In these fibers, the core is doped with active dopant material; it both guides and amplifies the signal light. The inner cladding and core together guide the pump light, which provides the energy needed to allow amplification in the core.
These fibers utilize a doped core that acts as both a guide and amplifier for signal light. The inner cladding and core collaborate to direct the pump light, which provides the energy required for signal amplification in the core.