A cascaded filter combining active and passive filters is proposed. The active filter acts as a low-coherence infinite-impulse-response (IIR) filter and achieves a sharp frequency response. The low-coherence IIR filter is realized by employing the cross-gain modulation (XGM) of the amplified spontaneous emission (ASE) spectrum of the semiconductor optical amplifier (SOA). The passive filter is an n-section unbalance Mach-Zehnder (UMZ) structure, which is used to increase free spectral range (FSR) and Q factor further. The low-coherence IIR filter cascaded with one section of UMZ passive filter is experimentally demonstrated, and a Q factor of 1268 is obtained.
A high-Q microwave photonic filter using a semiconductor optical amplifier (SOA)-based mode-locking fiber ring laser is proposed, analyzed and experimentally demonstrated. The proposed microwave photonic filter can realize a high-Q frequency response, it is compact without an optical source, and it can be easily tuned by adjusting an optical variable delay line in a ring cavity. A result with a Q-factor of about 236 and a rejection ratio of about 45 dB is obtained. The measured results and the theoretical estimations agree very well.
A novel photonic technique for instantaneous microwave frequency measurement based on hybrid microwave photonic filter(HMPF) is proposed and experimentally demonstrated. The HMPF is composed of an all-pass filter and a band-pass filter with negative coefficients. By properly controlling the power relationship between the all-pass filter and the band-pass filter, the HMPF can realize a monotonic frequency response, and then a unique relationship between the output power and the input frequency is established. A high measurement resolution can be achieved for a given frequency range.
