APC versus AFC

Most of the ends of the fibres in our lab are angle polished. This is done on the input side to reduce specular reflection back into the laser, and reduce optical feedback; and on either side angle polishing reduces an etalon effect that might modulate the transmission of the fibre when its length is less than the coherence length of the source (typically the case for us).

There are two standards for this: one has more reliable facet placement (AFC) so is better when swapping air-coupled fibres, and the other is smoother for better fibre-to-fibre coupling (APC). Please read the following to understand the difference between them, especially if you are ordering new fibre for the lab.

Note that some people have argued we only need angle polishing on one side to prevent etalon effects. I haven't seen any data to know how much better double-end angle-polished is compared to single-end.

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