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 laser-written LPFGs, we demonstrate a filter with a frequency spacing of and a notch depth of . Our experimental results agree well with theoretical calculations.
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