Bias Control Of Mach-Zehnder Electro-optic Modulator In RoF Links

Radio over fiber (RoF the:Radio over Fiber) technology refers to the use of fiber instead of the atmosphere as a transmission technology for wideband RF signals.The RoF technology combines the advantages of optical communication and wireless communication.and has found many applications.lt has become a hot topic of wireless communications and optical fiber communication.As a key component of the optical fiber communication systems, the LiNbO3Mach-Zehnder modulator loads an electrical signal on laser carrier by modulating the amplitude of laser to change with the high-speed microwave signals. The transfer function of Mach-Zehnder modulator is a nonlinear periodic function. In order to meet different application requirements, the modulator should operate at different bias point.However, the bias point is not stable, it will drift with time, ambient temperature, laser power and optical fiber insertion and coupling loss factors, etc., and leading to the deterioration of the output signal. Therefore the Mach-Zehnder modulator bias point control technology is of great importance. In this thesis, based on the traditional Mach-Zehnder electro-optic modulator bias control theory, a method of feedback control based on low-frequency response of dither tone signal is presented. An arbitrary working point controllable bias control system is brought out, by harmonic analysis of low frequency dither tone signal with FFT algorithm.This thesis mainly includes three parts. The first part is the introduction of radio over fiber (RoF) system. Mach-Zehnder modulator is used in the RoF system, so first the backgound and application of RoF system is introduced.The second part gives a brief introduction to the optical devices and modulation theory used in RoF system.And theoretical and mathematical model of the Mach-Zehnder modulator is especially analyzed.Finally, on the basis of the analysis of the link model and optical devices, specific Mach-Zehnder modulator bias point control system is designed and implemented. The RF transmission performance of the system in temperature changing circumstances is measured and analyzed. The results show that the system can meet a certain target range.