  Mid-infrared femtosecond optical frequency combs were produced by difference
frequency generation of the spectral components of a near-infrared comb in a
3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB
parametric gain in the 1.5 \mu m signal, which yields powers above 500 mW (3
\mu W/mode) in the idler with spectra covering 2.8 \mu m to 3.5 \mu m.
Potential for broadband, high-resolution molecular spectroscopy is demonstrated
by absorption spectra and interferograms obtained by heterodyning two combs.
