All-Fiber Tunable Microwave Photonic Filter Based on a Cladding-Mode Coupler

We demonstrate an all-fiber tunable microwave photonic filter based on the configuration of a Mach-Zehnder interferometer, which employs two fixed widely separated wavelengths and a cladding-mode coupler formed with a pair of matching long-period fiber gratings to provide continuous tuning of the time-delay difference between the two arms of the interferometer. The filter provides stable coherence-free operation and a free spectral range that can be tuned continuously and precisely over a wide range.