  We develop a high-power narrow-linewidth fiber amplifier based on a tapered
Yb-doped fiber. In the experiment, the narrow end of the tapered fiber is used
as the input port of both pump source and signal light to form a robust
all-fiber configuration. The stimulated Brillouin scattering (SBS) is
effectively suppressed owing to its gradually increased mode area as well as
continuously changed Brillouin frequency shift dependent on the core diameter.
As a result, an output power of 260 W without SBS and mode instability (MI) is
obtained, with a slope efficiency of 71.3 % and a narrow linewidth of ~2GHz.
However, the MI rather than SBS is observed when output power reaches 260 W.
The features of MI are experimentally studied in detail. It is pointed out
that, the MI seems the primary and durative limitation factor of the tapered
fiber amplifier for high-power narrow-linewidth output, although it has
advantage to reduce other nonlinear effect. To the best of our knowledge, this
is the first detailed experimental study on MI in a tapered fiber. At last,
some worthwhile discussion about the optimization of the tapered fiber and the
whole system for both SBS and MI suppression is conducted based on the present
and previous results.
