Why is cladding used in optical fiber?
Fiber optics work on the principle that light that is transmitted through the end of the fiber is reflected through the fiber’s core at the boundary (interface) between the core and the cladding. Without the cladding, most of the light energy would eventually be refracted through the core and be absorbed by the fiber’s exterior jacket.
In some cases, however, it may be desirable to use optical fiber that does not have cladding, or to remove it. When installing fiber optic lighting systems, an effect called sideglow can be achieved this way, where the fiber’s body to emits light, resembling a neon light.
(2014) The FOA Reference For Fiber Optics – Optical Fiber [Online]. Available: http://www.thefoa.org/tech/ref/basic/fiber.html