  We describe a tunable broadband mid-infrared laser source based on
difference-frequency mixing of a 100 MHz femtosecond Yb:fiber laser oscillator
and a Raman-shifted soliton generated with the same laser. The resulting light
is tunable over 3.0 um to 4.4 um, with a FWHM bandwidth of 170 nm and maximum
average output power up to 125 mW. The noise and coherence properties of this
source are also investigated and described.
