  We report on the development of a high-power mid-infrared frequency comb with
100 MHz repetition rate and 100 fs pulse duration. Difference frequency
generation is realized between two branches derived from an Er:fiber comb,
amplified separately in Yb:fiber and Er:fiber amplifiers. Average powers of 6.7
W and 14.9 W are generated in the 2.9 $\mu$m idler and 1.6 $\mu$m signal,
respectively. With high average power, excellent beam quality, and passive
carrier-envelope phase stabilization, this light source is a promising platform
for generating broadband frequency combs in the far infrared, visible, and deep
ultraviolet.
