This is the scattering of light due to small localized changes in the refractive index of the core and the cladding material. The changes are indeed very localized.
We are looking at dimensions which are less than the wavelength of the light. There are two causes, both problems within the manufacturing processes.
The first is the inevitable slight fluctuations in the ‘mix’ of the ingredients. These random changes are impossible to completely eliminate. It is a bit like making a currant bun and hoping to stir it long enough to get all the currants equally spaced.
The other cause is slight changes in the density as the silica cools and solidifies. One such discontinuity is illustrated in figure below and results in light being scattered in all directions.
All the light that now finds itself with an angle of incidence less than the critical angle can escape from the core and is lost. However, much of the light misses the discontinuity because it is so small. The scale size is shown at the bottom.
The amount of scatter depends on the size of the discontinuity compared with the wavelength of the light so the shortest wavelength, or highest frequency, suffers most scattering. This accounts for the blue sky and the red of the sunset.
The high frequency end of the visible spectrum is the blue light and this is scattered more than the red light when sunlight hits the atmosphere. The sky is only actually illuminated by the scattered light.
So when we look up, we see the blue scattered light, and the sky appears blue. The moon has no atmosphere, no scattering, and hence a black sky.
At sunset, we look towards the sun and see the less scattered light which is closer to the sun. This light is the lower frequency red light.
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