Nonuniform fringe characteristics of cascaded symmetrically chirped long-period fiber gratings

A novel scheme of cascaded symmetrically chirped long-period fiber gratings (CSCLPFG) is proposed in this paper. Two linearly chirped long-period fiber gratings with opposite chirp coefficients are concatenated without an in-between fiber. In this fiber Mach-Zehnder interferencer (MZI) configuration, the light of different wavelength experiences different propagation distance both in core and cladding due to the symmetrically chirped grating structure. As a result, the phase difference between core and cladding mode increases nonlinearly with wavelength, which gives rise to an increment of fringe spacing with wavelength and thus, a nonuniform fringe pattern in transmission spectrum. The fringe spacing increases with the decreasing slope of phase difference curve versus wavelength. A Fourier transform analysis of the nonuniform fringe shows that the frequency range is substantially enlarged by symmetrically chirping, as compared to the uniform fringe of the cascaded identically chirped gratings. The overall fringe frequency increases with the grating length, while the wavelength scope of the fringe envelope is dependent on both the length and the chirp. In addition, the fringe frequency can be further adjusted by inserting a separation fiber in between two gratings. The mode coupling mechanism and fringe characteristics are numerically investigated, which may provide a theoretical foundation for the potential applications of this fiber device in filtering and sensing areas.