What's the difference between 62.5 micron and 50 micron fibers?

Physically the two fiber types differ in the diameter of their cores, the light-carrying region of the fiber. This is signified by the numeric nomenclature. In 62.5/125 fiber, for example, the core has a diameter of 62.5 microns and the cladding diameter is 125 microns. In terms of performance, the difference lies in the fibers' bandwidth, or information-carrying capacity. Bandwidth is actually specified as a bandwidth-distance product with units of MHz·km. The bandwidth needed to support an application depends on the data rate. As the data rate goes up [MHz], the distance that rate can be transmitted [km], goes down. Thus, a higher fiber bandwidth enables you to transmit at higher data rates or for longer distances. 50 mm multimode fiber offers nearly three times more bandwidth (500 MHz·km) than FDDI-grade 62.5 mm fiber (160 MHz·km) at 850 nm. Network planners often choose 50 micron fiber when they know the network will need to carry high bandwidth applications over longer link distances, or when they anticipate running higher speed protocols in the future.