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Terahertz dual-comb spectroscopy (THz-DCS) 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 fiber laser. While greatly reducing the size, complexity, and cost of the laser source, THz-DCS maintains the spectroscopic performance comparable to a system equipped with dual stabilized fiber lasers, and can be effectively applied to gas spectroscopy.

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f THz of CW-THz radiation. Using an f rep-free-running PC-THz comb, the f THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2 × 10−13 and a relative precision of 8.8 × 10−12 to a rubidium frequency standard. Furthermore, f THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.

We demonstrate dual-THz-comb spectroscopy using a wavelength-multiplexed mode- locked fiber laser. While greatly reducing its size, complexity, and cost, the proposed system maintains the spectroscopic performance comparable to a system equipped with dual stabilized fiber lasers.

Asynchronous-optical-sampling THz-TDS using a free-running, dual-wavelength mode-locked fiber laser is demonstrated for highly cost-effective and compact THz spectrometer while maintaining its high spectroscopic performance.

THz dual-comb spectroscopy using a free-running, dual-wavelength-comb fiber laser is demonstrated for highly cost-effective and compact THz spectroscopy systems while maintaining its high spectroscopic performance.

Frequency measurement based on dual THz frequency combs of photocarrier (dual PC-THz combs) is a promising method for the real-time measurement of the absolute frequency in CW-THz radiation without the need for cryogenic cooling. However, use of dual femtosecond lasers hinders the practicability of this method. In this paper, we generated dual PC-THz combs using a free-running, dual-wavelength mode-locked fiber laser, and applied the resulting dual PC-THz comb for the real-time monitoring of the absolute frequency in CW-THz radiation.

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., Comment: 29 pages, 7 figures

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy. In this paper, we have demonstrated adaptive-sampling THz-DCS, allowing the use of unstabilized dual femtosecond lasers.

A free-running, dual-wavelength mode-locked fiber laser is used for the real-time absolute frequency measurement of CW-THz radiation based on dual photo-carrier-THz-comb with different frequency spacing. This opens the door for compact and low-cost dual-THz-comb systems.