  We report on the high power amplification of 1064nm linearly-polarized laser
in all-fiber polarization-maintained MOPA, which can operate at output power
level of 1.3kW. The main amplifier was pumped with six 915nm laser diodes, and
the slope efficiency is 65.3%. The beam quality (M2) was measured to be <1.2 at
full power operation. The polarization extinction rate of the fiber amplifier
was measured to be above 94% before mode instabilities (MI) sets in, which
reduced to about 90% after the onset of MI. Power scaling capability of
strategies for suppressing MI is analyzed based on a novel semi-analytical
model, the theoretical results of which agree with the experimental results. It
shows that mitigating MI by coiling the gain fiber is an effective and
practical way in standard double-cladding large mode area fiber, and, by tight
coiling of the gain fiber to the radius of 5.5cm, the MI threshold can be
increased to 3 times higher than that without coiling or loose coiling.
Experimental study has been carried out to verify the idea, which has proved
that MI was suppressed successfully in the amplifier by tight coiling.
