| Microwave filter is the core component of radio frequency system and link,which is used to separate the target signal and stray signal at the signal transmitter and receiver,and plays a vital role in reducing the impact of noise and clutter and improving the quality of signal transmission.The microwave photon filter carries the microwave signal through the optical carrier,realizes the selection,suppression or waveform processing of the microwave signal in the optical domain,and then gets the processed microwave signal in the electrical domain again by photoelectric conversion.This method effectively overcomes the problem of poor tuning of traditional electrical microwave filters due to structural reasons,and has the advantages of large tuning range,reconfiguration,low loss and anti-electromagnetic interference.According to the current research status of microwave photon filter,it is found that due to the ultra-high frequency characteristics of light wave itself and the limitation of filtering structure,microwave photon filter has two main technical difficulties: first,the filtering bandwidth is difficult to achieve ultra-narrow band;Second,the center frequency of the filter is difficult to achieve a wide range of tuning.In view of the above technical difficulties,the microwave photon filtering technology based on stimulated Brillouin fiber laser is studied in this paper.Firstly,accurate filtering is realized through the narrow line width Brillouin laser and target signal coherent amplification principle,breaking through the traditional electrical filtering mechanism of low resolution,can not be used in high frequency band,poor stability and other bottlenecks.Further,in order to reduce the threshold of stimulated Brillouin scattering(SBS),the Brillouin laser resonator was lengthened,and the secondary cavity was cascaded to form a double ring cavity structure.The vernier effect was used to suppress the comb tooth gain spectrum caused by the lengthened laser resonator.After the signal processing is completed in the optical domain,the signal to be filtered is demodulated by the vector network analyzer through photoelectric conversion.The line width of the Brillouin laser can be sub-k Hz,so as to realize the accurate identification of a large number of signals and improve the utilization of frequency band.In addition,a microwave source is used to apply a two-tone signal to SBS pump light through an intensity modulator,and a voltage source outputs a half-wave voltage at the same time to achieve carrier suppression.Finally,the double passband of the filter is realized.The dual laser structure is used to provide SBS pump light and signal optical carrier respectively.By simply adjusting the wavelength of pump light,the dual passband can be synchronously tuned.By changing the RF signal frequency imposed by the microwave source,the difference in central frequency of the double passband can be changed,and two tuning modes of filtering passband can be realized.It also has an out of band inhibition ratio of nearly 20 d B.The research work of this paper provides a useful reference for the passband bandwidth of the narrow-band filter,the increase of the out of band rejection ratio of the passband and the improvement of the filtering passband tuning mode. |
