  An optical frequency comb generated with an electro-optic phase modulator and
a chirped radiofrequency waveform is used to perform saturation and pump-probe
spectroscopy on the $D_1$ and $D_2$ transitions of atomic potassium. With a
comb tooth spacing of 200 kHz and an optical bandwidth of 2 GHz the hyperfine
transitions can be simultaneously observed. Interferograms are recorded in as
little as 5 $\mu$s (a timescale corresponding to the inverse of the comb tooth
spacing). Importantly, the sub-Doppler features can be measured as long as the
laser carrier frequency lies within the Doppler profile, thus removing the need
for slow scanning or a priori knowledge of the frequencies of the sub-Doppler
features. Sub-Doppler optical frequency comb spectroscopy has the potential to
dramatically reduce acquisition times and allow for rapid and accurate
assignment of complex molecular and atomic spectra which are presently
intractable.
