  Dual-comb spectroscopy (DCS) is useful for gas spectroscopy due to high
potential of optical frequency comb (OFC). However, fast Fourier transform
(FFT) calculation of a huge amount of temporal data spends significantly longer
time than the acquisition time of an interferogram. In this article, we
demonstrate frequency-domain DCS by a combination of DCS with lock-in
detection, namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from
a vast number of OFC modes without the need for FFT calculation. Usefulness of
LID-DCS is demonstrated in rapid monitoring of transient signal change and
spectroscopy of hydrogen cyanide gas.
