  High-power fiber laser amplifiers have enabled an increasing range of
applications in industry, medicine and defense. The power scaling for
narrow-band amplifiers is currently limited by the transverse modal
instability. Various techniques have been developed to suppress the instability
in a single or few-mode fiber in order to output a clean, collimated beam. Here
we propose to use a highly multimode fiber and equal modal excitation to
suppress the thermo-optical nonlinearity and instability. Our numerical
simulations and theoretical analysis predict a significant reduction of dynamic
coupling among the fiber modes with such excitation. When the bandwidth of a
coherent seed is narrower than the spectral correlation width of the multimode
fiber, the amplified light maintains high spatial coherence and can be
transformed to any target pattern or focused to a diffraction-limited spot by a
spatial mask at either input or output end of the amplifier. Our method
simultaneously achieves high average power, narrow spectral width, and good
beam quality, which are desired for fiber amplifiers in many applications.
