Novel multimode fiber for narrow-band Bragg gratings

摘要

We propose a novel multimode fiber structure with modalnpropagation characteristics tailored to facilitate the creation ofnnarrow-band high-reflectivity fiber Bragg gratings. The fiber structurenproposed consists of concentric cylindrical shells of higher and lowernrefractive index material. A full vector second-order finite-elementnmethod is used to analyze the proposed multimode fiber structure.nSimulations of the modal profiles show that high-order modes arenlocalized to particular high-refractive index shells. We present thentheoretical characterization of the modal propagation constant as anfunction of inner shell radius, shell separation, and harmonic-modenparameter. It is shown that a fiber with a minimum inner shell radius ofnat least 25Λ (where Λ is the vacuum wavelength), and anminimum shell separation of at least 10Λ provides a reasonablentradeoff between fiber size and grating performance. A simulation of thenmultimode fiber grating shows that a grating with a full-width atnhalf-maximum bandwidth on the order of 10-4Λ isntheoretically possible, if optical power is launched strictly into modesnwith angular harmonic parameter p=1