Novel multimode fiber for narrow-band Bragg gratings

We propose a novel multimode fiber structure with modal propagation characteristics tailored to facilitate the creation of narrow-band high-reflectivity fiber Bragg gratings. The fiber structure proposed consists of concentric cylindrical shells of higher and lower refractive index material. 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 index shells. We present the theoretical characterization of the modal propagation constant as a function of inner shell radius, shell separation, and harmonic-mode parameter. It is shown that a fiber with a minimum inner shell radius of at least 25λ (where λ is the vacuum wavelength), and a minimum shell separation of at least 10λ provides a reasonable tradeoff between fiber size and grating performance. A simulation of the multimode fiber grating shows that a grating with a full-width at half-maximum bandwidth on the order of 10^-4λ is theoretically possible, if optical power is launched strictly into modes with angular harmonic parameter p=1