Until recently the fiber optic cable used in most local area networks has a core diameter of 62.5 microns. However, improvements in fiber optic technology and a reduction in their associated expenses have led to increased popularity of 50-micron fiber.
Compared to the expensive lasers and fibers associated with singlemode fiber optic networking, the less expensive light-emitting diodes (LEDs) and 62.5-micron or 50-micron fibers associated with multimode fiber networking have become the standard of local area networking. However, recent technological improvements and cost reductions have changed this situation.
In the past, fiber core diameter and its numerical aperture (light receptivity of a fiber) were extremely important details in reliable fiber optic transmission, but improvements in fiber geometry, connector polishing, core alignment and LED power output have reduced their importance. In addition, new low-cost 850nm VCSELs (Vertical Cavity Surface Emitting Lasers) offer increased transmission rates and smaller spot size requirements in the fiber's core. These new VCSELs and the low cost and high-bandwidth capacity of 50-micron fiber in particular allowed both gigabit and 10-gigabit data transfer rates at an affordable price for the first time in history. Although 62.5-micron fiber is also capable of achieving gigabit and 10-gigabit data transfer rates, 50-micron fiber offers better reliability, lower attenuation rates and increased distances. (See diagram below.)
Another reason for the growing popularity of 50-micron fiber is the fact that most important standards organizations for fiber optics (IEEE, INCITS, InfiniBand, OIF, TIA, IEC and ITU-T) have specified high bandwidth laser-optimized 50-micron multimode fiber for new high-speed installations. 50-micron fiber has three times the bandwidth capacity of 62.5-micron fiber and this enables it to achieve longer distances when VCSELs are used. In addition, some of the new laser-based 50-micron fibers have 10 to 20 times the bandwidth of 62.5 micron fiber. 850nm is the preferred wavelength for such laser based multimode fiber applications.
Before deciding between 50-micron and 62.5micron fiber other factors must be taken into consideration. Namely, the choice between 62.5-micron and 50-micron fiber will depend on whether a new network segment is being installed or an existing network is being replaced or upgraded. When installing a new network segment 50-micron fiber offers numerous advantages over 62.5-micron fiber in all important areas: bandwidth, speed, distance and upgradability. However, existing networks, especially those with 62.5-micron fiber throughout, will usually offer better performance when not mixed with 50-micron fiber. Although not recommended, 62.5-micron and 50-micron fiber can be mixed in the same transmission line and numerous tests have been made to demonstrate this, however this does create attenuation where different size fibers are joined. The amount of loss which occurs where mixed fibers are joined can vary. For more details on this, please visit http://www.corning.com/docs/opticalfiber/an4256.pdf . Although uniformity in fiber size is important where fibers are mated using connectors or splices, fiber segments between the ports of two powered fiber optic devices may be upgraded to 50-micron fiber to accomodate future bandwidth needs.
The diagram below illustrates the speed and distance benefits of 50 micron fiber:
As a final note, the price difference between 62.5-micron and 50-micron fiber is small enough that it shouldn't be an important factor in determining which fiber size you choose.