  We investigated the effct of pump wavelength on the modal instabilities (MI)
in high power linearly-polarized Yb-doped fiber amplifiers. We built a novel
semi-analytical model to determine the frequency coupling characteristics and
power threshold of MI, which indicates promising MI suppression through pumping
at an appropriate wavelength. By pumping at 915nm, the threshold can be
enhanced by a factor of 2.36 as compared to that pumped at 976nm. Based on a
high power linearly-polarized fiber amplifier platform, we studied the
influence of pump wavelength experimentally. The threshold has been increased
by a factor of 2 at 915nm, which agrees with the theoretical calculation and
verified our theoretical model. Furthermore, we show that MI suppression by
detuning the pump wavelength is weakened for fiber with large core-to-cladding
ratio.
