Nonuniformly Spaced Photonic Microwave Delay-Line Filters and Applications

A finite impulse response (FIR) filter for microwave signal processing implemented based on an optical delay-line structure with uniformly spaced taps has been extensively investigated, but the realization of such a filter with negative or complex tap coefficients to provide an arbitrary frequency response is still a challenge. In this paper, an overview of photonic microwave delay-line filters with nonuniformly spaced taps, by which an arbitrary bandpass frequency response can be achieved with all-positive tap coefficients, is presented. We show that the nonuniform time delays provide equivalent phase shifts to the tap coefficients, while the all-positive-coefficient nature simplifies greatly the filter realization. Based on the theory, a 50-tap flat-top bandpass filter with a quadratic phase response is designed and analyzed. A seven-tap nonuniformly spaced photonic microwave filter with a flat-top and chirp-free bandpass response is then demonstrated. The use of the proposed technique for advanced microwave signal processing is then discussed. The generation of a chirped microwave signal and a phase-coded microwave signal are discussed and demonstrated. The use of the proposed technique to design a FIR filter for microwave matched filtering is also discussed and experimentally demonstrated.