| 摘要 |
A novel multimode fiber structure with modal propagation characteristics tailored to facilitate the creation of narrowband, high reflectivity, fiber Bragg gratings is disclosed. The fiber structure comprises 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 25lambd (where lambd is the vacuum wavelength), and a minimum shell separation of at least 10lambd provides a reasonable trade off between fiber size and grating performance. A simulation of the multimode fiber grating shows that a grating with a FWHM bandwidth on the order of 10-4lambd is theoretically possible if optical power is launched strictly into modes with angular harmonic parameter p=1.
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