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- Advanced Photonics
- Vol. 2, Issue 3, 036004 (2020)

Abstract

1 Introduction

Coherent spectroscopic techniques in the terahertz (THz) or far-infrared region (frequencies of 0.1 to 10 THz and wavelengths of ^{1}^{–}^{3}^{4}^{,}^{5}^{6}^{7}^{8}

Because optical frequency combs^{9}^{,}^{10}^{9}^{,}^{10}^{11}^{–}^{13}^{14}^{–}^{16}^{17}^{,}^{18}

Various schemes have been investigated to further reduce the complexity of THz-DCS and optical DCS systems. Recently, advances in the endeavor to generate a pair of frequency combs from a free-running single-cavity laser^{19}^{–}^{28}^{25}^{,}^{29}^{–}^{33}^{34}^{,}^{35}^{31}^{,}^{33}^{36}^{37}^{38}^{39}^{24}^{,}^{40}^{–}^{44}

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In this paper, we demonstrate an adaptive-sampling, near-Doppler-limited THz-DCS scheme based on a simple free-running SCDCL. Doppler-limit-approaching absorption features were investigated for low-pressure gas mixtures of acetonitrile and air.

2 Materials and Methods

2.1 Principle of Operation

THz-DCS can be performed in the frequency domain^{11}^{,}^{12}^{13}^{45}^{–}^{47}

Figure 1.Principle of operation. (a) Flowchart of time-domain THz-DCS. (b) Acquisition of the temporal waveform using the adaptive sampling method.

Details of the adaptive sampling method have been discussed elsewhere,^{39}

2.2 Single-Cavity Dual-Comb Laser

The upper-left part of ^{29}^{,}^{32}^{,}^{34}^{29}

Figure 2.Configuration of comb-mode-resolved adaptive sampling THz-DCS. SFG-X, sum-frequency-generation cross-correlator; BBO, beta-barium borate crystal; PC-THz comb, photocarrier THz comb; M, double-balanced mixer; FM, frequency multiplier (frequency multiplication factor

2.3 Adaptive Clock Generator

The adaptive clock generator (ACG) provides the adaptive clock to suppress the residual long-term drift and timing jitter in the repetition rate difference ^{39}

2.4 THz Dual-Comb Spectroscopy

The right part of

A portion of the separated

The temporal waveform of the amplifier output was acquired with a data acquisition board. The mode-resolved THz comb spectrum was obtained by taking Fourier transform of the temporal waveform accumulated in the time domain. A rubidium frequency standard (Stanford Research FS725,

2.5 Sample

Acetonitrile (^{48}

3 Results

3.1 Basic Performance of SCDCL

When the cavity EDF was pumped beyond its mode-locking threshold and the intracavity PC was properly adjusted, dual-comb mode-locking oscillation was achieved at two different center wavelengths with similar peak intensities, as shown in

Figure 3.Performance of the dual-comb fiber laser. (a) Output spectrum of the laser. (b) RF spectrum of the dual-comb pulses. (c) Fluctuations of

The temporal drifts of

3.2 Performance of Adaptive Sampling THz-DCS

To investigate the effectiveness of the proposed adaptive sampling THz-DCS method, 100,000 temporal waveforms of 10-consecutive THz pulse train were acquired and accumulated by the adaptive sampling method. For a comparison, similar temporal waveforms were acquired based on a constant sampling method, which is widely used for data acquisition of the previous DCS with SCDCL.^{31}^{,}^{33}^{31}^{,}^{33}

Figure 4.(a) Comparison of the temporal waveforms averaged 100,000 times obtained using different sampling clocks. Inset: a zoomed-in plot of the main THz pulse. (b) Comb-mode-resolved THz spectrum through air at room pressure. Inset: a zoomed-in plot around 0.5672 THz.

Next, the mode-resolved THz comb spectrum was obtained by calculating the FT of the temporal waveform of the adaptive sampling RF pulse train [see the lower part of ^{49}^{,}^{50}^{39}

3.3 Near-Doppler-Limited Spectroscopy of Low-Pressure Acetonitrile/Air Gas

To demonstrate the high-resolution spectroscopic capability of the proposed system, THz spectroscopy of mixed gas of acetonitrile (^{48}^{51}^{49}^{34}

Figure 5.Comb-mode-resolved THz spectroscopy of a mixture gas sample of

Each manifold is composed of a number of closely spaced rotational transitions, assigned by ^{51}^{51}

The validity of the adaptive sampling THz-DCS scheme was more precisely evaluated by measuring the pressure broadening characteristics of

Figure 6.Mode-resolved absorption characterization of

Total pressure of | Partial pressure of | FWHM linewidth of | ||

Result | Uncertainty | Result | Uncertainty | |

430 | 142 | 1 | 102 | 2 |

360 | 127 | 1 | 77 | 2 |

330 | 87 | 1 | 65 | 2 |

280 | 73 | 1 | 53 | 2 |

256 | 68 | 1 | 47 | 1 |

149 | 52 | 1 | 37 | 1 |

115 | 42 | 1 | 25 | 1 |

Table 1. Quantitative analysis of

4 Discussion

We first discuss the potential of the adaptive sampling THz-DCS scheme for Doppler-limited gas spectroscopy. We confirmed clear differences in the absorption linewidth as the gas pressure was reduced, as shown in ^{52}^{,}^{53}^{54}

Figure 7.Pressure broadening characteristic of

We next discuss the possibility of the proposed method for THz spectroscopy with further enhanced precision. Although we demonstrated the near-Doppler-limited gas spectroscopy in the THz region, the frequency comb spacing ^{49}^{,}^{50}

In summary, we demonstrated the adaptive-sampling near-Doppler-limited THz comb spectroscopy with the SCDCL. Using the free-running SCDCL with the adaptive sampling method, the long-term instability of the TMF was effectively suppressed, facilitating the long-term acquisition and temporal accumulation of THz temporal waveforms with a time window extending to multiple laser pulse periods. This results in a broadband, mode-resolved THz comb spectrum with a frequency sampling spacing of 48.8 MHz, a spectral resolution of 4.88 MHz, and a power dynamic range of 50 dB. Low-pressure

References

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[31] R. Liao *et al*. Dual-comb spectroscopy with a single free-running thulium-doped fiber laser**. Opt. Express, 26, 11046-11054(2018)**.

[54] S. Svanberg, M. W. Sigrist, J. D. Winefordner, I. M. Kolthoff. Differential absorption lidar (DIAL)**. Air Monitoring by Spectroscopic Techniques(1994)**.

Jie Chen, Kazuki Nitta, Xin Zhao, Takahiko Mizuno, Takeo Minamikawa, Francis Hindle, Zheng Zheng, Takeshi Yasui. Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser[J]. Advanced Photonics, 2020, 2(3): 036004

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