All-fiber bandpass filter with adjustable transmission using cladding-mode coupling

We demonstrate an all-fiber bandpass filter with 2-nm bandwidth and more than 25 dB of nonresonant light suppression (ignoring sidelobes) in a 60-nm-wide spectral range around its central wavelength. This novel device uses a pair of long-period gratings to couple resonant light from the fiber core into the cladding and then back into the core. The amplitude transmission through the filter is dynamically adjustable over a 20-dB range.