  A fully stabilized mid-infrared optical frequency comb spanning from 2.9 to
3.4 um is described. The comb is based on half-harmonic generation in a
femtosecond optical parametric oscillator, which transfers the high phase
coherence of a fully stabilized near-infrared Er-doped fiber laser comb to the
mid-infrared region. The method is simple, as no phase-locked loops or
reference lasers are needed. Precise locking of optical frequencies of the
mid-infrared comb to the pump comb is experimentally verified at sub-20 mHz
level, which corresponds to a fractional statistical uncertainty of 2x10-16 at
the center frequency of the mid-infrared comb. The fully stabilized
mid-infrared comb is an ideal tool for high-precision molecular spectroscopy,
as well as for optical frequency metrology in the mid-infrared region, which is
difficult to access with other stabilized frequency comb techniques.
