A Novel Low-Bias Method Research For Analog Optical Link

In the last decade,more and more attention has been paid to radio-over-fiber(ROF)technology due to its widely applications in wireless broad-band communications,intelligent transportation systems and so on.ROF technology,which combines the advantages of wireless communication and optical fiber communication,is essentially a communication solution of analog optical link.The stable control for bias of Mach-Zehnder modulator is an important research topic in the external modulation of ROF link.The bias drift will reduce the gain of RF link,increase the noise figure,at the same time reduce the Spurious-freeDynamicRange(SFDR),which will finally impact on the system ' s performance of ROF link.Therefore,the significance can be very important to find and automatically control the bias scientifically.Meanwhile,keep the modulator work under the low-bias angles will improve the efficiency of modulator,what' s more,it may also be advantageous for reducing the noise figure and increasing the SFDR more effectively.Thus,the research of low-bias control methods in the analog optical link possesses very important significance.This paper first introduces the development background of radio over fiber communication technology and explains the significance of low-bias control for the Mach-Zehnder modulator in the ROF system.Then we expound and summarize the existing two common technologies of automatic bias control:the method based on the optical power and the method based on the dither signals.Considering all the above techniques,we design a novel dither-free low-bias control skill for analog optical links.In the proposed scheme,the low-bias stabilization is achieved by adding the reverse transmitting lightwave with power locked to the Mach-Zehnder modulator.In this paper,we describe the theoretical analysis and make the simulation that this low-bias control method based upon.Through the Taylor expansion for the transmission function of Mach-Zehnder modulator,we explore the main factors which cause the bias drift.The simulation for transmission loss of Mach-Zehnder modulator is based on modulation depth and phase,which shows the method is reasonable in theory.Further,we explain the detailed design of each part in the feedback circuit which realizes this low-bias control method.Based on the above theories and circuit design,we did the experimental vertification for the dither-free low-bias control method through constructing the real experiment circuit.We measured the power of output optical signal and the bias angles for Mach-Zehnder modulator in the experiment,made comparison and finally analyzed the experimental results.We can see from the experimental results,with the increase of input signal power,the ratio of output signal power and input signal power to the Mach-Zehnder modulator was very close to the theoretical value,which was close to a constant.At the same time the bias angle remained to be constant,during the process that the input power of the modulator increased from 0 to 20dBm,the bias drift limited to only 2 to 3 degrees.This result show that,this design scheme can effectively achieve the automatic control of any low-bias point in the analog optical link.