Dispersion is defined as the spreading of a pulse as it propagates through a medium. It essentially causes different components of the light to propagate at different speeds, leading to distortion. The most commonly discussed dispersion in optical fibers is modal dispersion, which is the result of different modes propagating within a MMF (multimode fiber). The fiber optic cable supports many modes because the core is of a larger diameter than SMF (single mode fibers). Single mode fibers tend to be used more commonly now due to decreased attenuation and dispersion over long distances, although MMF fibers can be cheaper over short distances.
Let’s analyze modal dispersion. When the core is sufficiently large (generally the core of a SMF is around 8.5 microns or so), light enters are different angles creating different modes. Because these modes experience total internal reflection at different angles, their speeds differ and over long distances, this can have a huge effect. In many cases, the signal which was sent is completely unrecognizable. This type of dispersion limits the bandwidth of the signal. Often GRIN (graded index) fibers are employed to reduce this type of dispersion by gradually varying the refractive index of the fiber within the core so that it decreases as you move further out. As we have learned, the refractive index directly influences the propagation velocity of light. The refractive index is defined as the ratio of the speed of light to the speed of the medium. In other words, it is inversely proportional to the speed of the medium (in this case silica glass).
In order to mitigate the effects of intermodal distortion in multimode fibers, pulses are lengthened to overlap components of different modes, or even better to switch to Single mode fibers when it is available.
The next type of dispersion is chromatic dispersion. All lasers suffer from this effect because no laser is comprised of a single frequency. Therefore, different wavelengths will propagate at different speeds. Sometimes chirped Bragg gratings are employed to compensate for this effect. Doped fiber lasers and solid state lasers tend to have much thinner linewidths than semiconductor PIN lasers and therefore tend to have less chromatic dispersion, although the semiconductor lasers has several advantages such as lesser cost and smaller size.
Another dispersion type is PMD (Polarization mode dispersion) which is caused by different polarizations travelling at different speeds within a fiber. Generally, these travel at the same speed however spreading of pulses can be caused by imperfections in the material.
For SMF fibers, it is important to cover waveguide dispersion. It is important to note that since the cladding of the fiber is doped differently than the core, the core has a higher refractive index than the cladding (doping with fluorine lowers refractive index and doping with germanium increases it). As we know, a lower refractive index indicates faster speed of propagation. Although most of the light stays within the core, some is absorbed by the cladding. Over long distances this can lead to greater dispersion as the light travels faster in the core leading to different propagation velocities.