Graded-Index Multimode Fibers Solves the Problem of Modal Dispersion

Graded-index fiber’s refractive index decreases gradually away from its center, finally dropping to the same value as the cladding at the edge of the core. The change in refractive index causes refraction, instead of total internal reflection, which bends light rays back toward the fiber axis as they pass through layers with lower refractive index. No total internal reflection happens because refraction bends light rays back into the fiber axis before they reach the cladding boundary.

Different light modes in a graded-index multimode fiber still follow different lengths along the fiber, as in step-index multimode fiber. However their speeds differ because the speed of guided light changes with fiber core’s refractive index.

So the farther the light goes from the center of the fiber, the faster its speed. So the speed difference compensate for the longer paths followed by the light rays that go farthest from the center of the fiber. This equalizing of transit times of different modes greatly reduces modal dispersion.

The bandwidth of a typical off-the-shelf graded-index multimode fiber with a 50 μm core may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced.

But please note that modal dispersion may be considerably reduced, but never completely eliminated.

September 5, 2011

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