  Chip-based optical frequency combs address the demand for compact, bright,
coherent light sources of equidistant phase-locked lines. Traditionally, the
Fourier Transform Spectroscopy (FTS) technique has been considered a suboptimal
choice for resolving comb lines in chip-based sensing applications due to the
requirement of long optical delays, and spectral distortion from the
instrumental line shape. Here, we develop a sub-nominal resolution FTS
technique that precisely extracts the comb's offset frequency in any spectral
region directly from the measured interferogram without resorting to nonlinear
$f$-to-$2f$ interferometry. This in turn enables MHz-resolution spectrometry
with millimeter optical retardations. Low-pressure MHz-wide absorption lines
probed by widely-tunable chip-scale mid-infrared OFCs with electrical pumping
are fully resolved over a span of tens of nanometers. This versatile technique
paves the way for compact, electrostatically-actuated, or even all-on-chip
high-fidelity FTS, and can be readily applied to boost the resolution of
existing commercial instruments several hundred times.
