Coupling loss is usually expressed in the same units—such as watts or decibels—as in the originating circuit element or medium.
[2] In electronics, impedance mismatch between coupled components results in a reflection of a portion of the energy at the interface.
Likewise, in optical systems, where there is a change in index of refraction (most commonly at a fiber/air interface), a portion of the energy is reflected back into the source component.
As an example, two fibers coupled end-to-end may not be precisely aligned, with the result that the two cores overlap somewhat.
Similarly, even for two perfectly aligned cores, where there is a gap of any significant distance between the two fibers, there will be some geometric loss due to spread of the beam.