How green optical fiber connectors work

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Every time one inserts a connector into any communications channel, there will inevitably be some nonzero amount of signal loss.  Green optical fiber connectors use a clever bit of geometry to reduce greatly the signal loss that would otherwise be incurred.

What we see in the photograph above are the green connectors at two ends of a fiber optic patch cable.  The green color tells us that the connector is a so-called “SC/APC” connector.  Depending on whom you choose to believe, the “SC” might stand for “standard connector” or “subscriber connector” or “square connector”.  Although people cannot seem to agree on the meaning of “SC”, everybody seems to agree that the “APC” stands for “angled physical contact”.  (The green color of the connector also connotes that the fiber in the patch cable is single-mode fiber as distinguished from multimode fiber.)

We now direct our attention to the term “angled”.  As we can see when we look closely at the photograph above, the mating surfaces of the two connectors are each beveled at a slight angle (about 8°) away from being simply perpendicular to the axis of the fiber.

This angle is the bit of clever geometry I mentioned earlier.  The idea is that if any reflection of light occurs due to the interface between the two connectors (and some amount of reflection will inevitably occur), the path of the reflected light will be away from the axis of the fiber.  Any such reflected light will largely get absorbed by the cladding of the fiber.  This reduces greatly the extent to which such reflection will constitute “noise”, and thus reduces the extent to which such reflection will aggravate the signal-to-noise ratio.

In practical terms, this means that the green connector might cause a loss of as little 0.2 dB of signal.  In contrast, a connector that fails to use the “angled” geometry (by which we mean a connector having a face that is perpendicular to the axis of the fiber) might cause a much greater loss, perhaps 2 dB or more.

If you are so fortunate as to get your internet service from a fiber optic ISP, take a look at the fiber patch cable that delivers the service to your ONT (optical network terminal).  I predict that you will see that it has green connectors.

Are your connectors green?  Please post a comment below.

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