When coupling the radiation of an extended source into a multimode fiber, there are principle limitations.
An extended source, e.g. a LED source or a plasma typically emits a beam from a large area and with a large angular spectrum. This beam characteristics (area x angle) is given by the so-called beam parameter product BPM or, more precisely, the étendue.
The multimode fiber however -as every fiber type - only has a limited acceptance angle and core area (defined by the numerical aperture and the core diameter, respectively). Only light that is focused with the right angle (⇾numerical aperture) and focus size (⇾core diameter) is coupled into the fiber, everything else is not transmitted. The multimode fiber accepts beams with the corresponding BPM.
Every components has different BPMs it emits or accepts (source, optics, as well as fiber).
In classical optics, this BPM is an invariant, which means that it is not possible to change the BPM of an optical system by means of optical imaging.
An extended source, e.g. a LED source or a plasma emits a beam that automatically has a large BPM as both the emitter area as well as the angular spectrum is large. This also means that you need to choose a fiber that accepts a large BPM (large NA, large core diameter) in order to couple as much light as possible into the fiber.