For the transmission of light through a waveguide a two-layer structure is required. A core with a higher refractive index than the outer cladding. This can be achieved either by doping the core with elements which increase the refractive index. E.g. with Germanium like in telecommunication fibers. Or by doping the cladding with elements like Fluorine which lowers the refractive index.
The height of the refractive index step defines how confined the light is guided in the core and how many different modes (pathways of the light trough the fiber) can be guided.
The layer thickness of the cladding also influences the guiding properties as always some light penetrates into the cladding. If the layer is too thin some light gets lost. Especially if the fiber is bent.
In modern designs the refractive index profile shows several layers with different optical functions. E.g. to create a ring shape instead of a single spot or to create a pump cladding for laser fibers .