Dispersion in POF
Fiber optics works by transmitting of light through an optically transparent core that is surrounded by a cladding. The cladding has a lower refractive index than the core, causing the light energy to be reflected through the core, rather than refracted and dissipated.
POF works by reflecting rays of light, or modes, through a core surrounded by a cladding of lower refractive index. Based on the angle the ray enters the fiber, the ray may take one of many different paths through the fiber known as modes.
With large-core fiber, particularly POF, it is possible for modes to enter the fiber at a wide range of angles without precise light launch angle control. Modes that enter at a shallow angle end up reflecting off the cladding more often, causing them to take a longer path through the fiber than a ray that takes a more direct path. This causes rays of light launched at the same time to arrive at the other end of the fiber at different times. For example, if a short pulse of light is transmitted through a fiber, dispersion will cause it to appear to be wider and of a lower amplitude.
The effects of dispersion become more severe the longer the fiber, because different modes tend to spread out and disperse further. At low bandwidths, nanoseconds of dispersion may not be noticeable, but at higher bandwidths on the order of MHz and GHz, nanoseconds are significant and can be the difference between successful communication and a nonfunctional connection.
To achieve higher bandwidth communication, extraneous modes must be eliminated. Fiber with a lower numerical aperture (NA) has a smaller acceptance cone, preventing light from entering at shallow angles. With a small enough core diameter and a low NA, it is possible to produce a singlemode fiber capable of GHz-level bandwidth over long distance.
O. Ziemann et al., POF Handbook. Berlin, Germany: Springer-Verlag Berlin Heidelberg, 2008.