English for Specific Purposes

36 Electronic Engineering

that enters at less than the critical angle is guided along the fiber.

Three different lightwaves 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. 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 mode, causing the two modes to arrive at separate times. This disparity between arrival times of the different light rays is known as dispersion, and the result is a muddied signal at the receiving end. It is important to note that high dispersion is an unavoidable characteristic of multimode step-index fiber.

Graded-index⁵ refers to the refractive index ofthe core gradually decreasing farther from the core center. The increased refraction in the core center slows the speed of some light rays, allowing all the light rays to reach the receiving end at approximately the same time, reducing dispersion.

Figure 2 shows the principle of multimodegraded-index fiber. The core's central refractive index, nA, is greater than that of the outer core's refractive index, nB. As discussed earlier, the core's refractive index is parabolic, being higher at the center. The light rays follow a serpentine path being gradually bent back toward the center by the continuously declining refractive index. 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.

Single-mode fiber allows for a higher capacity to transmit information retaining the fidelity of each light pulse over longer distances and exhibiting no dispersion caused by multiple modes. Thus, more information can be transmitted per unit of time.

Single-mode fiber has disadvantages. The smaller core diameter makes coupling light into the core more difficult, as well as the tolerances for single-mode connectors and splices are much more demanding.

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