The interior one is called the core of the fibre and the exterior one is called the cladding.
For example, the timber cladding is oak, left to age naturally.
Explosives are now used in such diverse fields as welding, bulk cladding of plates, forming, sizing, powder compaction, hardening, and cutting.
There is amplification of the intensity in the core and no amplification of the intensity traveling in the cladding.
The small-signal gain along a disk diagonal is reduced, simplifying edge claddings for parasitics suppression.
The shells are designed as brickwork cladding with vertical windows.
Since the intensity profile is wider than the gain region the intensity profile experiences a step between core and cladding.
Why change the cladding from one facade to the next in such a playful and inconsistent fashion?