Theoretical And Experimental Researches On High-Q Microwave Photonic Filters

Compared with conventional microwave filters based on electrical components, microwave photonic filters have the advantages of high bandwidth, immunities to electromagnetic interference (EMI), frequency dependent losses, flexibility in design and inherent compatibilities with optical fiber microwave systems. A common objective is to increase the Q factor of the filters. Higher the Q factor, sharper the response peaks and better the selectivity. Supported by the National Basic Research Program of China (Grant No.2006CB302805) and the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-04-0715), we have done some theoretical and experimental researches on tunable high-Q microwave photonic filters, and some original results have been demonstrated:(1) Operation principle for microwave photonic filters with high Q factor is analyzed. Compared with finite impulse response (FIR) filters, infinite impulse response (IIR) filters are preferred for obtaining high Q factor. Microwave photonic IIR filters are in recursive structure. With the optical carrier traveling through the feedback loop back and forth, the microwave signal oscillates in it. The structure is amount to a resonant chamber for the microwave signal. The simulated results show improving Q factor requires:the loss of the signal in the resonant chamber should be as small as possible; the number of the received delayed signals on the detector should be as larger as possible.(2) A microwave photonic IIR filter is demonstrated using an SOA-based delay line loop. The influences of the pump current, input power of the SO A and the bandwidth of the optical filter on the Q factor are measured and calculated. It shows that the Q factor of the filter is limited by the amplified spontaneity emission (ASE) noises of the active components. By the accumulating of the ASE noises, which lay over the useful signals, the Signal-to-Noise of the microwave is reduced, resulting in the received number of the signal on the detector being decreased.(3) An improved setup is presented. By using cross-gain modulation (XGM) in SOA, the microwave signal is inversed copied onto the ASE. One frequency part of the ASE is extracted out as a converted signal, which oscillates in the loop chamber, yielding an IIR filter. This structure utilizes the commonly harmful ASE to approve the Q factor of a single IIR filter greatly. The max Q factor we measured is around 500, higher than those reported contemporaneously (around 300).(4) A cascade connection of the optical processing parts of two IIR filters is expected to increase the Q factor further. By adopting "vernier effect" technique in such a structure, the response peaks of the two IIR filters are set stagger. Then the FSR and the Q factor of the whole filter can be increased significantly. While, optical interferences between light beams traveling different paths will happen in those cascaded filters, if none especial technology is adopted, which should break the stability of this structure and the linearity of the electrical transfer function. In this thesis, by using XGM in SOA, two structures effectively removing optical interferences have been presented to cascaded IIR filters. In one structure two EDFA-based delay line loops are cascaded, and an SOA is inserted between them. The simulated and measured results prove that the whole filter demonstrates a cascade response of two IIR filters. In another structure, two SOA-based delay line loops are cascaded, and an erbium-doped fiber amplifier (EDFA) is inserted between them. The simulated and measured results show that the whole filter demonstrates a response given by a weak all-pass response subtracted from a cascade response of two IIR filters. Analysis show:larger the loop gains of the constituent single-loop filters, higher the Q factor and larger the rejection ratio of the whole cascaded filter; closer the lengths of two loops, higher the Q factor but smaller the rejection ratio of the whole cascaded filter; the weak all-pass response can reduce the welter of the stopband to a certain extent. The max Q factor we measured is 1870 with the rejection ration of 25dB. It meets the demand of the research and is higher than those reported contemporaneously (the max Q factor of the all-optical microwave filters is around 1000 with the rejection ration of 25dB, untunable). Both structures are designed tunable by setting an optical variable delay line (OVDL) in one of the delay loops. The experiment and analysis results show:in order to achieve semi-continuous tunability or to decrease the tuning step, the lengths of the two loops should be close.