We report a new approach to obtaining single-transverse-mode operation of a
multimode fiber amplifier in which the gain fiber is coiled to induce sign
ificant bend loss for all but the lowest-order mode. We demonstrated this m
ethod by constructing a coiled amplifier using Yb-doped, double-clad fiber
with a core diameter of 25 mu m and a numerical aperture of similar to 0.1
(V approximate to 7.4). When the amplifier was operated as an amplified-spo
ntaneous-emission source, the output beam had an M-2 value of 1.09 +/- 0.09
; when seeded at 1064 nm, the slope efficiency was similar to that of an un
coiled amplifier. This technique will permit scaling of pulsed fiber lasers
and amplifiers to significantly higher pulse energies and peak powers and
cw fiber sources to higher average powers while maintaining excellent beam
quality. (C) 2000 Optical Society of America OCIS codes: 060.2320, 140.3510
, 140.4480, 140.3570.