Graded-index multimode fiber

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Multimode graded-index fiber

Graded-index multimode fiber (GRIN) is a compromise between single-mode fibers and multimode fibers. It offers bandwidth potential higher than MMF, but lower than SMF, yet it has the light-gathering capability of MMF.

The core and cladding diameters are comparable to MMF, which accounts for the high numerical aperture. To combat the significant MMF dispersion problem, the refractive index is graded across the diameter of the fiber. The index profile in the image to the right illustrates this; the index increases gradually towards the center-line of the fiber. This has an interesting effect on light rays entering the fiber: as they try to travel away from the center, they are redirected toward the core’s axis because of the changing refractive index. In a sense, the light rays seek the higher refractive index.

This means that light entering the fiber, even though it enters at different modes, is rapidly tuned to follow a similar (usually parabolic) profile. It is this unification of light modes that counters the dispersion property of the optical fiber.

The large physical dimensions reduce manufacturing costs, although the graded index is more expensive than the stepped-index version of MMF.

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/grin-stin.mp3%7Cdownload</mp3> | Graded-index multimode fiber vs. Stepped-index multimode fiber