  Dual-comb spectroscopy utilizes two sets of comb lines with slightly
different comb-tooth-spacings, and optical spectral information is acquired by
measuring the radio-frequency beat notes between the sets of comb lines. It
holds the promise as a real-time, high-resolution analytical spectroscopy tool
for a range of applications. However, the stringent requirement on the
coherence between comb lines from two separate lasers and the sophisticated
control system to achieve that have confined the technology to the top
metrology laboratories. By replacing control electronics with an all-optical
dual-comb lasing scheme, a simplified dual-comb spectroscopy scheme is
demonstrated using just one dual-wavelength, passively mode-locked fiber laser.
Dual-comb pulses with a repetition-frequency difference determined by the
intracavity dispersion are shown to be sufficiently stable against common-mode
cavity drifts and noises. As sufficiently low relative linewidth is maintained
between two sets of comb lines, capability to resolve RF beat notes between
comb teeth and picometer-wide optical spectral features is demonstrated using a
simple data acquisition and processing system in an all-fiber setup.
Possibility to use energy-efficient, free-running fiber lasers with a small
comb-tooth-spacing could enable the realization of low-cost dual-comb
spectroscopy systems affordable to more applications.
