Investigation On Microwave Photonic Filters

Microwave photonic filters have attracted significant interest over the past few years due to their ability of overcoming the so-called electronic bottleneck and other sources of degradation as electromagnetic interference and frequency dependent losses, and providing in-built signal conditioning that can be integrated with the fiber-optic system. Under the support of National 973 Project, the key technologies of microwave photonic filters have been researched, including the implementations of high-Q value, negative coefficients and high-order filters. The main contents are as follows:(1) The fundamental concepts, applications, and limitations of microwave photonic filters have been introduced, and the implementations of high-Q value, negative coefficients and high-order filters have been summarized.(2) A structure of one-order infinite impulse response (IIR) microwave photonic filter using semiconductor optical amplifier (SOA) and a tunable narrow-band filter has been proposed, which can implement high-Q value and negative coefficients in different conditions.(3) A high-Q value microwave photonic filter can be implemented, when the center wavelength of the tunable filter is the same as that of the laser. Based on the theoretical analysis and simulated results, the factors which influence the Q value of the filter have been researched with the noise considered or not respectively.(4) When the center wavelength of the tunable filter detunes a little to the center wavelength of the laser, a negative coefficients microwave photonic filter could be implemented by employing the cross-gain modulation (XGM) of amplified spontaneous emission (ASE) spectrum in SOA, under the condition of the matched power. The characteristic of the filter is related to the matched power.(5) A high-order microwave photonic filter may be implemented, if a fiber Bragg grating (FBG) with the half reflection and the half transmission is used in the active loop, the high-order filters could improve the Q value and notch rejection of the filter.(6) Experiments have been carried out to demonstrate the concept of the proposed filter. The results show that a microwave photonic filter whose Q value is around 100 has been implemented. Also, a negative coefficients filter has been implemented when the center wavelength of the tunable filter detunes 0.2nm to the center wavelength of the laser, whose notch rejection is around 18dB.