Research On Modal Noise In Multimode Fiber Microwave Photonic Links

Modal noise, which is generated from multimode fiber, exhibits parasitic amplitude modulation of signals, and degrades the performance of digital and analog communication systems. By combining the advantages of optical fiber communication and wireless communication, Microwave photonic links (MPL) have advantages in large capacity, lossless, easy to access, and low system complexity. Recent years, multimode fiber based MPL has consentrated many attentions, for the feature of easy installing, low cost supporting devises of multimode fiber. Ways can be found to improve the link performance by analyzing the modal noise in multimode MPL.First, the background and present situations of multimode fiber MPL were introduced, and then, transfer properties of multimode and necessary condition of modal noise were analyzed. Frequence response of multimode fiber decreases with the increasing of frequence in low frequence area, there are many passband can be utilized in high frequence. Modal noise is a kind of multiplicative noise, the presence of modal noise will decrease the opening degree in eye diagram and increase the BER in digital communication systems, signal-noise ratio will be worse and nonlinear distortion will appear if analog communication systems were considered.The performance parameters in multimode fiber MPL mainly contain gain, signal-to-noise ratio, nonlinear distortion product, error vector magnitude, error symbols ratio and the Q factor. Some choices need to be selected when design a multimode fiber MPL, the chosing of fibre, light source, modulation technique and detection technique was introduced.By giving the structure of IM-DD multimode fiber MPL and analyzing the signals at every port of the link, the theory model of modal noise was acquired. And then the relationship between performance and modal noise, and more original, the relationship between frequency chirp, optical modulation index, launch condition, fiber length, detecting area and performance, were studied.The performance, including gain, carrier to noise ratio and nonlinear product, of multimode fiber MPL was analyzed by utilizing the theory model. The system using different digital modulation mode was simulated, and acquired the relationship between modulation index, modulation mode digital performance. Then, it can be concluded that lower frequence chirp, higher optical modulation index, central launch technical and bigger detecting area can improve transmission performance by reducing the effect of modal noise to gain, carrier to noise ratio and nonlinear product.At last, the thesis was concluded and several perspectives for the future work were made.