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50μ vs. 62.5μ Multimode Fiber
The glass fiber used in fiber optic transmission consists of a central
core upon which the signal is carried and a surrounding cladding which
has a lower refractive index to contain the signal within the core. The
cladding in most fiber today has an outside diameter of 125 microns. In
comparison, the diameter of an average human hair is 85 microns.
There are two commonly available core sizes for multimode fiber, 50μ and 62.5μ. The 50μ core was most common in the 1980s as the use of fiber optics was beginning to gain popularity. This fiber core size had a good ratio of bandwidth and power launch, which are both mutually exclusive. A 62.5μ core typically provides a lower bandwidth capability but accept more light from an inexpensive LED source. By the 1990s the increased loss budgets associated with using 62.5μ core fiber resulted in a dramatic increase in fiber installations using a 62.5μ core.
During the 2000s Ethernet data rates have increased from Mb/s to Gb/s. At this increased speed the bandwidth of 62.5μ fiber can sometimes limit the distance of high performance systems that require higher bandwidths. At the same time a new optical light source called a VCSEL has replaced the LED as the light source of choice. The VCSEL focuses its light onto a small spot in the center of the core, eliminating the need for the larger 62.5μ core fiber. These two factors have swung the preferred fiber type for new installations back to the 50μ core size, especially network installations. The majority of video security installations, which typically do not require high speed data, still employ 62.5μ core fiber to take full advantage of the additional optical loss budget. Fortunately, all American Fibertek multimode products can be used with either 50μ or 62.5μ fiber.
One concern resulting from the availability of both 50μ and 62.5μ fiber is the potential to mix the two core sizes within one link. It is important to remember that any time an optical signal traveling down a 62.5μ core strand of fiber is transferred into a 50μ core strand of fiber approximately 50% of the signal is lost into the cladding of the 50μ fiber. In terms of optical loss budget, this signal loss results in a decrease of 3dB to 4dB in the overall budget available for the link.
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