Optical Networking Best Practices Handbook

Section 4.7.1.2 - Multimode Graded-Index Fiber

Graded Index refers to the fact that the refractive index of the core gradually decreases farther from the center. The increased refraction in the center of the core slows the speed of some light rays, allowing all the light rays to reach the receiving end at approximately the same time, thus reducing dispersion. Figure 4.4 shows the principle of multimode graded-index fiber [2]. The core’s central refractive index, nA, is greater than the outer core’s refractive index, nB. As discussed earlier, the core’s refractive index is parabolic, being higher at the center. As shown in Figure 4.4, the light rays no longer follow straight lines; they follow a serpentine path, being gradually bent back toward the center by the continuously declining refractive index [2]. This reduces the arrival time disparity because all modes arrive at about the same time. The modes traveling in a straight line are in a higher refractive index, so they travel slower than the serpentine modes. These travel farther but move faster in the lower refractive index of the outer core region.

John Wiley & Sons, Inc.

Figure 4.4 Multimode graded-index fiber.

 

John Wiley & Sons, Inc.

Figure 4.5 Single-mode fiber.

 

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