Mixing Fibre Optics

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Reasons Not To Mix Fibre Optics – Network managers who are considering making the switch from 62.5-micron to 50-micron multimode fibre.

If and when Network managers choose to make the switch, 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 fibre.
  2. The bandwidth of 62.5-micron fibres 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 fibre types, upgradeability will be severely compromised.

The elevated loss problem occurs when transmitting from the larger-core 62.5-micron fibre into a smaller 50-micron core. It is comparable to a 100mm (4-inch) water pipe connecting to a 75mm (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.

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. Since most optical loss test sets use LED’s you should plan for the worst and assume you’ll see a 4-dB loss in one direction.

Increasingly, cabling system designers and end users are specifying 50/125-micron optical fibre into their networks. 50-micron optical fibre is not a new technology. Rather, it has been used extensively outside of North America, including Europe and Japan. The 50-micron optical fibre cable, as specified in the ANSI-approved TIA/EIA-568B.3 standard, offers more than three times the bandwidth of the standard 62.5/125-micron optical fibre in the short-wavelength window of 850 nanometers (nm; 500 to 62.5/125’s 160

The short wavelength is crucial to networks, because low-cost 850-nm vertical-cavity surface-emitting laser (VCSEL) sources are being deployed to support Gigabit Ethernet applications throughout premises networks. Characterizing these optical fibres with the right equipment and methods is extremely important to ensure a network’s cabling performance. When testing the optical-fibre cable for Gigabit Ethernet networks, Fluke Networks recommends that you use a tester equipped with a VCSEL source at 850 nm and a laser source at 1,310 nm.

Because optical connections are not perfect, loss issues frequently result from air gaps, angular misalignment, area mismatch, or area misalignment. Dirt also frequently contributes to optical loss.


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