Research On Rf Arbitrary Waveform Generation Technology Based On Microwave Photonics

Optical Arbitrary Waveform Generation(OAWG)technology generally refers to the technique that takes advantage of photonics to generate RF waveforms with specific shapes and specific needs.Compared with electrical arbitrary waveform generation methods,OAWG methods have the advantages of large bandwidth,high accuracy,high refresh rate,simple structure and large time scale,and have wide application prospects in many fields such as radar systems,high-speed optical communication,medical imaging,and radio measurement.Capable of all-weather,full-time,long-range target detection and identification,radar is considered an important sensor for the future intelligent society,and the waveform it employs largely determines system performance.In the past decades,efforts have been made to improve the functionality,accuracy and response time of radar,in which the generation,control and high-speed processing of broadband radar signals are the keys to achieve technological breakthroughs.Microwave photonic radar realized by introducing microwave photonic technology with the characteristics of wide frequency band,flat response,low-loss transmission,multi-dimensional multiplexing,ultra-fast signal processing and antielectromagnetic interference into radar systems has performance advantages of large instantaneous bandwidth,reconfigurability,low phase noise,arbitrary frequency carrier frequency,high-resolution imaging,etc.It can achieve performance in resolution,coverage and speed that is difficult to achieve by traditional electronic radar.In addition,the multiband radar based on microwave photonic technology can achieve or quickly switch multiple functions such as detection,tracking and imaging in the same radar,making multiple operation modes possible.In this paper,the key technology of RF waveform generation for microwave photonic multi-band radar is in-depth researched,and various waveform generation schemes that can be applied to microwave photonic multi-band radar are proposed,and the feasibility of the proposed schemes is verified by simulation system and experimental platform.The innovative work of the paper is carried out in two aspects as follows:1.In order to expand the multi-band radar bands and generate energy-balanced multiband radar signals so as to effectively enhance the radar’s operating range,the generation technology of high flatness and large bandwidth optical frequency comb(OFC)has been studied in depth.The main work and innovations are as follows:1)A high-flatness,large-bandwidth OFC generation scheme based on a dualpolarization Mach-Zehnder modulator(DPol-MZM)is proposed.Using the structural configuration of an embedded DPol-MZM cascaded the third MZM to generate the theoretically optimal flatness OFC by manipulating the polarization state of the lightwave and selecting the appropriate modulation depth and bias point of the push-pull mode MZMs.The scheme does not use additional optical filters,providing free frequency range and flexible tunability.The simulation achieves flat multi-carrier generation up to 66-lines with a flatness of 2.85dB,a bandwidth of 455GHz,and a side mode suppression ratio(SMSR)of 10.13dB.Changing the radio frequency(RF)modulation mode of the two sub-MZMs in the DPol-MZM and using orthogonally polarized optical tones without cross beating during photodetection,can make the power ratio of harmonics in the output photocurrent to be matches the coefficient ratio of the ideal waveform Fourier series expansion,and frequencydoubled triangular and square waveforms with full duty cycles of 5,10 and 20GHz repetition rates were also successfully obtained in the simulation using the optimized modulation indices.2)An ultra-wide spectrum OFC generation scheme based on electro-optic modulation(EOM)optical frequency shifting loop(OFSL)is proposed.The dual-parallel Mach-Zehnder modulator(DP-MZM)is used as the frequency shifter in the loop,the erbium-doped fiber amplifier(EDFA)cascaded with it performs cyclic frequency shifting and power compensation,so that the gain can just compensate the loss in the loop,and thus a large bandwidth OFC with good flatness is obtained in the loop output.By properly configuring the DP-MZM and adjusting the RF drive power,the generation of high-order sidebands during a single frequency shift in the open-loop can be effectively suppressed,thus reducing the impact of higher-order crosstalk components on the closed-loop output OFC spectrum.Compared with the acousto-optic modulation(AOM)frequency shifter,the EOM frequency shifter can provide higher modulation frequency,larger bandwidth and more flexible tunability,which can further improve the performance of OFSL.In the experiment,an OFC with a flatness of less than 4dB,a line spacing of 12GHz and a bandwidth up to 12.6nm(1575GHz)was generated,and the output spectrum will be further broadened with the increase of RF frequency.2.In order to improve the resolution of the radar system,the frequency modulation signal with a large time-bandwidth product(TB WP)can be generated based on microwave photonic technology,so that the radar system in the premise of ensuring the detection range both good range resolution.The generation techniques of multi-band linear dual-chirp signals and nonlinear dual-chirp signals have been studied in depth.The main work and innovative achievements are as follows:1)A multi-band multi-bandwidth dual-chirp waveform photonic generation scheme is proposed,and the parameters are further optimized to propose an improved scheme.In the scheme,the OFC generated by the first MZM are divided into two beams by an optical coupler.One of the beams is modulated by the baseband single-chirp signal through the second MZM biased at different bias points,and the modulated lightwave and the other unmodulated lightwave are heterodyne beating through a balance photodiode(BPD)to generate dual-chirp waveforms with center frequency multiplication and bandwidth multiplication without high frequency clutter components.Five-band quadruple-bandwidth dual-chirp waveforms covering the C-,X-,Ku-,K-,and Ka-bands were generated by simulation,and the quadruple-bandwidth and multi-bandwidth dual-chirp waveforms were generated simultaneously in three different central frequency bands.The generated waveforms have high TBWP and all show good pulse compression and range-Doppler resolution.2)A photonics scheme for generating multi-band dual-chirp waveforms with linear or nonlinear chirp rates based on phase modulation is proposed.The cascaded polarization modulator(PolM)is used to generate a flat OFC as the multi-frequency reference light,while a phase modulator(PM)is driven by a power-function-type baseband signal to obtain a phase-chirp modulated lightwave with different orders.The multi-frequency reference lightwave and the phase-chirped lightwave are orthogonally coupled by a polarization beam combiner(PBC),and then recombined into two in-phase and anti-phase branches by a polarization beam splitter(PBS).After balanced detection,dual-chirp signals of different orders covering six frequency bands are obtained,and the interference clutter are effectively suppressed.The simulation generates six-band linear dual-chirp signals with center frequencies of 10,30,50,70,90,and 110GHz,respectively,each with a bandwidth of 4.1 GHz,the pulse compression ratio(PCR)of 465.45,and the peak-to-sidelobe ratio(PSLR)of 12.8dB.A six-band nonlinear dual-chirp signal with an increased bandwidth of 12.8 GHz and a PSLR boost of 20.1 dB was generated at the same center frequency.The simulation results show that the generated nonlinear chirp signals have better sidelobe suppression ability and improve with increasing nonlinearity.