Because of their large comparative bandwidths, fibers can carry large amounts of information. A single graded index fiber can easily carry 500 million bits/second (Mb/s) of information. However, bandwidth capacity limits exist for all types of fibers and depend on the fiber and type of emitter employed.

To accurately reproduce data, light pulses must be kept separate and distinct with correct shape and spacing during transmission. Yet, the rays comprising each pulse travel in many different paths within a multimode fiber. For step index fibers, modes traveling at different angles as they zigzag down the fiber arrive at the receiver end at different times.

This arrival time variance results in distorted and overlapping pulses at the receiver end. This "modal dispersion," or spreading of the light pulses, limits the frequency that can be transmitted, because the detector cannot tell where one pulse ends and the next begins.

The time difference between the fastest and the slowest mode of light entering the fiber at the same time and traveling a kilometer may only be 1 to 3 nanoseconds, yet this modal dispersion causes major limitations on the system's operating speeds over long distances. Doubling the distance, doubles the dispersion effect.

Just as optical power loss reduces signal performance, a system can be bandwidth limited when the shape of the light pulse is distorted beyond specified limits.

Modal dispersion is often expressed in nanoseconds per kilometer, e.g. 30 ns/km. The same effect can also be expressed as a frequency, such as 200 MHz-km. This indicates that the fiber or system will operate efficiently up to 200 MHz before dispersion adversely affects signal performance over a one kilometer length. The same system could transmit a 100 MHz signal as far as two kilometers.

Dispersion makes the multimode step index fiber the least bandwidth efficient of the three types. It is therefore used for shorter runs and lower operating frequencies, e.g. 20 MHz-km.

Single mode fiber has small core sizes of 8 to 10µm diameter in order to allow only one lightwave ray to propagate down the fiber. Because modal dispersion is completely eliminated, this fiber has much greater bandwidths which can exceed several hundred gigahertz-kilometer (GHz-km).

However, fibers are susceptible to another type of dispersion problem caused by the fact that different wavelengths travel at different velocities through a medium.

This "spectral dispersion" is evident when white light decomposes into a rainbow of colors by a glass prism. Each wavelength travels at a different speed leading to unequal amounts of bending of the rays associated with each color.

If the fiber system's spectral source emitted a single frequency of light, this spectral dispersion, or material dispersion (or chromatic dispersion, as it is also often called) would be eliminated. However, an LED light source has a spectral range of about 20 times that of a laser, and thus has much greater spectral dispersion. Dispersion in glass fiber has a minimum around 1.3µm, allowing monomode fibers extremely large bandwidth capacities at this wavelength.

Single mode fiber is typically used with laser emitters, because of their greater spectral purity. Precision connectors and splicing are required.

Because of their low loss, and high capacity qualities, single mode fibers are the choice for constructing long, high data rate links, such as cross-country telecommunications.

Between monomode and step index fibers, there are graded index fibers. Rays in a graded index fiber are gradually redirected back towards the core's axis during propagation to reduce the effects of modal dispersion. Graded index have much greater bandwidth capacities than step index fibers. A 600 MHz-km graded index fiber can transmit a 20 MHz modulation signal as far as 30 km. The cost of the glass fiber is currently one of the lowest. Its low loss plus high bandwidth make it the fiber of choice for most local area network applications.

Bandwidth Summary

Different propagation pathways cause delays, or modal dispersion in multimode fibers.

Modal dispersion provides the principle bandwidth limitation for laser-based multimode fiber systems at 850 nanometers, and for both laser and LED systems at 1300 nanometers.

Spectral dispersion provides the principle bandwidth limitation for LED based systems at the first window of 850 nanometers of about 100 MHz-km, and for single mode laser-based systems typically more than 50 GHz-km at the 1300 nanometer region.

The basic loss mechanism, or attenuation, within fibers is caused by light scattering which varies by wavelength. The 1300 nanometer wavelength is important because not only is the attenuation low at this point, but spectral dispersion is generally a minimum at this wavelength.

Fibers have a constant loss over a wide range of modulation rates, but there is a rapid increase in effective loss when pulse dispersion becomes comparable to the pulse period at or near maximum bandwidth limits. Contrast this with baseband metallic systems where attenuation increases as the square root of the modulation rate. Provided dispersion is small, fiber systems do not require equalization and line amplifiers which are necessary with metallic systems.