Solid glass bonded around the core with a refractive index about 0.4 percent lower in singlemode and 1 to 2 percent lower in multimode, which creates the boundary for total internal reflection. Almost every telecom and datacom fiber has a cladding diameter of 125 µm regardless of core size, which is why the same cleavers, strippers and ferrules work on singlemode and multimode. Light that leaks into the cladding is lost within a short distance.
Ferrule hole's sized for 125 micron cladding, so if the fiber won't go in, you didn't strip all the coating off.
Do not mix it up with
- Buffer: Buffer is the plastic protection added outside the coating, not glass, and it plays no part in guiding light.
Related terms
Core
The central glass region of an optical fiber where the light actually travels.
Two-fifty
Splicer slang for fiber that has only its 250 µm acrylate coating, with no tight buffer over it.
Numerical aperture
A measure of how wide a cone of light a fiber can accept and guide, written as a number such as 0.20.
Macrobend
A bend in a fiber tight enough to see, which lets light escape the core and causes loss.