Network Dictionary — Ferrule

The cor­rect term for Ferrule is the metal ring that holds things together, such as the ring that holds the rub­ber onto a pen­cil, or the metal ring that hold the brush bristles on a paint brush.

In net­work, we use the term to describe the bit that sticks out of the end of of a fibre optic con­nector since it “goes over the top of the fibre” or holds the fibre in place. [holds the fibres in place being the joke… ged­dit… just like a paint brush fibres are held in place.…jeesh, why do I bother]

Note that Ferrules have many shapes (at least, I thought it was interesting):

Connector Ferrule Shapes & Polishes¹

Fiber optic con­nect­ors can have sev­eral dif­fer­ent fer­rule shapes or fin­ishes, usu­ally referred to as pol­ishes. early con­nect­ors, because they did not have keyed fer­rules and could rotate in mat­ing adapters, always had an air gap between the con­nect­ors to pre­vent them rotat­ing and grind­ing scratches into the ends of the fibers.

Beginning with the ST and FC which had keyed fer­rules, the con­nect­ors were designed to con­tact tightly, what we now call phys­ical con­tact (PC) con­nect­ors. Reducing the air gap reduced the loss and back reflec­tion (very import­ant to laser-​​based sin­glemode sys­tems ), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflec­ted back up the fiber. While air gap con­nect­ors usu­ally had losses of 0.5 dB or more and return loss of 20 dB, PC con­nect­ors had typ­ical losses of 0.3 dB and a return loss of 30 to 40 dB.
Soon there­after, it was determ­ined that mak­ing the con­nector fer­rules con­vex would pro­duce an even bet­ter con­nec­tion. The con­vex fer­rule guar­an­teed the fiber cores were in con­tact. Losses were under 0.3dB and return loss 40 dB or bet­ter. The final solu­tion for sin­glemode sys­tems extremely sens­it­ive to reflec­tions, like CATV or high bitrate telco links, was to angle the end of the fer­rule 8 degrees to cre­ate what we call an APC or angled PC con­nector. Then any reflec­ted light is at an angle that is absorbed in the clad­ding of the fiber.

Footnotes

1. Reference [back]

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