| Microwave photonics technology,which can realize microwave signal generation,transmission,processing,control,frequency conversion and other functions,is regarded as an indispensable and essential module in the fields of next generation wireless communications,radar detection,satellite communications,deep space exploration,satellite payload,electronic warfare and so on.In addition,it can be widely used in high frequency microwave signal generation,optical fiber dispersion compensation,image rejection,Doppler frequency shift(DFS)measurement,and other technologies.With the increase of business volume of electronic systems,much higher information transmission rate is required,and it is inevitable for modern electronic systems to realize the technique development toward higher performance,integration and multi-band signal processing.However,the traditional microwave links are faced with serious electronic bottlenecks,such as the limited spectrum resources in low frequency band,low bandwidth,high transmission loss,poor signal isolation and weak electromagnetic interference ability,which makes it difficult to meet the development of future communication systems.To this end,the microwave photonics link is proposed,where microwave signal is modulated onto optical wave,and then the microwave signal processing could be realized in the optical domain.Due to the advantages of the microwave photonics link,such as large bandwidth,tunable in broadband,small loss and strong electromagnetic interference ability,it could provide good application prospects for the development of electronic field in the future.In this dissertation,the key technologies of high gain,high linearity and high performance are studied in view of the technical difficulties in microwave photonics link.(1)In order to solve the problems of high frequency microwave signal generation in microwave photonics link and period power fading in antenna remote systems,an upconversion approach based on Sagnac loop and optical phase shifter(OPS)is proposed.In the approach,photonic microwave mixing technology and fiber transmission link are combined to realize long-distance transmission of high-frequency microwave signals,and achieve the period power fading compensation of the signal.The dispersion compensation performance of the scheme is verified by simulating the frequency response curves under different phase shifts and different fiber lengths.The simulation results show that by adjusting the phase shift angle of OPS,the power compensation of the up-converted microwave signal can be realized.When the phase shift angle is set to 90°,the signal power at the notch points can be fully compensated,and the compensation ranges are about 40 dB.The frequency spectra before and after dispersion compensation verify the up-conversion function of the scheme.Among them,the input local oscillator(LO)signal frequencies are about half of the output radio frequency(RF)signal frequencies,which reduces the bandwidth requirement of LO signal and phase modulator(PM).In addition,the spurious free dynamic range(SFDR)of the scheme can be increased by about 13.54 dB after dispersion compensation.Subsequently,the link stability is analyzed and explained.The results show that when the phase jitter changes from-30°to 30°,the output signal amplitude fluctuates only 1.26 dB;when the power distribution ratio changes from 0.3 to 0.7,the output The signal power fluctuation is only 0.77 dB,which shows that a certain degree of phase jitter or slight power distribution imbalance has little effect on the link performance,which verifies the stability of the system.(2)In order to solve the problem of image interference in microwave photonic frequency conversion link,an image rejection approach based on a Sagnac loop is proposed.Meanwhile,the theoretical analysis and simulation verification of the approach are carried out.In the simulation,by observing the time-domain waveforms of the output electrical signals from the balanced photodetectors(BPDs),the phase shift function of the approach can be clearly illustrated.Besides,by connecting the spectrum analyzer and oscilloscope after an electric low-frequency 90°hybrid coupler(90°HC),the spectrums and waveforms of the output intermediate frequency(IF)signal are obtained,and then the image rejection ability can be well explained by observing these figures.The results show that the power of the image IF signal is lower than that of the desired IF signal about 57 dB,that is,the image rejection ratio is 57 dB.Afterwards,compared with the image rejection mixing approach based on parallel PMs,conversion efficiency is improved by about 6 dB.And by obtaining the image rejection ratios at different RF frequencies or different IF frequencies,it can be seen that when the RF signal frequencies change at 10 GHz?40 GHz,the IF signal image rejection ratios are greater than 45 dB;when the output IF signal frequencies change in the range of 0.6 GHz?1.2 GHz,the image rejection ratios are greater than 50 dB.Therefore,the broadband processing ability of the proposed approach can be verified.(3)Aiming at the problem of DFS measurement in radar and other systems,a DFS signal discrimination approach based on parallel PMs is proposed.In this approach,the direction of DFS can be distinguished by comparing the phase difference between the DFS signals from two photodetectors(PD).Meanwhile,the direction of DFS can also be distinguished by observing the output spectrums of 90°HC.(4)In order to further improve the spectrum utilization,a DFS discrimination approach is proposed by using a Sagnac loop and a bidirectional PM.This approach not only can distinguish the value and direction of DFS signal simultaneously,but also has some other advantages.For example,the impact of phase imbalance in parallel link can be reduced effectively by using Sagnac loop,and the spectrum utilization could also be improved through the full utilization of upper and lower sidebands.Meanwhile,using BPD can increase the output power of fundamental frequency signal,and suppress the direct current terms,second-order intermodulation(IMD2)terms and common mode noise.Consequently,the measurement accuracy will be improved.The simulation results show that the power of IMD2 terms is suppressed by 63.98 dB,and the direct current offset is suppressed from 7.99 mV to-0.09 mV.Meanwhile,the discrimination of DFS signal value and direction at ±1 MHz is simulated and analyzed when the transmitted signal is set to 16 GHz. |
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