Reaearch On Generation Technology Of Linear Chirp Signal With High Frequency And Large Time-bandwidth Product Based On Microwave Photonics

From the perspective of modern warfare,radar has become a very important "soft" weapon in air combat,land warfare and naval warfare,with multi-target search,interception,tracking and identifiction.With the continuous improvement of the level of modern technology,the requirements for performance indicators such as radar resolution,detection range are getting higher and higher.Linearly chirped signals have been widely used in radar systems for their own advantages.However,most of the linearly chirped signals currently applied are based on electronic methods.Limited by the limited sampling rate,they have the disadvantages of low center frequency,narrow bandwidth and small Time-Bandwidth Product(TBWP),which is difficult to adapt to the application requirements of high-frequency and large-bandwidth waveforms required by radar.Microwave photonics technology has solved this problem well.The generated Millimeter Wave(MMW)signal has the advantages of high-frequency,wide-bandwidth,low phase noise,compact structure and tunability,which provides a new way for the generation of high-quality high-frequency broadband linearly chirped signals.Driven by the demand for higher resolution and longer range detection,how to obtain linearly chirped signals with higher-frequency and larger-TBWP based on microwave photonics technology has become a research hotspot in this field.This paper first reviews the research background and research status of thesis work,then expounds the basic characteristics of linearly chirped signals,radar resolution and the principle of pulse compression.Based on the comprehensive analysis of the generation principle of optical MMW signal,the method of generating multi-frequency millimeter wave signals and optical frequency comb(OFC)based on the nonlinearity of phase modulator(PM)and polarization modulator(PolM)has been deeply studied.On this basis,two linearly chirped signal generation schemes are proposed in this paper:(1)On the basis of the study of optical MMW frequency multiplication generation technology and the phase information characteristic of linearly chirped signal,a scheme for generating linearly chirped signal by using cascaded PolM is proposed.According to the characteristics of the Bessel function,by optimizing the modulation index of the first PolM,the ±3rd and the ±4th-order sidebands of the output optical modulation signal have equal and large amplitudes.Then the orthogonally polarized-4th and +3rd sidebands are abstracted by an interleaver(IL).Then the second PolM introduces an opposite parabolic phase for the two orthogonally polarized optical sidebands,and a linearly chirped signal with septupled frequency of the radio frequency(RF)driving signal is generated after the heterodyne beating.In order to improve the TBWP of the generated signal,on the one hand,the parabolic electrical driving signal is split into multiple pieces to increase the bandwidth;on the other hand,stretching the parabolic signal is used to increase the temporal width of the generated signal.The linearly chirped signal generated by this scheme not only has high-frequency and large-TBWP,but also can effectively improve the sidelobe performance.Simulation results show that chirped microwave waveform with the carrier frequency of 70 GHz,bandwidth of 29.49 GHz,TBWP of 19859.40,Peak Side Lobe Rate(PSLR)of 32.19 dB is generated by dealing with the electrical parabolic driving signal.(2)On the basis of the study of fiber Bragg grating(FBG)delay line,a linearly chirped signal generation scheme based on OFC and FBG delay line is proposed.The Continuous Wave(CW)is intensity modulated to obtain an optical pulse signal with a duty ratio of 1/16,and the optical signal is respectively modulated by RF local oscillator signals via two cascaded PolMs to generate 16-line flat OFC.The OFC is completely separated in the time domain via a time delay line composed of 16 discrete FBGs,and then using a PM driven by a periodic parabolic electric signal to introduce parabolic phase.After the heterodyne beating between the phase-modulated optical signal and another local oscillator source,a linearly chirped signal is generated.The scheme is simple in principle and tunable.Simulation results show that the generated chirped microwave signal has the bandwidth of 31.78 GHz,TBWP of 1365.33,and its center frequency can be tuned by varying the wavelength of the TLS.