Research On Tunable Microwave Photonic Filters

Microwave photonic filters (MPFs) own incomparable advantage over traditional filters in terms of bandwidth and flexibility, thus hold great potentials in applications such as wireless communication networks, sensing, and radars. MPFs are used to perform the same tasks as traditional filters, and the difference is that the electrical signal is first modulated to an optical carrier, and then filtered in the optical domain; finally, the signal with filtering functions is converted back to the electrical domain. By doing so, we explore an excellent nature of the optical fiber that its response for different RF frequency is "flat", which means that it is easier to realize wideband frequency tuning and passband reconfiguration in the MPFs. Therefore, research on the theories and key technologies of novel MPFs has significant importance.The dissertation is mainly focused on novel MPF techniques. At first, the background and operation principle of MPF are introduced. Then, we propose three different frequency tunable and spectrum reconfigurable MPFs, their operaton principle are proposed and experimentally verified. Finally, based on previous studys on the tunable MPF, a widely tunable optoelectronic oscillator (OEO) based on a composit MPF is proposed. The major innovations and contributions are as follows:1. A positive coefficient MPF based on cascaded fiber-based Mach-Zehnder interferometers (MZIs) is proposed. By adjusting the variable delay line (VDL) and variable optical coupler (OC), the center frequency,3-dB bandwidth, side lobe suppression and other key parameters can be changed, which shows that it is very flexibile in the application.2. To solve the problem of filter passband performance degradation in positive coefficient MPFs while frequency tuning, a complex coefficient MPF based on DPMZM is proposed. It is based on addition of carrier suppressed single sideband (CS-SSB) signal and phase-controllable optical carrier, which forms a phase shifter. By incorperating the phase shifer to a traditional multi-tap delay line filter, complex-valued frequency tuning is achieved. The proposed structure features wideband RF response and tuning range.3. To solve the periodic spectrum problem of traditional MPFs, a single pass, composite MPF induced by spectral subtraction technique of two high-pass filters is proposed. Thanks to the special nature of the phase modulation signal, a composite band-pass MPF is constructed by the subtraction between two high-pass filters, which are formed by two lasers of different wavelengths passing through a phase modulator (PM) and tunable optical filter (TOF) seperately. Theoritically, the passband of the proposed MPF can be tuned continuously up to dozens of GHz, or even hundreds of GHz.4. By an in-depth study on the tunable MPF, a tunable OEO based on a composite MPF induced by spectral subtraction technique is proposed. The previously proposed single pass, composite MPF is used to choose the oscillation mode of the OEO. And when the OEO loop is closed, a single oscillation mode is choosed by the MPF and thus enables single-mode oscillation of a tunable OEO.