Step-Index Multimode Fiber vs Graded-Index Multimode Fiber

Multimode fiber can be divided into step-index fiber and graded-index fiber according to the fiber refractive index distribution. Since the two types of multimode fibers differ in working principles, they are used in different scenarios. Read through this post to get the details of the two in working principles and applications as well as their differences.

Step-Index Multimode Fiber Working Principles and Applications

In optical fibers, a step-index fiber is a fiber where a uniform refractive index exists within the core and a sharply decreased refractive index exists in the core-cladding interface because of the lower refractive index in cladding. For step-index multimode fiber, the light propagates in the shape of a zigzag along the fiber/core axis according to the principle of total reflection. Light entering the fiber at different angles of incidence will go through different paths. Although the incident lights propagate at the same speed simultaneously at the input, the time to reach the output of the fiber is different, resulting in a temporal dispersion called modal dispersion.

Since digital communications use light pulses to transmit signals down the length of the fiber, the modal dispersion causes the pulse to widen severely and spread out when they travel along with the fiber. The more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers. What’s more, the modal dispersion is not good for optical fiber communication as well. For digital optical fiber systems, when the dispersion is serious, it will cause pulses to overlap with each other, causing inter-symbol interference (ISI) and increasing the bit error rate (BER). Therefore, fiber dispersion not only affects the transmission capacity of the fiber, but also limits the relay distance of optical fiber communication systems. Because of that limitation, the step-index multimode fiber is normally used in short-distance (within a few kilometers) and low-speed (8 Mb/s or less) communication systems with a relatively lower cost. However, things differ in step-index single mode fibers. In a step-index single mode fiber, it can only transmit light of one mode. Therefore, the modal dispersion is very small, causing less impact on the transmission distance. Normally, single mode fibers are all step-index fibers.

Graded-Index Multimode Fiber Working Principles and Applications

Graded index multimode fiber is a type of optical fiber where the refractive index is higher at the axis of the core and then it decreases gradually towards the core-cladding interface. That is to say, the refractive index of a graded-index fiber gradually decreases from its center, and eventually decreases to the same value as the cladding at the core edge. The change in refractive index causes refraction rather than total internal reflection. When light passes through a layer with a lower refractive index, the light will fold back to the fiber axis. Total internal reflection does not occur because refraction folds the light back into the fiber axis before it reaches the cladding boundary.

For graded-index multimode fiber, the light travels forward in the form of sinusoidal oscillation. Like step-index multimode fibers, different lights in a graded-index multimode fiber travel along different paths. However, the speed of light propagation in graded-index multimode fibers is different because the speed of guided light varies with the refractive index of the fiber core. The farther the light goes from the center of the fiber, the faster its speed is. The speed difference compensates for the longer paths followed by the light rays that go farthest from the center of the fiber. This equalization of the transmission time of different modes reduces the mode dispersion greatly, making a higher bandwidth in graded-index fiber than step-index fiber. Therefore, most of the multimode fiber today is graded-index fiber. Compared to step-index fiber, the graded-index fiber is usually used in medium-distance (10~20 km) and relatively higher-speed (34~140 Mb/s) communication systems with higher cost.

Step-Index Multimode Fiber vs Graded-Index Multimode Fiber

From all the above, the step-index multimode fiber and graded-index multimode fiber mainly differ in the following aspects:

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