Two major types of optical fibers exist for communication system: multimode fiber and single mode fiber. The difference between them is that multimode fiber can carry many modes of light rays while single mode fiber can only carry one mode of light ray.

This is a big difference. This fact determines that multimode fiber can only be used in short distance link, usually within a few hundred meters, while single mode fiber is used on much longer, usually 40~120km, distance.

Based on this fact, it is now easier to understand that multimode fiber is used in LAN network and video surveillance while single mode fiber is used in long distance telephony applications.

Another consequence of this difference is the information carrying capability difference between these two types of fibers. Single mode fiber can carry hundreds times of more information than multimode fiber.

Now let's examine the fiber types in detail.

:: Multimode fiber

Light ray travels in the fiber core at discrete angles within its acceptance cone. Multimode fiber has 50um or 62.5um diameter core, which is much larger than a 9um diameter single mode fiber core. Thus a large number of modes (light rays injected into the fiber at different angles) can be coupled into multimode fiber.

Now let's look at two light rays that travel along a multimode fiber. One light ray travels straight down the fiber core center which is the shortest path. A second light ray travels at a steep angle and bounces back and forth by the fiber core side wall (a phenomenon called total internal reflection) while traveling down the fiber length which is a longer path than the first light ray.

Since the second light ray travels a longer path than the first light ray, they arrive at the fiber end at separate time. (the second light arrives later than the first)

This disparity between arrival times of the different light rays is called dispersion. The consequence of this disparity is a muddied signal at the receiving end. In order to properly receive the signal, the signal must run at a slower rate and that is why multimode fiber's bandwidth is limited.

:: Single mode fiber

Single mode fiber, on the other hand, only accepts one light ray, which is the first light ray that travels straight down the fiber core center. So there is no arrival time disparity between different fiber modes which makes a cleaner signal at the receiving end. This is the reason why single mode fiber can run signals at much higher speed resulting in its much higher bandwidth.

Single mode fiber does have some disadvantages though. The smaller fiber core diameter makes it much harder to couple light into the fiber. This increases the manufacturing cost of many single mode fiber optic components such as isolator, attenuator, etc. The tolerances for single mode connectors, mechanical splices are also much more demanding.

One important variety of single mode fiber is polarization maintaining fiber, or also called PM fiber. PM fiber carries only one polarization (the light's electronic field direction) of the light. PM fiber's major applications include coherent communication system and electro-optic modulators which serves as a light transmitter in high speed fiber optic system.