  Long-wavelength mid-infrared frequency combs with high power and flexible
tunability are highly desired for molecular spectroscopy, including
investigation of large molecules such as C60. We present a high power,
phase-stabilized frequency comb near 10 {\mu}m, generated by a
synchronously-pumped, singly-resonant optical parametric oscillator (OPO) based
on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 {\mu}m, with a
maximum average idler power of 100 mW at the center wavelength of 8.5 {\mu}m.
Both the repetition rate (frep) and the carrier-envelop offset frequency (fceo)
of the idler wave are phase-locked to microwave signals referenced to a Cs
clock. We describe the detailed design and construction of the frequency comb,
and discuss potential applications for precise and sensitive direct frequency
comb spectroscopy.
