  We investigate the frequency-resolved intensity noise spectrum of an Yb-doped
fiber amplifier down to the fundamental limit of quantum noise. We focus on the
kHz and low MHz frequency regime with special interest in the region between 1
and 10 kHz. Intensity noise levels up to >60 dB above the shot noise limit are
found, revealing great optimization potential. Additionally, two seed lasers
with different noise characteristics were amplified, showing that the seed
source has a significant impact and should be considered in the design of high
power fiber amplifiers.
