Hello again!
Let us learn on propagation mechanism in an optical fiber :
Optical fiber is a tubular structure through which light energy of some sort would be guided in the form of waves.
Since light source can be guided through a fibre it is called as light guide or wave guide.
In any Optical fibre the refractive index of the Cladding is always less than the refractive index of it's Core.
The light source which enter into the Core will strike the interface of the Core and the Cladding with angle greater than Critical Angle to undergo total internal reflection.
The light source undergoes reflection after reflection within the core and propagate through the fibre.
The propagation of light continues as long as the fibre is not bend too sharply, since for sharp bends the light will fail to undergo total internal reflection and light signal strength drops drastically.