Research On Key Technologies Of Microwave Photonic Radar Signal Generation And Processing

Future radar systems will gradually develop in the direction of multi-function,multifrequency band,wide bandwidth and miniaturization.The traditional electronic radar systems are limited by the working bandwidth and frequency tuning range of electronic components,so it is difficult to meet the development demands of the future radar systems.Microwave photonics technology is a potentially transformative technology in the field of information technology,which has the advantages of large bandwidth,wide frequency tuning range,low loss,small size,light weight and anti-electromagnetic interference.Using microwave photonic technology can effectively overcome the bottleneck of electronic technology for future radar systems,improve the technical performance of radar systems,and promote the development of radar to high frequency band,large bandwidth and multi-function.Therefore,it is of great significance to study the key technologies of microwave photonic radar systems.To satisfy the development demands of multi-function,high frequency band,wide bandwidth and miniaturization of future radar systems,I study the key technologies of microwave photonic radar systems in this paper,such as pulse compression radar signal generation and downconversion processing,so as to improve the detection ability of radar.The specific work completed in the paper are as follows:1.A reconfigurable phase-encoded signal generator is proposed,and theoretical analysis and experimental demonstration are carried out.The scheme mainly includes a laser,a modulator and a photodetector,which has a simple structure.This scheme can generate binary phase-encoded signals with a frequency equal to or twice LO frequency.Without using any filters,the system has high reconfigurability and large frequency tuning range.In the experiment,two methods are used to generate phase-encoded signals with the same frequency.The results showed that the waveform quality of the phase-encoded signals produced by the two methods is similar,and the electrical spectra of the generated signals are relatively pure.After pulse compression,the pulse compression ratio can reach 123,which is close to the ideal value of 127.The peak sidelobe ratio can reach 9.3 d B,which is of practical significance for radar detection of weak pulses.2.A photonic approach to generating dual-band dual-chirped linearly frequencymodulated(LFM)signal with flexible frequency selectivity and good reconfigurability is proposed based on a dual-polarization quadrature phase-shift keying(DP-QPSK)modulator.This LFM signal generator can not only generate dual-band dual-chirped LFM waveforms,but also when changing the phase shift introduced by the PC,it can also independently select any frequency band signal from the generated dual-band dual-chirp LFM signals without using any filters.In addition,when changing the center frequency of the low-frequency LFM signal input to the modulator,this method can also generate multi-band single-chirped LFM waveforms.The system has good reconfigurability.This scheme adopts all-optical operation,so the system has a large working bandwidth and frequency tuning range.Experimental results demonstrate that the scheme has flexible frequency selectivity and good reconfigurability.The generated dual-chirped LFM signals have good PSR and PCR values,which is suitable for distributed radar and multi-function radar systems.3.A multi-functional down-conversion processing scheme based on polarization control is proposed.This scheme uses a dual-polarization Mach-Zehnder modulator(Dpol-MZM),in which two RF ports in one sub-modulator are driven by two RF signals representing two different functions.The LO signal drives the other sub-modulator to produce ±1 order optical sidebands.The polarization states of the modulated signals from the two sub-modulators are orthogonal to each other.After polarization combination,the linearly polarized light wave enters the balanced detector for photoelectric conversion.The down-conversion signal of different RF signals can be obtained.Then,by adjusting the phase difference between the two orthogonal polarization states introduced by the PC,the down-converted signal of any function can be selected independently and flexibly,and they will not interfere with each other.The system shows flexible frequency selection characteristics.And the system has large frequency tuning range due to no use of filters.In addition,the bias voltages of the modulator can be automatically controlled by a commercial bias controller,so the system has high stability.Experimental results show that for a single tone signal,the channel rejection ratio of the two functions is larger than 36 d B,and for wideband signals,the channel rejection ratio is larger than 25 d B.This multifunctional downconversion system has potential application value in the integrated radar system.4.A microwave photonic sub-harmonic down-converter which can simultaneously suppress image interference and anti-chromatic dispersion transmission is proposed based on a DP-QPSK modulator.The RF signal and LO signal are respectively modulated in two arms of the DP-QPSK modulator by carrier-suppressed single sideband(CS-SSB)modulation mode and sub-harmonic single sideband(SH-SSB)modulation mode.The signals based on CS-SSB and SH-SSB modulation are not affected by optical fiber dispersion,so they are suitable for fiber remoting scenarios.The frequency of LO signal is doubled,which reduces the requirement of LO sources.The scheme uses microwave photonic phase-shifting technology to construct the Hartley structure,so as to suppress the image signal.Experimental results show that the image rejection ratio of this down-converter can reach 50 d B for single-tone signals and 33 d B for broadband signals.The proposed microwave photonic sub-harmonic downconverter has potential applications in such as radio over fiber(Ro F)and remote radar systems.5.A linear optimization scheme for down-conversion links is proposed,which can simultaneously compensate the chromatic dispersion-induced power fading.The amplitudefrequency response curve of the signal shows a trend of periodic fading due to fiber dispersion.Based on this feature,the proposed approach takes advantage of the fact that IF and IMD3 signals have different power change trends affected by fiber dispersion.By introducing additional phase shift to the optical sidebands,the power of IMD3 term falls at the maximum power fading point of the transmission response curve,while the power of the IF signal falls near the minimum power fading point after link optimization.Thus,the approach achieves the purpose of suppressing IMD3 while compensating the dispersion-induced power fading.Experimental results show that,for a 25-km fiber transmission links,SFDR can be improved more than 15.5 d B.The scheme does not need bias control and the system is robust.In addition,the proposed approach does not need digital processing,which improves the processing bandwidths.No filters and frequency-dependent electrical or optical devices are used,the system gets a large frequency tuning range.Benefiting from the mentioned advantages,the approach has potential application value in antenna remoting receiver systems.