Research On Frequency Conversion Of Broad-Band Signals Based On Microwave Photonics

Frequency conversion technology is the core module in microwave communication.The indicators of traditional electrical mixers at high frequencies have reached the limits.In recent years,microwave photonics has attracted much attention due to its advantages of low loss,anti-electromagnetic interference and large bandwidth.The idea based on microwave photonics to achieve frequency conversion has become one of the most promising technologies to break through the "electronic bottleneck" of traditional frequency conversion.So it is of important scientific significance and practical value to make an in-depth research on microwave photonic conversion.In this thesis,the frequency conversion technology based on microwave photonics is deeply studied.Firstly,the background of microwave photonics and the advantages of microwave photonic conversion technology compared with traditional electronical frequency conversion are introduced.Next,the research status and application prospects of microwave photonic conversion are analyzed.Then,a study of the core devices and the parameters of system performance is carried out.Finally,we compare three kinds of microwave photonic frequency conversion structures commonly used in the recent years and summarize the advantages and disadvantages of above schemes.Based on the aforementioned analysis,a novel up-conversion scheme using optoelectronic oscillator(OEO)and optical coherent QPSK modulation is proposed.The double-sideband modulated optical signal is provided by the photoelectric oscillator,where one first-order sideband is modulated by the baseband digital signal using dual-parallel Mach-Zehnder modulator.The up-conversion scheme eliminates the need for external local oscillation(LO)signals injection and get the frequency-doubled wideband signals due to optical heterodyne mixing.At the same time,this scheme avoids the step of pre-modulating the baseband signal to the low frequency carrier,which can effectively improve the bandwidth of' digital signals.In the experiment,the QPSK baseband signal is directly up-converted to K-band,and the vector error magnitude of the wideband signal is less than 7%.In addition,a kind of down-conversion structure based on two cascaded Mach-Zehnder modulators is studied.On the one hand,the frequency of the local oscillator signal can be reduced by half.On the other hand,since the carrier suppression modulation reduces the DC component of the center carrier,the efficiency of conversion can be effectively improved.Through simulation on Optisystem platform and experiments,the effects of the LO signal and the radio frequency(RF)signal on system gain and spurious free dynamic range(SFDR)are further analyzed.In the experiment,the error vector magnitude of the signal after down-conversion and 17.7 km fiber transmission is less than 10%.