Optical fiber consists of a core and acladding layer, selected for total internal reflection due to the difference inthe refractive index between the two.
In practical fibers, the cladding isusually coated with a layer of acrylate polymer or polyimide.
This coatingprotects the fiber from damage but does not contribute to its optical waveguideproperties.
Individual coated fibers (or fibers formed into ribbons or bundles)then have a tough resin buffer layer or core tube(s) extruded around them toform the cable core.
Several layers of protective sheathing, depending on theapplication, are added to form the cable. Rigid fiber assemblies sometimes putlight-absorbing ("dark") glass between the fibers, to prevent lightthat leaks out of one fiber from entering another.
This reduces cross-talkbetween the fibers, or reduces flare in fiber bundle imaging applications.