What is Communications Fiber?

Step-index multimode fibers

Step-index multimode fibers with cores not quite as large can be used for some types of communications. One smaller type, has a 100-p.m core surrounded by a cladding 20 um thick, for total diameter of 140 um. It is typically called 100/140 fiber, with the core diameter written before the overall diameter of the cladding.

Large Core fiber

Typically an outer plastic coating covers the whole fiber, protecting it from mechanical damage and making it easier to handle. The large core is attractive for certain types of communications, because it can collect light efficiently from inexpensive light sources such as LEDs.

Step-index fibers for communication

If you think of light in terms of rays, you can see an important limitation of large-core step-index fibers for communication. Light rays enter the fiber at a range of angles, and rays at different angles travel different paths through the same length of fiber. The larger the angle between the light ray and the axis, the longer the path. For example, a light ray that entered at 8° from the axis (the maximum confinement angle in the earlier example) of a perfectly straight 1-m length of fiber would travel a distance of 1.0098 m (1 m/cos 8°) before it emerged from the other end. Thus light just inside the confinement angle would emerge from the fiber shortly after light that traveled down the middle. This pulse-dispersion effect becomes larger with distance and can limit data transmission speed.
In fact, the ray model gives a greatly simplified view of light transmission down optical fibers. As I mentioned earlier, an optical fiber is a waveguide that transmits light waves transmission modes. Stay tuned for the next section, and I’ll explain more about these modes. The larger the fiber core, the more modes it can transmit, so a step index fiber with a core of 20 um or more is a multimode fiber. Light rays enter the fiber at different angles, and the various modes travel down the fiber at different speeds. What you have as a result is modal dispersion, which occurs in all fibers that carry multiple modes. It is largely irrelevant for imaging and guiding illuminating beams, but it is a serious drawback for communications. To understand why, we need to take a closer look at modes.