance.It is shown that the partially coherent light is one of the effective ways to improve the performance of wireless laser communication because of its ability to suppress the turbulence effect in the turbulent atmosphere.In recent years,the theoretical analysis have been systematically studied,but the experimental investigation is relatively lagging.Therefore,it is of great theoretical significance and application value to set up a partially coherent optical communication system and to carry out the experimental study in the turbulent atmosphere.This thesis is based on the study of partial coherent optical communication technology to design and build the optical communication system,adopting random-phase-modulation fiber array to produce partially coherent light.Based on the theory of laser propagation in atmospheric turbulence,the propagation properties of partially coherent light in turbulent atmosphere are studied and the relationship between BER and SNR of optical communication system is analyzed.This optical communication experimental system is based on the method of intensity modulation and direct detection,using OOK modulation to produce signal light.At the transmitter,the 10 X beam expander is used to collimate the beam and transmit the partially coherent optical signal.Then the optical signal is coupled by one fiber coupler into a 200-micron-core multimode fiber linked into In Ga As detector of 25 M bandwidth at the receiver and be converted into electrical signals.Through the comparator circuit and FPGA signal processing,the communication signal is attached into the computer in the end.The transmitter and receiver structures of the partially coherent light communication experimental system are simple and the design of modulation circuit and signal processing are of good performance and stability,which can meet the experimental study on optical communication performance.Experimental study of partially coherent optical communication in the laboratory is carried out by using the experimental system of hot air turbulence simulation and the optical communication system,in which the partially coherent light with different coherence length can be obtained by adjusting the amplitude of the random-phase-modulation voltage in the fiber array.The scintillation characteristics of the light beam after the atmospheric turbulence simulation device are measured and compared with that of the array beam without random phase modulation,which indicates that the intensity fluctuation will become smaller and the quality of the received beam will become better when the difference of coherence become terrible.And the results show that partially coherent light can suppress the turbulence effect to some extent.At the same time,the OOK intensity modulation of partially coherent light with different coherence length is carried out in the different turbulence simulation environment and oscilloscope is used to observe and test the communication performance of partially coherent light.Through the analysis of the eye height,eye width and voltage noise in the eye diagram of received signal,It can be concluded that high quality signal eye diagram can be obtained in the certain atmospheric turbulence by adjusting the coherence length of partially coherent light,and it is further shown that the partially coherent light can improve the performance of long range atmospheric laser communication to some extent.