We present experimental results for a single mode, narrow linewidth, high power fiber amplifier which employs a novel large mode area photonic crystal fiber structure in the power amplification stage. The waveguide has been modified such that the fundamental mode has a flattened intensity profile. The redistribution of intensity increases the Stimulated Brillouin Scattering (SBS) threshold. A theoretical prediction of the SBS threshold is calculated for the same core dimensions with a Gaussian intensity profile. The power out of the flattened mode fiber exceeds the calculated threshold, however, SBS was not observed due to pump limitations.