QUESTION: How do I measure the concentricity of, or tune, an APC connector?
ANSWER:
The short answer is you can’t measure concentricity with an APC connector end-face. Producing tuned APC terminations comes down to technique. Concentricity can only be determined with a PC tip. So, usually an APC connector is selected with a non-pre-angled step-ferrule. It’s terminated then polished PC, typically only up to the D1 step, and then measured with a concentricity set up. After tuning, the ferrule base will be assembled into the housing. The rotational placement of the ferrule base (4, 6 or even 8 positions) provided by the tuning step will optimize the fiber center placement with respect to an ‘ideal’ connector. Afterwards, the APC polish is done.
It should be noted the measured value at the UPC point isn’t valid anymore because of the ferrule exit hole angle. During APC generation, considerable ceramic material (as related to the fiber and fiber core) is removed, and this offset alters the actually concentricity.
However, even though the UPC concentricity numbers aren’t ‘valid’, APC tuning is still done to minimize insertion loss. By determining the offset at the PC level, ‘tuning’ it so the fiber center is at the same spot as the OD center, selecting good ferrules (tight fit with the SM fiber), minimizing material removable during the APC polish, you can get a low connection loss; on the order of less than 0.05 dB.
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