Research On Frequency Stability And Regulation Technology In Heterodyne Photogenerated Microwave

With the continuous development of Internet technology,such as automatic driving,intelligent medical treatment,intelligent city and other application technologies that can change human life are becoming more and more mature,and the requirements for wireless communication speed are becoming higher and higher.The existing frequency band of wireless communication can not meet the requirements of information transmission rate.The application of microwave signal and millimeter wave in higher frequency band has become a new requirement of ultra wideband wireless communication.Microwave and millimeter wave signals in the electric field are getting close to the bottleneck gradually,and the transmission loss is very large,so it is difficult to transmit through wireless or long-distance cable.In recent years,radio over fiber(ROF)provides a good transmission medium for high-frequency microwave and millimeter wave signals with the help of low loss optical fiber transmission,which is expected to achieve higher rate wireless communication applications.Photogenerated microwave is one of the most important technologies in ROF system,in which optical heterodyne method has the advantages of simple structure,large tunable range and so on.However,the frequency of the free running laser is easy to drift,which leads to the instability of the microwave signal,so it is necessary to stabilize the frequency of the laser.In this paper,the problem of microwave signal instability caused by laser frequency drift in optical heterodyne method is studied.Two main tasks were carried out:1.A method of using 3×3 coupler to build interferometer to monitor the frequency drift of laser is proposed.The phase difference of 3×3 coupler's three ports is 120 °,so it has the advantages of large measurement range and high sensitivity,and it can detect the direction of phase drift.When the frequency of the laser drifts,the phase difference between the two arms of the interferometer changes,so the frequency drift of the laser can be detected by the principle of interference.Two 3×3 couplers are used to build an unbalanced Mach-Zehnder interferometer.Two lasers are detected by the same interferometer,avoiding the influence of the drift of the interferometer on the beat frequency signal measurement.The scheme is verified by simulation.The interferometer is built and the output coefficient of the coupler is calculated by ellipse fitting.The phase demodulation of the asymmetrical 3×3 coupler is realized.LabVIEW is used to simulate the system through PID feedback control,which realizes the phase stability and regulation of the system.2.The method of linear mapping high frequency optical signal to low frequency electrical signal by using the frequency shift characteristic of stimulated Brillouin scattering is proposed,so as to realize the function of wavelength meter,which can monitor the frequency change.The stimulated Brillouin frequency shift is directly proportional to the frequency of the pump light.When the laser frequency changes,the Brillouin frequency shift changes accordingly.Therefore,the frequency shift of the laser can be reflected by detecting the frequency of the electrical signal.In order to acquire and process the digital signal,a scheme is put forward to make the laser produce twice stimulated Brillouin scattering and two Stokes light beat frequency through two scattering media.The optical signal is mapped to about 180 MHz,and the function of the wavelength meter is realized.