Filtering characteristics of film-coated long-period fiber gratings operating at the phase-matching turning point

We present novel filtering characteristics of film-coated long-period fiber gratings (LPFGs) operating at phase-matching turning point (PMTP). The 3 dB bandwidth of a single broadband dip depends greatly on the film refractive index and thickness. A π-phase-shift in the grating center produces two band-rejection peaks between which the separation reaches 375 nm. This separation increases with the number M (M > 1) of π-phase-shifts that divide the LPFG uniformly. The central loss is constant at zero for odd M, whereas decreases with M for even M. Increasing film thickness and refractive index result in blue shifts of the dual peaks and enlarge the separations, which exhibits excellent tunabilities.