High Tunable Microwave Photonic Filter Base On Multiple Lasers

Recently,microwave photonic filter(MPF)which processes microwave singal with optical method has become a hot research topic.The use of microwave photonic filter can overcome the bottlenecks in the processing of high frequency microwave in electrical appliances with its advantages of low-loss and insensitivity to electromagnetic interference.Today,MPF is widely used in radio-over-fiber(RoF)system and plays an important role in it.MPF's tunability,as one of the most important parameters of MPS,has great research significance.The increasing of tunability of MPF will improve RoF system's flexibility and scope of application.At present,microwave photonic filters are mainly divided into time-delay and non-delayed microwave photonic filters.The time-delay-based microwave photonic filter includes the finite impulse response and infinit impulse response filter.The frequency response of these filters is periodic,and it needs kilometers optical fiber to realize the delay.The non-delayed microwave photonic filter is mainly based on phase modulation to intensity modulation conversion.There are many ways to achieve this conversion,such as,stimulated Brillouin scattering,semiconductor optical amplifier,and phase-shifted fiber Bragg gratings.The method based on phase-shifted fiber Bragg grating is relatively simple and feasible.However,the low tunability of the passband is the main limitation in practical applications.In addition,dual-passband filters and band-notch filters are sometimes desired.In this paper,a way to improve the tunability is proposedand demonstrated.The main contents of this article are:1.A tunable band-pass MPF implemented by two tunable laser sources and dual-phase-shifted fiber Bragg grating(DPS-FBG)is proposed.In this MPF,the two laser sources are modulated by a phase modulator.One of the sideband of each optical modulated signal is suppressed by each notch of the DPS-FBG,and the phase-modulation to intensity-modulation conversion is performed.By tuning the wavelengths of the two tunable laser sources,the switchability between the single-passband filter and the dual-passband filter can be achieved.Moreover,the central frequency and the 3-dB bandwidth of the single-passband filter can be tuned,and the frequencies of the dual-passband filter can be simultaneously tuned or one of them can be tuned.In the experiment,the 3-dB bandwidth of MPF can be tuned from 180 MHz to 319 MHz,and the frequency tuning range from 1 to 7GHz is obtained.2.A tunable band-stop MPF implemented by adding another laser sources acting as all-pass path is proposed.The all-pass filter is achieved by using an apodized fiber Bragg grating(FBG)and a new laser source.Since the allpass filter and the bandpass filter are out of phase,when they are combined,a band-stop MPF is implemented.Through theoretical calculation,a band-stop filter with a 3-dB bandwidth increased from 178 MHz to 281 MHz,and a frequency tuning range from 5 to 8GHz is obtained.