Design of flat-top bandpass filters based on symmetric multiple phase-shifted long-period fiber gratings

We analyze the transmission response of symmetric multiple phase-shifted long-period fiber gratings (LPGs). In particular, we present the optimal design parameters for two and three phase-shifted LPGs having a flat-top passband response. Similarities and differences between symmetric multiple phase-shifted LPGs and fiber Bragg gratings (FBGs) are highlighted.