Research On Improving The Performance Of Weak Microwave Signal Detection Based On Microwave Photonics

With the demand of microwave radar signal for larger bandwidth,radar signal is exploring to higher frequency.Microwave photonic radar with the advantages of broadband,array and miniaturization,has become a research hotspot.High-frequency microwave signals are prone to loss during atmospheric transmission,especially in a humid environment.This will make the signal power and signal-to-noise ratio(SNR)to be weak during radar detection.Therefore,the processing of high frequency weak microwave signal in the receiver of microwave photonic radar is one of the important research directions.However,the traditional electro-optic modulator has the requirements for the large SNR and driving power of the modulated signal,the weak signal cannot meet the requirements of the modulation depth,that the weak echo signal cannot be utilized directly,which limits the development of the microwave photonic radar.In this dissertation,a weak microwave signal detection system with high electro-optic conversion efficiency based on microring resonator is proposed.We first designed a receiver for the weak microwave signal,and researched the electro-optical conversion between the input microwave signal and the up-converted optical signal.The results show that when the input microwave signal meets the weak signal condition,the up-converted optical signal has a linear relationship with the input microwave signal.By studying the sufficient-necessary conditions,that the interaction strength in the resonator is affected by the nonlinear coefficient and the electric field superposition factor,the optimized parameters have obtained.As the instantaneous bandwidth of the system is 19.34 MHz,the weak microwave echo signal can be detected below microwatt(-30 dBm).The power conversion efficiency in the microring resonator is 4.37×10⁴,and the photon up-conversion efficiency is 6.78×10^-2.The limitation of photon up-conversion efficiency and conversion bandwidth in the whispering gallery mode resonator is overcome.It can meet the detection demand of weak microwave signal under strong background noise.To reduce the influence of noise on the weak microwave signal detection system and improve the output performance of the optical signal in the weak microwave signal detection system,a microwave photonic tunable bandpass filter based on the graphene-assisted silicon waveguide(GSW)is proposed.The output SNR of the weak signal detection system is improved by this filter.In the research,the relationship between the effective refractive index of the GSW and the graphene chemical potential was simulated,and the adjustment mechanism of the GSW based on the electrical control model was established.The spectral response model of the filter is obtained,and the output performance of the filter is optimized.The designed microwave photonic filter has a 3 dB passband width of 3.74 GHz,a rectangle coefficient of 1.29 and an out-of-band suppression of more than 36 dB.Through the external voltage,the passband tuning performance above 70 GHz can be obtained.The filter can effectively filter out the pump light and stray light that carried in the output up-conversion signal.Based on the filter's noise suppression effect,the optical amplifier can be further added to the optical link to enhance the signal.The minimum detectable power of the optimized weak signal detection system is-93.34 dBm.Compared with direct detection input SNR,the output SNR of the received microwave signal is improved by 8.66 dB.To reduce the bandwidth limitation in the weak microwave signal detection system and improve the performance of the system,a wide band weak microwave signal detection system based on radius-coded microring resonators array is proposed.The structure of the microring resonators array with encoding radius is adapte to the weak stepped frequency pulse signal.Though the instantaneous bandwidth of the single resonator remains unchanged,the ability to receive broadband microwave signals is greatly improved in this system.The research gives the conversion relationship between the frequency-encoded pulse signal and the output optical signal.The relationship among the coupling rate of the array receiving system,the radius of the resonators and the center frequency of the received signal is analyzed.The minimum detection power spectral density of the system is-166.2 dBm/Hz,which can realize the weak microwave signal detection with the input microwave signal power spectral density below-96.0 dBm/Hz and the dynamic range is 70.2 dB.The simulation realizes the receiving bandwidth of the stepped frequency pulse signal of 193.41 MHz.The weak signal receiving system is not limited to a fixed-frequency-encoded pulse signal.By encoding the radius of the resonators array,it can be adapted to receive pulse signals under multiple frequency encoding.These results increase the minimum detectable power of the electro-optical conversion process and improve the output SNR of the system,which solve the problem of microwave photonic radar cannot directly detect and process weak microwave signals.It provides support for the development of microwave photonic radar in the detection of weak signals,and provides a reference for the detection and processing of weak signals in the microwave photonic communication field.