Only the light that is coupled into the receiving fiber’s core will propagate, while the rest of the light is not transmitted through the splice and is radiated out of the fiber. Ideally, the loss will be minimized if the two fiber cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Unfortunately, both the fiber and connector are subject to manufacturing tolerances that create less than perfect alignment.
Connector and splice loss is caused by many factors. For example, end gaps will influence insertion loss and return loss. Therefore, optical connectors will adopt a number of polishing techniques to insure physical contact of the fiber ends to minimize back reflection. In addition, light from a fiber with a larger numerical aperture (NA) will be more sensitive to angularity and end gap, so transmission from a fiber of larger NA to one of smaller NA will be higher loss than the reverse. In other words, connecting larger fibers to smaller ones results in substantial losses, not only due to the smaller cores size, but also the smaller NA of most small core fibers.