Generation Scheme And Performance Analysis Of Optical Frequency Comb Based On External Modulator

Along with the general progress and development of modern communication network systems,traditional microwave wireless communication technology has been difficult to meet the increasing demand for communication system capacity and information transmission rate.In recent years,optical fiber communication has gradually attracted people's attention due to its advantages of high bandwidth,low loss,high resistance to electromagnetic interference,large capacity,and high confidentiality.Scientists are trying to combine the merit of microwave wireless communication and optical fiber communication to cope with the current dilemma of capacity's supply under the environment of communication,and thus emerged a new type of interdisciplinary subject called microwave photonics.Microwave photonics is suitable for a variety of application scenarios and is one of the core technologies in the future communication.Optical frequency comb,as one of the important part of signal generation in microwave photonics,is the research object of this thesis,and it is quite of value in both throretical research and actual practice.By the means of theoretical analysis,formula deduction and simulation verification,this thesis tries to construct system that can generate high quality optical frequency combs starting with several indexes(high flatness,high off-band rejection ratio,more number of comb lines and adjustable frequency interval)which can be used to judge the performance of optical frequency comb.Referring to the above research,the following works has been carried out:(1)First,two schemes are proposed for generating flat optical combs based on external modulator,and the basic modules in the two schemes are analyzed in detail.That is,a single tone signal is modulated based on a single intensity modulator and a single dual parallel Mach-Zehnder modulator,respectively.A structure is designed in scheme 1 which two intensity modulators are cascaded with a dual parallel Mach-Zehnder modulator.Multi-line flat optical combs will be generated by controlling the working states of the modulators and the parameters of the input driving signals at various stages.Finally,50-line optical frequency combs with flatness of 0.45 d B and out-of-band rejection ratio of 15 d B are obtained through the simulation in this programme.In the scheme 2,the structure based on a dual parallel Mach-Zehnder modulator is used.By controlling the relationship between the operating points of the upper and lower arms of the modulator and the respective input signals,the power of the center component of the output spectrum is compensated to generate an ultra-flat optical comb.In the end,4-line ultra flat frequency combs with flatness of 0.07 d B and out-of-band rejection ratio of 47 d B are generated through the simulation.(2)Next,the scheme is proposed that the structure of two-tone signals modulated by a phase modulator which can generate 9-tone flat combs is used as an optical frequency comb generator embedded in a recirculating frequency shifter.The basic module in this scheme is analyzed in detail,that is,a system that modulates single tone signals and dual tone signals based on a single phase modulator.The system uses the amplifier in the loop to compensate for the power loss of the signal and the filter to eliminate the loop noise,thereby forming a loop system that can generate an ultra-wide spectrum cyclically.The simulation results show that when the number of cycle is 15 times,the output of system can be an ultra wide spectrum consisting of about 150-tone optical combs with flatness of 6d B.With the increase of cycle number,the flatness performance of the optical combs does not deteriorate.