Optical Networking Best Practices Handbook

Section 4.7.1.1 - Multimode Step-Index Fiber

Figure 4.3 shows how the principle of total internal reflection applied to multimode step-index fiber [2]. Because the core’s index of refraction is higher than the cladding’s index of refraction, the light that enters at less than the critical angle is guided along the fiber.

John Wiley & Sons, Inc.

Figure 4.3 Total internal reflection in multimode step-index fiber.

Three different light waves travel down the fiber: one mode travels straight down the center of the core; a second mode travels at a steep angle and bounces back and forth by total internal reflection; and the third mode exceeds the critical angle and refracts into the cladding. Intuitively, it can be seen that the second mode travels a longer distance than the first, causing the two modes to arrive at separate times [2]. This disparity between arrival times of the different light rays is known as dispersion,1 and the result is a muddied signal at the receiving end.

 

1. High dispersion is an unavoidable characteristic of the multimode step-index fiber.

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