Microwave photonic filter based on circulating a cladding mode in a fiber ring resonator.

We propose a microwave photonic filter based on circulating a cladding mode in a fiber ring resonator, where the cladding mode is injected into and extracted from the resonator with a pair of matching long-period fiber gratings (LPFGs). The filter has a compact configuration and allows the frequency spacing and the notch depth to be tuned easily. The use of LPFGs also provides the capability of wavelength selection and tuning. Using a standard single-mode fiber and a pair of CO(2) laser-written LPFGs, we demonstrate a filter with a frequency spacing of approximately 1.3 GHz and a notch depth of approximately 17 dB. Our experimental results agree well with theoretical calculations.