  In this letter, the impact of pump configuration on the modal instabilities
(MI) threshold of high power fiber amplifiers is investigated. Using a time
dependent computer model based on the optical beam propagation method (BPM),
the effect of bidirectional pumping on the MI threshold of high power fiber
amplifiers is explored and compared to that of a co-pumped configuration.
Simulation results indicate that the bidirectional pumping increases the
instabilities threshold by a factor of 30% with respect to the co-pumped
scheme. We attribute this threshold enhancement to a weaker strength of the
temperature induced refractive index grating due to a more homogeneous pump
profile along the fiber length. This technique demonstrates that the
engineering pump scheme is a promising approach leading to an appreciable
threshold enhancement of fiber amplifiers designed for high power applications.
