The ferrule hole bore must be slightly larger in diameter than the fiber to allow the fiber to be inserted into the hole. The smaller the ferrule hole diameter, the more precisely the fiber will be centered in the ferrule (thus, lower Insertion Loss can be expected). However, smaller/tighter-tolerance ferrules are more expensive, and can be more difficult to insert the fiber into. Conversely, the larger the ferrule hole, the more opportunity for the fiber to not sit perfectly centered within the ferrule; thus higher Insertion Loss can be expected (but such large-bore holes will be very easy to insert a fiber into).

Knowing the hole specifications for the ferrules you are using is critical to achieve desired IL results. If you are targeting minimum Insertion Loss, you should use the tightest-tolerance ferrule hole diameter available. If you can see a clearly defined “epoxy ring” around the fiber OD when viewing after polishing (usually seen as a thin “crescent-moon” around the fiber), this is the result of the fiber sitting off-center within an oversized ferrule hole. When using a terminated 125um fiber, a 126.0um +/- .5um should be the largest ferrule hole used for most low-loss applications. If your ferrule hole specification is larger than this and you are facing IL performance problems, consider purchasing ferrules with more exacting tolerances.




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