• Advanced Photonics
  • Vol. 2, Issue 3, 036004 (2020)
Jie Chen1、2, Kazuki Nitta2、3, Xin Zhao1, Takahiko Mizuno3、4、5, Takeo Minamikawa3、4、5、6, Francis Hindle7, Zheng Zheng1、8、*, and Takeshi Yasui3、4、5、6、*
Author Affiliations
  • 1Beihang University, School of Electronic and Information Engineering, Beijing, China
  • 2Tokushima University, Graduate School of Advanced Technology and Science, Tokushima, Japan
  • 3JST, ERATO MINOSHIMA Intelligent Optical Synthesizer, Tokushima, Japan
  • 4Tokushima University, Institute of Post-LED Photonics, Tokushima, Japan
  • 5Tokushima University, Graduate School of Technology, Industrial and Social Sciences, Tokushima, Japan
  • 6Tokushima University, Research Cluster on “Multi-scale Vibrational Microscopy for Comprehensive Diagnosis and Treatment of Cancer”, Tokushima, Japan
  • 7Université du Littoral Côte d’Opale, Laboratoire de Physico-Chimie de l’Atmosphère, Dunkerque, France
  • 8Beihang University, Beijing Advanced Innovation Center for Big Data-based Precision Medicine, Beijing, China
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    Dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution with rapid data acquisition. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS. We demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with the adaptive sampling method. While the use of a free-running single-cavity dual-comb fiber laser eliminates the need for two mode-locked lasers and their frequency control, the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter in the free-running dual-comb laser. Doppler-limit-approaching absorption features with linewidths down to 25 MHz are investigated for low-pressure acetonitrile/air mixed gas by comb-mode-resolved THz spectroscopy. The successful demonstration clearly indicates its great potential for the realization of low-complexity, Doppler-limited THz spectroscopy instrumentation.

    1 Introduction

    Coherent spectroscopic techniques in the terahertz (THz) or far-infrared region (frequencies of 0.1 to 10 THz and wavelengths of 30  μm to 3 mm) are the enabling technology for a wide variety of important applications.13 Among these techniques, photonic-based THz time-domain spectroscopy (THz-TDS)4,5 has been applied to the study of a diverse range of samples, such as the rotational transitions of polar gas molecules,6 the hydrogen bonding signature in aqueous systems,7 and self-assembly of proteins,8 because it takes advantage of the broad spectral bandwidth of THz emitters/receivers pumped by ultrafast lasers. This well-established THz-TDS scheme uses pump pulses and delayed probe pulses from a femtosecond mode-locked laser for THz radiation and THz detection, respectively. However, it has limited spectral resolution and accuracy due to constraints on the travel range, repeatability, and speed of the mechanical delay lines. Thus, the use of the photonic THz techniques for Doppler-limited spectral measurements and quantitative analysis of molecular rotational transitions, which are attractive for many applications including gas sensing, has not yet been well explored.