Exemplary embodiments of the disclosure include a fiber optic amplifier system and a fiber optic oscillator system having a first stage, comprising a first core fiber having a first core diameter and a first cladding size; a second stage, comprising a second core fiber having a second core diameter and a second cladding size; and a double mode adapter connecting the first stage to the second stage, wherein the double mode adapter is configured to provide transitions for the cores and the claddings of the first and second core fibers.