Research On Key Technologies For Microwave Photonic Radar Signal Generation And Recieving

Radar is the primary method to realize all-weather,all-time and long-distance target detection and recognition.In order to adapt to the increasingly complex electromagnetic environment,the next generation radar system should be featured with high-frequency,broadband,multifunctional integration and software definable.In the current radar systems,a direct digital synthesizer or a voltage-controlled oscillator(VCO)is used to generate the baseband signal or the intermediate frequency(IF)signal.Then,multistage frequency multiplication and up-conversion are generally employed to obtain the radar signal in the expected frequency range due to the well-known electronic bottleneck.Correspondingly,multistage down-conversion is implemented to the high-frequency echo signal before its recording and processing in the receiver.With the increasing stage of frequency multiplication and conversion,the size,the power consumption and the maintenance cost of the radar system are greatly increased.Especially,the phase noise and the amplitude-phase consistency of the radar signal degrade seriously,which inevitably deteriorates the radar system performance.Photonic technology is featured with advantages such as ultra-wide bandwidth,low transmission loss and immunity to electromagnetic interferences.Hence,microwave photonic radar is recognized as a potential candidate to realize the next generation radar system in recent years.In this dissertation,the key technologies of the microwave photonic radar are researched.Special attentions are paid to the broadband radar signal generation and receiving technologies,and the verification of the microwave photonic radar system.The main contents of this dissertation are listed as follows.(1)A parallel electro-optic modulation architecture is proposed to generate frequency-definable and bandwidth doubling radar signals.In this scheme,an IF linear frequency modulated(LFM)signal and a single-tone microwave signal are applied to a dual-parallel Mach-Zenhder modulator(DPMZM)and a Mach-Zenhder modulator(MZM),resepectively.The two modulators work in the carrier-suppression mode.Through beating the two optical signals from the two modulators,LFM signals from Ku band to Ka band can be generated,whose bandwidth is twice that of the IF signal,and center frequency can be tuned by varying the single-tone microwave signal frequency.In addition,complementarily chirped signals from X band to Ka band can be generated through applying a baseband symmetric triangular frequency modulation signal and a single-tone microwave signal to the MZM and the DPMZM,respectively.The proposed scheme is experimentally demonstrated,where complementarily chirped signals centered at 1.5 GHz and with bandwidths of 100 MHz and 200 MHz are generated by using baseband symmetric triangular frequency modulation signals with bandwidths of 50 MHz and 100 MHz,respectively.(2)A Fourier domain mode locking(FDML)optoelectronic oscillator(OEO)based on stimulated Brillouin scattering is proposed to generate tunable multi-format radar signals with low phase noise.In this scheme,a tunable pump light and a fast-tuning probe light with a definable frequency difference are generated by using a single laser source based on electro-optic frequency shifting technique.By using the pump light and the probe light,a microwave photonic bandpass filter with definable and fast-tuning passband is realized through phase-modulation-to-intensity modulation conversion based on stimulaterd Brillouin scattering.Hence,the microwave photonic filter can be used in the OEO cavity to generate tunable multi-format radar signals with low phase noise when the period of microwave photonic filter is set to satisfy the FDML condition,i.e.,the OEO loop delay is equal to an integer multiple of the tuning period of the microwave photonic filter.In the experiment,complementarily chirped microwave signals are generated by using a two-tone pump light,whose operation frequency range can be freely tuned from C band to Ku band.In addition,by using an open-loop VCO to control the frequencytime relationship of the probe light,Costas-coded signals and Costas-coded LFM signals from C band to X band are successfully generated.(3)The dynamics of the FDML OEO is intensively studied for further improving the performance of the generated radar signals.Firstly,the theoretical model of the FDML OEO is estabilished.Based on the model,the initiation oscillation process of the FDML OEO is numerically analyzed in the time domain,the frequency domain and the fractional domain,respectively.Based on the simulation,the FDML mechanism is revealed.Especially,the phase relationship between the longitudinal modes is clearly recognized.The results indicate that the quality of the generated radar signal is determined by the net gain of the OEO cavity and the bandwidth of the microwave photonic filter.In the experiment,the transient initiation oscillation processes of the FDML OEO from noise and from single-tone oscillation are captured and analyzed.The experimental results fit in with the simulation results,indicating that the model is a powerful tool to study the FDML OEO.(4)An inverse synthetic aperture radar(ISAR)is demonstrated based on an FDML OEO and microwave photonic dechirping technique.The FDML OEO is employed to generate the transmitted LFM microwave signal and the reference frequency-sweep optical signal for dechirping.In the receiver,the echo signal is loaded onto the reference optical signal via broadband electro-optic modulation.After photodetection,dechirping is achieved.In the experiment,an LFM microwave signal with an instantaneous bandwidth of 4 GHz is generated in the Ku band.By using this LFM signal,a range resolution of 3.8 cm and a two-dimensional ISAR image resolution of 3.75 cm×2.02 cm are realized.In addition,a Kramers-Kronig receiver is proposed to suppress the ghost targets introduced by the square law detection of the photodetector in the microwave photonic radar system.This scheme is verified by implementing numerical simulation.The results indicated that the ghost target suppression ratio is more than 15 d B and 9 d B in the dual-target and three-target ranging,respectively.