Research On Performance Of Microwave Photonic Link Based On Phase Modulation

Microwave photonics combines the advantages of high flexibility of radio frequency communication and large bandwidth of optical communication.It has many advantages such as low transmission loss,strong signal processing ability and strong anti-electromagnetic interference ability.It is very important for the development of mobile communication in the future and now.However,the RF signal in the link will produce a variety of non-linear effects such as intermodulation,intermodulation,mixing,dispersion,which will cause signal distortion and affect the link stray-free dynamic range（SFDR）.In this paper,the theoretical calculation method of microwave photonic link,and how to effectively suppress the third-order intermodulation distortion（IMD3）and expand the dynamic range of the link are studied.The specific research contents are as follows:1.Firstly,the working principle and performance parameters of several common devices of the link are introduced,as well as the basic concepts of link gain,noise figure,nonlinear distortion and SFDR.On this basis,a link analysis method based on Taylor expansion method is proposed.Taking the phase modulation link as an example,the Taylor series expansion results of the link output signal under different modulation coefficients are studied,and the gain,noise coefficient and SFDR of the link are deduced.Finally,the calculation results of Taylor series expansion method andBessel expansion method are compared by MATLAB.Compared with the experimental data,in the range of modulation coefficient 0<m<1.74,the gain peak obtained by Taylor series expansion method is 1-2dB different from the experimental data,which is more accurate than Bessel expansion;When the background noise is-155dBm/Hz,the dynamic range obtained by Bessel expansion is about3dB·Hz^2/3 higher than that of Taylor expansion.2.According to the gain characteristics of link,a method of IMD3 suppression is studied.Firstly,the periodic change of link gain with signal frequency is analyzed theoretically,and then the Optisystem is used to analyze the suppression effect of link IMD3 by adjusting the matching relation between MZI delay time and input dual-tone signal frequency under the dual-tone signal input.The experimental results show that when the dual tone signal frequency of the input phase modulator is similar and the IMD3 frequency of the output signal is near the cut-off frequency,the IMD3suppression ratio of the output fundamental frequency of the link increases by about 14dB.3.The phase modulated microwave photonic link with feedback is studied.The feasibility and stability of the nonlinear feedback loop are analyzed theoretically,and the link is simulated and analyzed combined with MATLAB and Optisystem.The research shows that the intermodulation term of the feedback loop can offset the third-order intermodulation term generated after link demodulation and linearize the link.By adjusting the appropriate feedback times,the dynamic range of the link can be improved.When the feedback times N=30,the suppression ratio of link IMD3 can reach 48dBm,and the SFDR of link is128dB·Hz^2/3,which is15dB·Hz^2/3 higher than that of traditional phase modulation link.