Research Of Theoretical Model And Key Technologies Of Phtonic Time-stretched Coherent Radar System

Microwave photonics technology is an important technical method for the new generation radar system to possess the characteristics of broadband,miniaturization and reconfiguration.With the development of microwave and digital electronic technology,modern radar systems have the operating advantages such as multi-function,multi-band and digitization.However,limited by the problems such as transit time,electromagnetic interference and time jitter,the radar systems implemented by microwave and digital electronic technology usually work at a specific frequency and limited bandwidth,making the resolution unsaticfactory in the applications such as detection and imaging.Light waves and microwaves both belong to the electromagnetic waves and optical devices have outstanding performance in the aspects of volume,weight,bandwidth and electromagnetic shielding.By combining the advantages of microwave and light waves effectively,microwave photonics technology can not only solve the problems on size,weight and power,but also help the new generation radar systems break through the restrictions on frequency and bandwidth,accelerating the upgrading process of radar systems.For the microwave photonic technology,it is common to use the dispersion effect of light waves to implement microwave signal processing.When light waves with different frequencies are propogating in the medium,they will be influenced by the material and structural characteristics of the medium,thereby possessing different transmission speeds.The photonic time-stretched coherent radar system based on the dispersion effect,which was proposed by our research group,is one of the important implementation schemes of all-optical microwave radar systems.For signal generation,high-frequency broadband signals can be achieved by means of interference on signals with unbalanced dispersion.For signal reception,the frequency and bandwidth of microwave signals can be compressed severalfold by time stretch technology.Compared with the conventional microwave radar systems,high-frequency broadband microwave signals can be directly generated in the photonic time-stretched coherent radar system and it is tunable in the aspects of frequency and bandwidth,and the microwave signals can be received and processed by the low-speed high-precision sampling and quantization devices,which solves the issue of data acquisition rate.This dissertation mainly studies the theory and key technologies of the photonic time-stretched coherent radar system.Based on the physical mechanism of the dispersion effect,the theoretical model and simulation of the system architecture and system performance are achieved.Focused on the limitations and deficiencies such as detection distance and range resolution,we made breakthrough on key technologies and optimized the system architecture.Moreover,experiments were made for theory and simulation verification.Finally,we designed and implemented the principle prototype of the photonic time-stretched coherent radar system and achieved the high-resolution detection on a small drone as the target.The main contributions and achievements of the dissertation are as follows:First,the related physical mechanism of the photonic time-stretched coherent radar system is researched and the theoretical model is established.Several key performance parameters of the pulsed radar system,such as detection distance,range resolution and time-bandwidth product,are analyzed,then the theoretical model of the photonic time-stretched coherent radar system is made.Based on the physical mechanism of the dispersion effect,the generation and reception process of the microwave signal is deduced and analyzed.The relationships between the system paramters and the system performance are especially focused,such as pulse repetition rate and signal bandwidth.Finally,the whole process of signal generation and reception in the photonic time-stretched coherent radar system is simulated,providing the theroretical basis for key techonologies breakthrough and priniciple prototype development.Second,aiming at the contradiction between the detection distance and the receiving window of the system structure,we made researches on the technology of detection distance extension.The pulse repetition rate of the laser will confine the detection distance of the radar system,and the adjustment range of the fiber delay line will also limit the receiving window of the receiver.In the transmitter,by adopting the method of optical switch synchronous control to reduce the pulse repetition rate of the transmitted signal,long-distance target detection can be achieved.In the receiver,by the combination of wavelength-to-time mapping and high pulse repetition rate,echo signals can be received in separate sections,eliminating the restriction on the receiving window.In principle,the proposed scheme can increase the detection distance from several meters to several kilometers,which is originally limited by pulse repetition rate of the laser.A tenfold extension of the detection distance is demonstrated by experiment.Further more,single and dual targets near the longest detection distance are used to verify the detection ability.Third,in the face of the challenges on the power fading and range resolution degradation when receiving broadband signals,we made researches on the technology of broadband characteristic compensation.The way of double sideband modulation in the receiver is under the influence of the dispersion effect,which will cause the amplitude distortion of the received echo signal and leading to peak power loss and range resolution deterioration of the output results after matched filtering.Based on the analysis of the transmission charactersitics of various modulators,an innovative broadband reception scheme based on dual channel combination with quadrature phase is proposed to realize the amplitude recovery of the broadband signal and eliminate peak power loss and range resolution deterioration.The proposed method can realize the reception of high-frequency broadband signal while maintaining the peak power and range resolution characteristic of the signal in the meantime.The experiment demonstrates the peak power compensation of broadband signal with the bandwidth of 4 GHz at X band and achieve the range resolution of 4.8 cm,which agrees with the ideal result.Fourth,focusing on the shortcoming of the short pulse width generated by dispersion effect,we made researches on the technology of time-bandwidth product enhancement.The pulse width generated in the transmitter is limited by the available filtering bandwidth and dispersion coefficient,resulting in a low time-bandwidth product of the signal and then affecting the signal to noise ratio of the output results after matched filtering.By analyzing the evolution process of the optical pulse under the influence of the dispersion effect,a novel signal generation scheme based on the optical pulse recirculation is proposed.The signal to noise ratio change of the pulse signal in the dispersive loop is theoretically analyzed.The pulse width can be multifold increased and the time-bandwidth product of the signal can be effectively improved.Broadband signals with a pulse width of100 ns and bandwidth of 4 GHz are demonstrated in the experiment and the operating band is switchable between X band and Ku band.Fifth,based on the theoretical model of the system architecture and researches on the key technologies,we designed and developed a principle prototype of the photonic time-stretched coherent radar system and verified the performance.First,the key paramters of the device in the prototype are tested,then a small drone is selected as the detection target and the detection results are discussed and analyzed to prove the high resolution characteristic of the system.The pulse repetition rate of the linear frequency modulated signal generated in the prototype is 1 k Hz and the time bandwidth product reaches 400(pulse width 100 ns and bandwidth 4 GHz),leading to an improvement of 26 d B on the peak power of the output results after matched filtering.The radar system can work at either X band or Ku band and the achieved range resolution is 4 cm for the drone(typical reflection cross-sectional area is 0.01 m~2).The experimental results also indicate that the small drone has better reflection characteristics at X band than at Ku band.