For light-emitting diodes(LEDs) used in the infrared range, the diameter of the emitting surface is between 50 µm and 100 µm. For laser diodes it is about 5 µm.
If the radiating surface and the angle of radiation produce a beam diameter that is larger than the feed area of the optical waveguide, i.e. larger than the diameter of the core glass, then the radiated light beam over-radiates the core glass. In this context, the term Overfilled Launch( OFL) is used. If, on the other hand, the emitted light beam is considerably smaller than the core glass, this is called an underfilled launch (UFL).