Three reasons not to mix 50- and 62.5-micron multimode fibers

Problems arise from the signal's inability to get from one fiber core to the other.

In an article he authored for Cabling Installation & Maintenance magazine, OFS's John Kamino details how and why network managers may make the switch from 62.5-micron to 50-micron multimode fiber. If and when users choose to make the switch, he warns that they should avoid mixing 50- and 62.5-micron cables in a single system, for three primary reasons.

  1. The difference in core sizes could cause high loss when transmitting from the 62.5-micron into the 50-micron fiber.
  2. The bandwidth of 62.5-micron fibers is typically much lower than that of 50-micron, further degrading system performance.
  3. Even if a low-speed application operates over a link made up of mixed fiber types, upgradability will be severely compromised.

"The elevated loss problem occurs when transmitting from the larger-core 62.5-micron fiber into a smaller 50-micron core," Kamino writes. "It is comparable to a 4-inch water pipe connecting to a 3-inch pipe. There is no problem going from the smaller pipe to the larger pipe, but going in the opposite direction can lead to a lot of lost water, or in this case, light."

He adds that the amount of connection loss users experience is about 4 dB for LED-based systems, and can be anywhere from 0 to 4 dB for a VCSEL-based system. He adds, "Since most optical loss test sets use LEDs, you should plan for the worst and assume you'll see a 4-dB loss in one direction."

Look for the full article in the May issue of Cabling Installation & Maintenance.

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