Any material that has a dynamic response to light (generally an output gain) can be regarded as an active medium. In specialty optical fibers, the doped region in the core where optical interactions occur to produce the output gain can be considered as an active medium. The active medium in optical fiber is created by doping the fiber with different materials such as Erbium,Ytterbium,Neodymium, etc. Depending on the concentration and type of dopants added to the fiber, its optical response would be different. Active optical fibers are generally applied in the manufacture of lasers or optical amplifiers.