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 CO₂ laser-written LPFGs, we demonstrate a filter with a frequency spacing of ∼ 1.3 GHz and a notch depth of ∼17 dB. Our experimental results agree well with theoretical calculations. © 2010 Optical Society of America.