Research On Nonlinear Compensation Technology Of Semiconductor Laser Based On FPGA

With the accelerated pace of urbanization in China,the population is constantly gathering in the direction of the central city,afterwards,there is an increasing demand for communication rate.Fiber-Optic Communication undoubtedly play an incomparable role in the communication arena.The semiconductor laser is the most important source of optical transmitter module.However,its nonlinear distortion will cause unfavorable factors such as waveform distortion in the process of analog modulation in the Sub Carrier Multiplexing(SCM)communication system,reduce the transmission efficiency of data.In view of this phenomenon,this article proposes a digital predistortion technique based on FPGA to compensate the nonlinear distortion of semiconductor lasers.In this thesis,the nonlinear distortion characteristics and compensation techniques of semiconductor lasers are described in a progressive way.The specific research contents are as follows.The theoretical knowledge related to the nonlinear distortion characteristics of semiconductor lasers is introduced in this thesis.The author has studied the relevant literature at home and abroad,some methods and ideas for solving the problem are stated.The P-I curves of the semiconductor laser was determined by direct modulation method.A model of the P-I response characteristics of semiconductor lasers is established,that is,the fourth-order Volterra polynomial model.The transfer equation of laser response is fitted by the least-squares algorithm.The predistortion equation is obtained by inverse operation of the response characteristic equation.The fast algorithm of wavelet filtering and Least Mean Saquare(LMS)adaptive filtering algorithm are analyzed theoretically.The P-I characteristics of the measured semiconductor laser are filtered in MATLAB,and the filtering results of the collected data are compared with the LMS adaptive filtering and the wavelet filtering.Finally,the data processed by LMS auto-adapted filter are selected.The effect of pred istortion equation on semiconductor laser compensation is accomplished by MATLAB.The coefficients of the predistortion equation of the fourth-order polynomial in the Volterra model are calculated in FPGA.The Coordinate Rotational Digital Computer(Cordic)algorithm is introduced to calculate trigonometric functions in the predistortion equation.In the top-level module,the calculation of modulation waveforms containing predistortion information is accomplished by iterative method.The correctness of the algorithm is simulated and verified in Modelsim and on chip logic analyzer Signal Tap II.Finally,the cross-correlation coefficient and variance are used to analyze the effect of predistortion.