Study Of Image-reject Frequency Conversion Technology In Microwave Photonic Signal Processing

Microwave photonics(MWP)is a cross-disciplinary field developed to meet the growing demand for information,in which the optical devices and techniques are used to generate,process,control and distribute microwave and millimeter-wave signals.Thanks to the inherent advantages of photonics technology,MWP link exhibits low transmission loss,large bandwidth and immunity to electromagnetic interference(EMI).The functionalities of microwave signal generation,true time delay,microwave photonic filtering(MPF),phase shift,frequency conversion,arbitrary waveform generation,beamforming,frequency measurement and analog-to-digital conversion have been realized with MWP techniques.Microwave photonic frequency conversion is an important function in an MWP link.For example,in a radio-over-fiber link,the intermediate frequency(IF)signal is up-converted to a wireless transmission frequency signal by mixing with the local oscillator at the central office(CO).The up-converted signal is then transmitted to the base station(BS)by the optical fiber.At the remote BS,the signal is detected,amplified,filtered and directed to an antenna for free-space transmission to the customer unit(CU).The radio frequency(RF)signal from the CU is converted to the optical domain at the BS and then transmitted back to the CO for down-conversion and further processing.Compared with conventional electrical mixer,an MWP frequency conversion link has the advantages of large bandwidth,high port isolation and immunity to EMI.In heterodyne receivers,the interference from the image frequency is a common problem.To solve this problem,MWP image-reject frequency conversion technology is studied in this thesis.The work of this thesis is summarized as follows:(1)The key devices and theories involved in this thesis are introduced,which lays a foundation for the research work.Specifically,the theoretical model of lithium niobate modulator is established by mathematical derivation,and the principle of electro-optic modulation and photodetection is introduced.In addition,the basic theory of stimulated Brillouin scattering(SBS)is described.(2)An MWP image-reject frequency conversion link based on SBS-MPF are proposed.Theoretical analysis and experiments are both carried out to verify the proposed scheme.The key to this scheme is to recover the desired signal by converting the phase-modulated desired signal to an intensity-modulated signal,which is achieved by using SBS.At the same time,the image is eliminated because it still maintains the phase modulation format.The feasibility of this scheme is verified by theoretical simulations and experiments.In experiment,the influence of the optical pump power and DC bias voltage on system performance are investigated,and the operating frequency range,spurious-free dynamic range of the proposed scheme are tested.(3)An integrated MWP down-conversion system with image rejection is proposed,which is implemented by two cascaded silicon-on-insulator(SOI)-based Mach-Zehnder interferometers(MZIs)and a microring resonator(MRR).The proposed scheme is verified theoretically and experimentally.The principle of image rejection for over-coupled MRR,critically coupled MRR and under coupled MRR is illustrated,respectively.The image rejection ratio and conversion gain when an RF signal within the frequency range of 6-24 GHz is down-converted to a 1-GHz IF signal are measured.In addition,the low conversion gain is analyzed.