Research On The RF Photonic System Based On Microwave Network Theory

With the development of modern information science and technology,the application of microwave technique has been widely used in daily life,particularly in military and national defense.In the meantime,there is an increasing demand for higher operating frequency and bandwidth,which can be achieved by introducing photonic technology into traditional electronic system.Therefore,the radio frequency(RF)photonic technology emerges,which combines the microwave engineering and optical fiber technology.It absorbs the advantages of both the microwave and optical technologies in order to generate,transmit,and operate the high frequency and broadband microwave signal on optical domain.Compared with the conventional microwave module,the RF photonic system has peculiar advantages including smaller size,lighter weight,broader band,the immunity to electromagnetic interference,and reconfigurability.Since the 90s of the last century,RF photonic technology has gained great research attention worldwide.Many developed countries in the west spend heavily on relevant devices,equipment,and systems to support such a high technology field with growing potential.Based on the theory of microwave network,the thesis focuses on the technologic problems in performance index and realization of functions of RF photonic module and aims at the application requirements of RF photonic technology in the high frequency and broadband microwave system.The work is carried out from three levels,that is,the pre-microwave module,RF photonic module and post-microwave module.The RF photonic module comprises RF photonic link and signal processing.Firstly,the RF photonic system is analyzed from the perspective of microwave network.Then,compared with the traditional two ports microwave amplifier,the RF photonic link is linearized on optical and electrical domains respectively based on the microwave amplifier linearization techniques.At last,the forward transfer characteristic S parameter of RF photonic link is used to measure the instantaneous microwave frequency.The main innovations are as follows:(1)The model of RF photonic systemThe present study establishes a mathematical model for RF photonic system from the theoretical perspective of microwave network,which can be divided into two ports network and multiple ports network.The RF photonic system can also be regarded as the cascade microwave network,including the pre-microwave module,RF photonic module and post-microwave module.Our model can help to analyze the cascade module affecting the degree of system performance,such as gain,noise figure,and nonlinear distortion.(2)RF photonic link modelThe classical RF photonic link is similar to traditional microwave amplifier,which can be regarded as two-ports microwave network or cascade network comprised by E/O unit,O/O unit,and O/E unit.The modeling analysis of system performance can be done based on envelope parameter/performance of each module.In this paper,we build a traditional opto-electrical model of link performance index according to the physical parameter of each module,and then investigate how does the optical output power and relative intensity noise of the laser,the half-wave voltage and bias angle of the external modulator affect the link performance.Based on the two types of modeling,we propose the approaches for designing and optimizing the RF photonic link,including gain enhancement,noise reduction and non-linearization suppression.The non-linearization suppression methods are similar to the traditional microwave amplifier linearization technology.(3)Link linearization based on nonlinear opto-electrical device on optical domainTaking advantage of photonic technology(high bandwidth)and the LINC technology for the microwave amplifier,we discuss the RF photonic link linearization based on nonlinear opto-electrical device,and mainly focus on the methods using parallel intensity modulators.Then the concept is proved by the scheme which combines the dual parallel intensity modulator and the balanced detection together.The two-tones test shows that the third-order distortion can be suppressed about 23dBc and the dynamic range is about 115dB/Hz2/3 at 2 GHz.Besides,the nonlinear can be suppressed about 34dBc at 16GHz,which means the scheme meets the broadband linearization requirement.(4)Link linearization based on RF pre-distorter on electrical domainBased on the system design method of microwave cascade network,we propose an electrical pre-distorter according to differential amplified circuit,which can be used for the linear electro-optical modulation module to improve the link performance.Due to the low noise and amplification function of BJT transistor,it can not only solve the problem of insertion loss from the traditional pre-distorter by additional gain,but can also improve the noise figure of RF photonic link.We simulate and design the differential pre-distorter by the ADS software in this study and manufacture the circuitusing the subtractive etching process.With the assistance of differential pre-distorter,the RF photonic link power gain increases by 12dB and the noise figure reduces by 10dB at 2.5 GHz.Moreover,the SFDR of the link can be improved by 13dB.(5)RF photonic instantaneous frequency measurement systemThis study also discusses RF photonic instantaneous frequency measurement using the forward transfer characteristic S parameter of RF photonic links.RF photonic instantaneous frequency measurement system can be divided into two types,microwave power mapping and optical power mapping.We propose the real-time and tunable schemes based on incoherent RF photonic filter and MZ interferometer for the microwave power mapping,while the optical power mapping schemes are based on dual outputs MZ interferometer and Sagnac filter.The measurement frequency range can achieve more than 5GHz and even 12 GHz.The schemes based on incoherent RF photonic filter and MZ interferometer effectively avoid the requirements for precise control of light polarization.In addition,the schemes based on dual outputs MZ interferometer and Sagnac filter simplifies the frequency measurement system by using single light source and complementary optical filter,which can also help to reduce the measurement error sources.