The core transmit the light. In order to contain the light within the core, both cladding and the core are both manufactured with two difference refractive index values as shown in the above diagram. Generally the core refractive index is higher than the cladding refractive index.
I have been in the field of fiber optics since early 1990s. I gained fiber optics skills and knowledge via my working experience as end-user, main contractor and sub-contractor and finally as an optical fiber enterpreneur.