We report the development and operation of a frequency-comb-assisted
high-resolution mid-infrared molecular spectrometer combining high spectral
purity, SI-traceability, wide tunability and high sensitivity. An optical
frequency comb is used to transfer the spectral purity of a SI-traceable 1.54
μm metrology-grade frequency reference to a 10.3 μm quantum cascade
laser (QCL). The near-infrared reference is operated at the French
time/frequency metrology institute, calibrated there to primary frequency
standards, and transferred to Laboratoire de Physique des Lasers via the
REFIMEVE fiber network. The QCL exhibits a sub-10 --15 frequency stability from
0.1 to 10 s and its frequency is traceable to the SI with a total uncertainty
better than 4 x 10 --14 after 1-s averaging time. We have developed the
instrumentation allowing comb modes to be continuously tuned over 9 GHz
resulting in a QCL of record spectral purity uninterruptedly tunable at the
precision of the reference over an unprecedented span of 1.4 GHz. We have used
our apparatus to conduct sub-Doppler spectroscopy of methanol in a multi-pass
cell, demonstrating state-of-art frequency uncertainties down to the few
kilohertz level. We have observed weak intensity resonances unreported so far,
resolved subtle doublets never seen before and brought to light discrepancies
with the HITRAN database. This demonstrates the potential of our apparatus for
probing subtle internal molecular processes, building accurate spectroscopic
models of polyatomic molecules of atmospheric or astrophysical interest, and
carrying out precise spectroscopic tests of fundamental physics