Research On Tunable Bandpass Microwave Photonic Filter Based On SBS

As a cross disciplinary studying the interaction between microwave and light wave,Microwave photonics fuses the superiority of microwave and photonics and has made remarkable progress in recent years.The function of microwave photonic filter is to suppress the noise and filter the clutter signal and make the microwave signal with desired frequency range pass only.It processes the signals of traditional microwave field by the methods of photonics,finally gains a new photoelectric composite system with large bandwidth,low transmission loss,fast processing speed,anti-electromagnetic interference and other advantages.The advantage of MPF is that it can break the electronic bottleneck,and make up for the deficiencies of the electrical filter,with large bandwidth,low loss,small volume,anti electromagnetic interference,tunable and reconfigurable and excellent characteristics,which attracted much attention and has been widely used in millimeter wave communication,radar,high performance optical wireless communication system and wireless local area network.In order to improve the tuning range of filters,a variable bandpass microwave photonic filter is proposed in this paper.The scheme is based on stimulated Brillouin scattering,combining two modulators and a fiber grating to generate two pump signal.By adjusting the frequency interval between the two pump signals,a dual passband filter or a four passband filter with a variable passband can be obtained respectively.Under the condition that the difference of the two pump frequency is constant,the continuous tuning of the central frequency of the filter can be realized by adjusting the frequency of the pump signal.Simulation of influence of the difference of the two pump signal frequency on the passband number and interval and the center frequency of the filter is given,and the relationship between bandwidth and pump power and SBS gain medium length is discussed.In order to make the frequency response of filter remains stable during the process of tuning,a phase shifter with 360 degree phase shift is proposed.On the basis of this structure,a six-tap complex coefficient microwave photonic filter is introduced.The required phase shift can be obtained by adjusting the two signal generators on each optical channel,so as to realize the tunability of the microwave photonic filter.The filter can be continuously tunable in the whole free spectral range while the phase shift is varied from negative 180 degree to 180 degree.Since the phase shift is independent of the RF signal,the frequency response of the filter remains constant throughout the tuning process.