This paper presents the concept and design of medium to high power fiber radiation sources in MOPA (Master Oscillator Power Amplifier) configuration. Research results are presented on a two-stage power amplifier for MOPA applications for the third telecommuni-cations window at 1550 nm based on active erbium-doped and erbium/ytterbium-doped fibers. The amplifier is fabricated entirely in fiber technology and delivers several watts of output power.