  We present detailed studies of the effect of polarization on thermal-induced
mode instability (MI) in ytterbium-doped fiber amplifiers. Based on a
steady-state theoretical model, which takes both electric fields along the two
principal axes into consideration, the effect of polarization effects on the
gain of Stokes wave was analyzed, which shows that the polarization
characteristics of the fiber laser have no impact on the threshold of MI.
Experimental validation of the theoretical analysis is presented with
experimental results agreeing well with the theoretical results, in which
polarization-maintained and non-polarization-maintained fiber lasers with
core/inner cladding diameter of 30/250um and core NA of 0.07 were employed. The
MI threshold power is measured to be about 367~386W.
