While light-emitting diodes have a relatively large emission area and a wide emission angle, the emission area of lasers is very small and the light emission is largely coherent.
As a result, light-emitting diodes are used with multimode fibers and over-radiate their core glass, which has a diameter of 50 µm and 62.5 µm. This over-radiation is known as overfilled launch (OFL) and results in a portion of the radiant energy not being used for signal transmission. This loss fraction is referred to as gap loss.
In general, it can be stated that surface-emitting LEDs overfire the core glass, whereas edge-emitting LEDs, lasers and VCSEL lasers have an extremely small radiation area and do not fully illuminate the core glass, which is referred to as underfilled launch (UFL).