The data-carrying capacity of an optical fiber (just as with a wire or coax) is called bandwidth. It is normally expressed as a distance-frequency product such as MHz-km (megahertz-kilometers) or more often as GHz-km (GigaHertz-kilometers). What this really means is, if you need to transmit a 27MHz signal (common frequency for CB radio) up to 1 kilometer (0.6 miles) and still detect it as a useful signal at the other end, you need an optical fiber with a bandwidth of at least 27MHz-km. Step-index, multi-mode fiber would just barely do it. If you wanted to transmit at the frequency of the newer wireless telephones, 900MHz, you would probably have to go to step-index, singlemode fiber to get a 1km range.

The main cause of bandwidth limiting is pulse broadening caused by modal and chromatic dispersion in the fiber. This is why step-index, singlemode fiber does so well over long distances. But, due to the very small core diameter, it does have major limitations when it comes to getting light into the fiber efficiently.

For step-index, multimode fiber pulse-broadening is a very difficult parameter to calculate, since macro bending, fiber length and the number of modes initially injected, all have a bearing on the pulse spreading.

Typical bandwidths for the different types of fiber are: