  We report the measurement of sub-Doppler double-resonance transitions in
methane using a 3.3 $\mu$m continuous wave optical parametric oscillator to
pump transitions in the fundamental $\nu_3$ band and a 1.67 $\mu$m frequency
comb to probe ladder-type transitions in the 3$\nu_3$ band over 200 cm$^{-1}$
of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions
with average center frequency accuracy of 1.7 MHz, limited by the pump
frequency stability. The lines are assigned using the TheoReTS line list, and
the intensity ratios for different relative pump/probe polarizations. This
method provides accurate assignment of highly excited energy levels that cannot
be done using high temperature spectra.
