  We report on the generation of a train of ~ 2 ps, 10 um laser pulses via
multiple four-wave mixing and compression of an infrared laser beat-wave
propagating in the negative group velocity dispersion region of bulk GaAs and a
combination of GaAs and NaCl. The use of a 200 ps, 106 GHz beat-wave, produced
by combining laser pulses amplified on the 10P(20) and 10P(16) transition of a
CO2 laser, provides a novel method for generating high-power, picosecond,
mid-IR laser pulses at a high repetition rate. By using 165 and 882 GHz
beat-waves we show that cascaded phase-mismatched difference frequency
generation plays a significant role in the four-wave mixing process in GaAs.
