THz Dual‐Comb Spectroscopy
Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low- pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5MHz and enhanced spectral accuracy of 8.39x10^−7 in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.
 Y.-D. Hsieh, Y. Iyonaga, Y. Sakaguchi, S. Yokoyama, H. Inaba, K. Minoshima, F. Hindle, Y. Takahashi, M. Yoshimura, Y. Mori, T. Araki, and T. Yasui, "Terahertz comb spectroscopy traceable to microwave frequency standard," IEEE Trans. THz Sci. Tech., 2013, 3, 322-330.
 Y.-D. Hsieh, Y. Iyonaga, Y. Sakaguchi, S. Yokoyama, H. Inaba, K. Minoshima, F. Hindle, T. Araki, and T. Yasui, "Spectrally interleaved, comb-mode-resolved spectroscopy using swept dual terahertz combs,” Sci. Rep., 2014, 4, 3816.
 T. Yasui, Y. Iyonaga, Y.-D. Hsieh, Y. Sakaguchi, F. Hindle, S. Yokoyama, T. Araki, and M. Hashimoto, "Super-resolution discrete Fourier transform spectroscopy beyond time window size limitation using precisely periodic pulsed radiation,” Optica, 2015, 2, 460-467.
 T. Yasui, R. Ichikawa, Y.-D. Hsieh, K. Hayashi, H. Cahyadi, F. Hindle, Y. Sakaguchi, T. Iwata, Y. Mizutani, H. Yamamoto, K. Minoshima, and H. Inaba, "Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers," Sci. Rep., 2015, 5, 10786.
 Y.-D. Hsieh, S. Nakamura, D. G. Abdelsalam, T. Minamikawa, Y. Mizutani, H. Yamamoto, T. Iwata, F. Hindle, and T. Yasui, "Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy," Sci. Rep., 2016, 6, 28114.
 G. Hu , T. Mizuguchi , R. Oe , K. Nitta , X. Zhao, T. Minamikawa, T. Li, Z. Zheng, and T. Yasui, "Dual terahertz comb spectroscopy with a single free-running fibre laser," Sci. Rep., 2018, 8, 11155.
 J. Chen, K. Nitta, X. Zhao, T. Mizuno, T. Minamikawa, F. Hindle, Z. Zheng, and T. Yasui,"Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser," Adv. Photon., 2020, 2, 3, 036004.