| In modern electronic countermeasures and communication systems,traditional electronic devices have gradually been unable to meet the requirements of current and future systems due to their "electronic bottlenecks" such as poor tunability,small operating bandwidth,and severe electromagnetic interference.Microwave photonics combines the advantages of microwave technology and optical technology.Compared to traditional single microwave or optical technologies,it has stronger electromagnetic interference resistance,greater operating bandwidth,higher sensitivity,and lower transmission losses.It gradually provides more innovation and technology for radar,satellite communication,and optical fiber sensing.Mixers are an important part of electronic warfare,wireless communication and radar system receivers.Compared with the traditional use of electronic devices for mixing,mixers based on microwave photonics are becoming more and more popular because of their advantages such as large working bandwidth,local oscillator isolation and strong anti-electromagnetic interference ability.Due to the intricacy of the electromagnetic field during the process of reception.,the signals received by the antenna will have other additional interference signals in addition to the required radio frequency signals,including the image frequency signals with the same frequency interval between the radio frequency signals and the local oscillator signals.The radio frequency signal will be interfered when mixing,so the image-reject technology should also be considered in the down-conversion system.The thesis focuses on the research of image reject mixer(IRM)based on electro-optical modulation and microwave photonics.The primary focus of the research is as follows.Firstly,the development and significance of microwave photonics are introduced.Then it introduces microwave photonic mixing technology and image-reject technology,expounds the necessity of image rejection,and focuses on the research progress of microwave photonic image rejection by domestic and foreign scholars at present.Secondly,the structure and principle of several important devices in microwave photonic link are introduced.It paves the way for the system scheme proposed in this thesis.In this thesis,a microwave photonic image rejection mixing scheme based on 90°optical coupler(OHC)is first proposed.The 90° OHC can simultaneously introduce accurate 0,π,π/2 and 3 π/2 phase shifts to the input signal.A balanced Hartley structure based on 90° OHC and balanced photodetector(BPD)is built to suppress the image interference.After the principle analysis and simulation of the scheme,it is verified that the image rejection ratio can reach about 66 d B in the frequency range of 10 ～ 40 GHz and has good amplitude equalization.Finally,it is verified by experiments that the image rejection ratio can reach about 36 d B in the wide bandwidth range of the signal,which is higher than the traditional electronic image rejection mixing technology.Then,a microwave photonic image-reject down-conversion scheme with longdistance transmission is proposed.Firstly,the background and necessity of antenna stretching application are introduced.Secondly,an analysis is conducted on the principle of the proposed scheme.Based on the principle of Hartley classic structure,the optical delay line is used to realize phase shift and eliminate the image interference.The simulation results show that the structure can achieve image rejection downconversion in the operating frequency range of 10 ～ 40 GHz,and the image rejection effect is basically maintained at about 48 d B,and the image rejection effect of the system is still obvious when the antenna is extended for transmission. |
