  Dual THz comb spectroscopy has the potential to be used as universal THz
spectroscopy with high spectral resolution, high spectral accuracy, and broad
spectral coverage; however, the requirement for dual stabilized femtosecond
lasers hampers its versatility due to the bulky size, high complexity, and high
cost. We here report the first demonstration of dual THz comb spectroscopy
using a single free-running fibre laser. By tuning the cavity-loss-dependent
gain profile with an intracavity Lyot filter together with precise management
of the cavity length and dispersion, dual-wavelength pulsed light beams with
slightly detuned repetition frequencies are generated in a single laser cavity.
Due to sharing of the same cavity, such pulsed light beams suffer from
common-mode fluctuation of the repetition frequency, and hence the
corresponding frequency difference between them is passively stable around a
few hundred hertz within millihertz fluctuation. This considerably stable
frequency difference enables dual THz comb spectroscopy with a single
free-running fibre laser. While greatly reducing the size, complexity, and cost
of the laser source by use of a single free-running fibre laser, the dual THz
comb spectroscopy system maintains a spectral bandwidth and dynamic range of
spectral power comparable to a system equipped with dual stabilized fibre
lasers, and can be effectively applied to high-precision spectroscopy of
acetonitrile gas at atmospheric pressure. The demonstrated results indicate
that this system is an attractive solution for practical applications of not
only THz spectroscopy but also THz-pulse-based measurements.
