R&M Data Center Handbook

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As already mentioned, return loss is a measurement for the amount of light at the connection point that is reflected
back to the light source. The higher the RL decibel value, the lower reflections will be. Typical RL values lie at 35
to 50 dB for PC, 60 to 90 dB for APC and 20 to 40 dB for multimode fibers.
Initially the end surface of fiber optic connectors was polished to a 90° angle to
the fiber axis. Current standards require Physical Contact or Angled Physical
Contact. The expression HRL (High Return Loss) is also used fairly often, and
means the same thing as APC.
In a PC surface polishing, the ferrule gets a convex polished end surface so fiber
cores can make contact at their highest elevation points. As a result, the creation
of reflections at the contact point is reduced.
PC Physical Contact
An additional improvement in return loss is achieved by means of APC angled
polishing technology. Here, the convex end surfaces of the ferrule are polished
angled (8°) to the axis of the fiber.
SC connectors are also offered with a 9° angled poli shing. They have IL and RL
values that are identical to the 8° version and hav e therefore not gained acceptance
worldwide.
APC Angled Physical Contact
As a comparison: the fiber itself has a return loss of 79.4 dB in case of a 1310 nm fiber, 81.7 dB for 1550 nm and
2.2 dB for 1625 nm (all values for a pulse length of 1 ns).
Different amounts of light or modes are diffused and scattered back, depending on the
junction of two fibers, eccentricities, scratches and impurities (red arrow).
APC connector that is polished and cleaned well has about 14.7 dB RL against air and 45
to 50 dB when plugged in.
8°
With APC connectors, modes are also scattered back as a result of the 8° or 9° polishing,
though at an angle that is greater than the angle of acceptance for total reflection. As a
result, modes are not transmitted. The calculation of the angle of acceptance using
−1
−1
NA G . 652 D
= sin Θ ⇒ Θ = sin ( NAG.652
D
) = sin (0.13) = 7.47°
shows that all modes which have an angle greater than 7.5° are decoupled after a few
centimeters and therefore do not reach the source and interfere with it. A quality APC
connector has at least 55 dB RL against air and 60 to 90 dB when plugged in.
Quality Grades
A channel’s attenuation budget is burdened substantially by the connections. In order to guarantee the compatibility
of fiber optic connectors from different manufacturers, manufacturer-neutral attenuation values and geometric
parameters for single-mode connectors were defined in 2007 via the IEC 61753 and IEC 61755-3-1/-2
standards. The attenuation values established for random connector pairings, also known as Each-to-Each or
Random-Mate, come significantly closer to actual operating connections than the attenuation values specified by
manufacturers.
One novelty of these Quality Grades is their requirements for mean (typical) values. This provides an optimal
basis for the calculation of path attenuation. Instead of calculating connectors using maximum values, the specified
mean values can be used.
A grade M was also considered for multimode connectors in the drafts of the standards, but it was rejected in the
standard that was adopted. Since then, manufacturers and planners have been getting by using information from
older or accompanying standards to find guideline values for multimode connectors. R&M uses Grade M, in the
way it was described shortly before the publication of IEC 61753-1. These values were supported by ISO/IEC with
a required insertion loss of < 0.75 dB per connector.
These standards for multimode connector quality already were no longer sufficient with the widespread
introduction of 10 Gigabit Ethernet, but especially with future Ethernet technologies which provide 40 Gbit/s and
100 Gbit/s.
Page 120 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0