We report a master-oscillator fiber power-amplifier system consisting of a
diode-pumped monolithic nonplanar ring laser as the master oscillator and a
Yb-doped large-mode-area double-clad fiber as the power amplifier. The sys
tem emits up to 20.1 W of single-frequency radiation at a wavelength of 106
4 nm with diffraction-limited beam quality (M-2 less than or equal to 1.3).
The optical emission spectrum and amplitude-noise behavior are investigate
d. Furthermore, the power-scaling possibilities are discussed. (C) 2001 Opt
ical Society of America.