Novel multimode fiber for narrow-band Bragg gratings

In this paper, we propose a novel multimode fiber structure with modal prop agation characteristics tailored to facilitate the creation of narrow-band high-reflectivity fiber Bragg gratings. The fiber structure proposed consis ts of concentric cylindrical shells of higher and lower refractive index ma terial. A full vector second-order finite-element method is used to analyze the proposed multimode fiber structure. Simulations of the modal profiles show that high-order modes are localized to particular high-refractive inde x shells. We present the theoretical characterization of the modal propagat ion constant as a function of inner shell radius, shell separation, and har monic-mode parameter. It is shown that a fiber with a minimum inner shell r adius of at least 25 lambda (where lambda is the vacuum wavelength), and a minimum shell separation of at least 10 lambda provides a reasonable tradeo ff between fiber size and grating performance. A simulation of the multimod e fiber grating shows that a grating with a full-width at half-maximum band width on the order of 10(-4) lambda is theoretically possible, if optical p ower is launched strictly into modes with angular harmonic parameter p = 1.