Research On Generation And Applications Of Photonics-aided Terahertz Radar Signals

With the development of social informatization,the demand for resolution of radar has gradually increased.Correspondingly,the radar system needs to provide signals with larger bandwidth,and the carrier frequency has also been continuously improved.Terahertz(THz)wave is the electromagnetic wave with frequency ranging from 0.1 to 10 THz.Compared with microwaves,THz wave locates in higher frequency band and can carry waveforms with larger bandwidth,yielding ultrahigh resolution imaging of objects.Compared with lidar and infrared radar,THz radar is superior to penetrate the smoky and dusty,which is more suitable for battlefield environments.At present,research on THz radar has received extensive attention from various countries and has become a new strategic commanding height for competition of military technology.The current research on THz radars is mainly based on electronic methods,while limited by the bandwidth of electronic devices,it is difficult to give full play to its advantage of frequency band.In comparison,photonics-based methods have the ability to process broadband signals with high speed,and have advantages of low loss and anti-electromagnetic interference.Therefore,research on the generation of THz radar signals based on photonics has important scientific significance and practical value.In this dissertation,the generation and applications of THz radar signals based on photonics,including phase-coded signals,linear frequency modulated(LFM)signals and dual-chirp signals,are studied.First,a scheme for generating phase-coded THz signals with high rate is proposed based on optical phase modulation and heterodyne technique.Then,a scheme for generating THz broadband LFM signals based on frequency-to-time mapping is provided,which releases the requirements for instantaneous bandwidth and sampling rate of receiving devices through bandwidth synthetic technique,and realizes high range resolution.Next,a system of THz LFM signal generation and de-chirp processing based on optical frequency quadrupling is proposed to achieve target positioning with high resolution and long-term stability.In addition,a scheme for generating dual-chirp THz signal is proposed via the frequency doubling method.This dissertation also conducts the two-dimensional imaging exploration of THz inverse synthetic aperture radar(ISAR)based on photonics.The major innovations and contributions of this dissertation are as follows.(1)We propose a scheme for the generation of phase-coded THz signal based on optical phase modulation and heterodyne technology with good stability and frequency tuning flexibility.Optical frequency comb with flat and broad spectrum is generated via optical modulation,and a dual-parallel Mach-Zehnder modulator is used to encode and modulate the target optical carriers respectively,which realizes the high coding rate of 15 GHz and carrier frequency of 352 GHz.The scheme has been verified by experiments,and the generated signal has high pulse compression ratio and range resolution of 1.57 cm.(2)In order to further improve the range resolution,a system of dual-band synthetic THz LFM pulses based on frequency-to-time mapping and bandwidth synthetic technique is proposed,which achieves a range resolution of 3.9 mm.The system uses bandwidth synthetic technique to process multi-band pulses offline,which not only releases the requirements for instantaneous bandwidth and sampling rate,but also breaks the bandwidth limitation of THz receiver in enabling ultrahigh resolution in the THz region.(3)We propose a scheme for the generation of photonic THz LFM signal based on optical frequency quadrupling with high positioning accuracy and stability.By beating the high-order sidebands generated by optical modulation,THz LFM signal which has four times the bandwidth of the input radio frequency signal is generated,achieving bandwidth extension of 40 GHz and a large time bandwidth product of 40,000.By performing photonic de-chirp processing,the requirements of receiving instantaneous bandwidth and sampling rate are reduced.(4)Aiming at the range-Doppler coupling effect of the LFM signal,a photonic generation scheme of THz dual-chirp signal based on frequency doubling is proposed to eliminate the ranging error.The scheme uses a Mach-Zehnder modulator to generate a THz dual-chirp signal,the bandwidth of which is twice that of the input RF modulated signal,which not only eliminates the range-Doppler coupling effect,but also has good performance of pulse compression and high range resolution rate.It is verified by simulation that the THz dual-chirp signal can reduce the range-Doppler coupling effect when measuring the moving target,avoiding the ranging error,and determine the moving speed of the object.(5)A photonics-based THz ISAR system is built to conduct a preliminary exploration on two-dimensional imaging.In the experiment,the system realizes a range resolution of 7 mm,and achieve the two-dimensional positioning of the objects based on the turntable imaging model and the far-field imaging algorithm.In conclusion,the generation and applications of THz radar signals based on photonics are studied in this dissertation.The advantages of high carrier frequency and large bandwidth of THz wave,combine with the characteristics of large operational bandwidth and anti-electromagnetic interference possessed by photonics methods,make the research results above be valuable in the exploring and further improving the resolution of the radar system,which can provide reference for subsequent work.