Ultra-wide bandpass filter based on long-period fiber gratings and the evanescent field coupling between two fibers

We demonstrate a fiber-based bandpass filter with an ultra-wide spectral bandwidth. The ultra-wide band feature is achieved by inscribing a long-period fiber grating (LPG) in a specially-designed low index core single mode fiber. To get the bandpass function, the evanescent field coupling between two attached fibers is utilized. By applying strain, the spectral shape of the pass-band is adjusted to flat-top and Gaussian shapes. For the flat-top case, the bandwidth is obtained similar to 160 nm with an insertion loss of similar to 2 dB. With strain, the spectral shape is switched into a Gaussian one, which has similar to 120 nm FWHM and similar to 4.18 dB insertion loss at the peak. (c) 2007 Optical Society of America.