A Study On Photonic-Assisted Radar Pulse Compression Signal Generation And Performance Analysis

With the progress of modern science and technology,the demands for radar systems are increasing,while the single-carrier radars have difficulties in achieving both large detection range and high range resolution.Pulse compression signal effectively solves the contradiction between the detection range and the range resolution of the singlecarrier radar and has attracted extensive attention from researchers all over the world.Conventionally,the pulse compression signal generation,transmission,and processing are completed in the electrical domain.However,these methods are limited by the "electronic bottleneck",and have problems such as limited bandwidth,serious electromagnetic interference,etc.With the rise of optoelectronics and optical fiber communication technology,photonic technology has more and more connections with the conventional microwave technology,and finally,a new research field of microwave photonics has been developed.Microwave photonics combines the advantages of the two methods,such as large bandwidth,immunity to electromagnetic interference,low transmission loss,small size,and light weight.Therefore,the signal generation,transmission,and processing schemes based on microwave photonics have obvious advantages.In this thesis,the generation of pulse compression signal is studied firstly.Secondly,it is found that the performance of the radar signal generated by heterodyning two free-running laser sources will be affected by the linewidths of the two laser sources.Taking the generation of linear frequency modulated(LFM)signal as an example,a method to evaluate the influence of laser linewidth on the quality of the generated signal is proposed and demonstrated.Finally,in order to obtain accurate laser linewidth,measurement methods of laser linewidth are also studied.The main contents of this thesis are as follows1.Two external modulation based phase-coded microwave signal generation schemes with a frequency multiplication factor of two or four are proposed and studied by simulation.Two orthogonally polarized ±first order or ±second order optical sidebands are firstly generated by different modulators in the two systems.Then in the two polarization directions,a polarization modulator(Pol M)is employed to implement complementary phase modulation on the two wavelengths.Beating the two phasemodulated optical sidebands at a balanced photodetector,a phase-coded microwave signal with a frequency multiplication factor of two or four is generated.The frequency multiplication characteristic of the two systems significantly reduces the requirement for the frequency of the local oscillator signal and lowers the system cost.The two systems feature simple structure,good frequency tunability and pulse compression performance,and have a good application prospect.2.Taking the generation of LFM signal as an example,a method to evaluate the influence of the laser linewidth on the signal generated by heterodyning two freerunning laser sources is proposed.When studying the generation of pulse compression signals,it is found that the performance of the signal generated by heterodyning two free-running laser sources will be affected by the linewidths of the two lasers,and the larger the linewidths,the worse the performance of the generated signal.If the quality of the generated signal is too poor,the signal may not be used in the radar system at all.The Pearson correlation coefficient is introduced to quantify the quality of the generated LFM signal.Various factors that may affect the signal's performance are finely quantified,and it is found that the key affecting factor is the ratio of the LFM signal bandwidth to the square root of the total linewidth of the two lasers.Theoretical analyses and simulations are performed to demonstrate the correctness of the proposed scheme and lay a foundation for practical applications.3.On the basis of studying the influence of laser linewidth on the generated radar signal,a laser linewidth measurement scheme is further studied.The linewidth can be obtained directly by calculating the digital characteristics of the frequency jitters of the generated signal.It is verified by simulation that the system can realize the measurement of linewidth of single frequency laser source and swept frequency laser source in the range of 10 Hz to 5 GHz,and the measurement error does not exceed 3%.Meanwhile,the influence of other factors,such as frequency observation interval and relative intensity noise,on this linewidth measurement scheme is also studied.The advantage of this scheme is that the linewidth can be obtained directly without using a signal spectrum analyzer,and the measurement accuracy is good.