  Dependence of mode instabilities (MI) on ytterbium dopant concentrations in
high power fiber amplifiers has been investigated. It is theoretically shown
that, by only varying the fiber length to maintain the same total small-signal
pump absorption, the MI threshold is independent of dopant concentration. MI
thresholds of gain fibers with ytterbium dopant concentration of 5.93X10^25/m3
and 1.02X10^26/m3 have been measured, which exhibit similar thresholds and
agree with theoretical results. The result indicates that heavy doping of
active fiber can be adopted to suppress nonlinear effects without decreasing MI
threshold, which provides a method of maximizing the power output of fiber
laser, taking into account the stimulated Brillouin scattering, stimulated
Raman Scattering, and MI thresholds simultaneously.
