Research On Performance Improvement Of RoF Optical Transmitter Based On Direct Modulation DFB Laser

Radio over Fiber(RoF)technology combines the characteristics of wireless communication technology and optical fiber communication technology.It utilizes the advantages of small size,light weight and large bandwidth of optical fiber communication.It uses analog fiber link to transmit RF signals,and has been widely used in wireless communication,radar and other systems.The direct-modulation optical transmitter module is an important component of the photoelectric conversion in the directmodulation RoF link,and its performance determines the overall performance of the RoF link.This thesis aims at the problems of RF signal integrity and the best working point of the laser.Based on a self-developed direct-modulation optical transmitter module,main performance indicators of the RoF link are tested.By utilizing the high-frequency simulation software HFSS and the simulation software matlab,the link performance of the S-parameter,noise figure and nonlinear performance of the optical transmitter are studied.And combined with the actual experimental test results,we propose optimization schemes for the optical-emission module.The feasibility and optimization of the solution will eventually lead to an optimization scheme.Utilizing the semiconductor laser rate equation,the S-parameters in the RoF link are deduced,the laser operating state in the steady state and modulation state of the laser is simulated.The amplitude response of the RF fundamental in the direct-modulation link and the influence parameters related to the modulation bandwidth of the laser are obtained from the rate equation.Optimized model design is obtained and a theoretical scheme for optimizing the noise figure is proposed by Utilizing HFSS to simulate the return loss of the laser chip carrier model.The three kinds of noise existing in the RoF link are analyzed and the noise signal model of the small signal input of the laser is established.The relationship between the inherent gain of the link and the modulation frequency is derived.According to the simulation results,the factors that affect the noise figure in the modulation link are obtained,and a scheme is proposed to optimize the frequency bandwidth.The main nonlinear influencing factors in the link are analyzed,and the effects of second harmonic distortion and third-order intermodulation on the output spectrum of RoF link are studied.Through the derivation of the semiconductor laser rate equation and the simulation of the results,the related influencing factors of second harmonic distortion and third-order intermodulation distortion are obtained.An optimization scheme for system nonlinearity is proposed.Through the comparison of the performance parameters of the proposed S-parameter,link noise figure and harmonic distortion optimization scheme,the feasibility and optimization degree of the optimization scheme are obtained,and the optimization result of the performance improvement of the optical transmitter is obtained.