Inherent gain flattening due to two mode interference in erbium doped coaxial fibers

•Erbium doped fiber (EDF) in coaxial configuration can be used for gain flattening.•Coaxial fiber has two non-identical concentric cores; supports two symmetric modes.•Interference among modes leads to exchange of power between inner and outer cores.•Use of coaxial EDF results in Inherent gain flattening across the C-band.•There is also an increase in the average gain across the 1540–1560 nm band.

In this paper we propose the use of an Erbium doped fiber (EDF) in coaxial configuration for gain flattening. The coaxial fiber supports two symmetric modes and can be designed to exhibit exchange of power between the amplifying doped inner core and the undoped outer core around the gain peak (1530 nm) due to interference among the two modes as it propagates along the coaxial fiber. This results in inherent gain flattening across the C-band as well as an increase in the average gain in the 1540–1560 nm band.