Abstract-Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser

Jie Chen, Kazuki Nitta, Xin Zhao, Takahiko Mizuno, Takeo Minamikawa, Francis Hindle, Zheng Zheng, Takeshi Yasui,

https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-2/issue-3/036004/Adaptive-sampling-near-Doppler-limited-terahertz-dual-comb-spectroscopy-with/10.1117/1.AP.2.3.036004.full

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.

Abstract-Combination of lock-in detection with dual-comb spectroscopy

Hidenori Koresawa, Kyuki Shibuya,  Takeo Minamikawa,  Akifumi Asahara,  Kaoru Minoshima,  Takeshi Yasui

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10925/109251A/Combination-of-lock-in-detection-with-dual-comb-spectroscopy/10.1117/12.2509619.short?SSO=1

Dual-comb spectroscopy (DCS) is a powerful tool for gas spectroscopy due to high resolution, high accuracy, broadband spectral coverage, and rapid data acquisition, based on optical frequency comb (OFC) traceable to a frequency standard. In DCS, after a temporal waveform of interferogram is acquired in time domain, the corresponding mode-resolved OFC spectrum is obtained by fast Fourier transform (FFT) calculation of the acquired interferogram. However, FFT calculation of huge-sized temporal data spends significantly longer time than the acquisition time of interferogram, making it difficult to response the transient signal change. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection (LID), namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation by the synchronous detection at a LID reference frequency while maintaining high resolution and high accuracy. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas by comparing with usual DCS.

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Abstract-Dual terahertz comb spectroscopy with a single free-running fibre laser

• Guoqing Hu, 
• Tatsuya Mizuguchi, 
• Ryo Oe, 
• Kazuki Nitta, 
• Xin Zhao, 
• Takeo Minamikawa, 
• Ting Li, 
• Zheng Zheng,  
• Takeshi Yasui, 

https://www.nature.com/articles/s41598-018-29403-9

Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. 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 comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb 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. 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 THz spectroscopy and other applications.

Abstract-Real-Time Amplitude and Phase Imaging of Optically Opaque Objects by Combining Full-Field Off-Axis Terahertz Digital Holography with Angular Spectrum Reconstruction

• Masatomo Yamagiwa, 
• Takayuki Ogawa, 
• Takeo Minamikawa, 
• Dahi Ghareab Abdelsalam, 
• Kyosuke Okabe, 
• Noriaki Tsurumachi, 
• Yasuhiro Mizutani, 
• Testuo Iwata, 
• Hirotsugu Yamamoto, 
• Takeshi Yasui

https://link.springer.com/article/10.1007/s10762-018-0482-6

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials.

Abstract-Scan-less hyperspectral dual-comb single-pixel-imaging in both amplitude and phase

Kyuki Shibuya, Takeo Minamikawa, Yasuhiro Mizutani, Hirotsugu Yamamoto, Kaoru Minoshima, Takeshi Yasui, and Tetsuo Iwata

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-18-21947

We have developed a hyperspectral imaging scheme that involves a combination of dual-comb spectroscopy and Hadamard-transform-based single-pixel imaging. The scheme enables us to obtain 12,000 hyperspectral images of amplitude and phase at a spatial resolution of 46 µm without mechanical scanning. The spectral resolution given by the data point interval in the frequency domain is 20 MHz and the comb mode interval is 100 MHz over a spectral range of 1.2 THz centered at 191.5 THz. As an initial demonstration of our scheme, we obtained spectroscopic images of a standard test chart through an etalon plate. The thickness of an absorptive chromium-coated layer on a float-glass substrate was determined to be 70 nm from the hyperspectral phase images in the near-infrared wavelength region.

© 2017 Optical Society of America

Abstract-Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser

Guoqing Hu , Tatsuya Mizuguchi, Xin Zhao, Takeo Minamikawa, Takahiko Mizuno, Yuli Yang, Cui Li, Ming Bai, Zheng Zheng & Takeshi Yasu

http://www.nature.com/articles/srep42082

A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). Two independent mode-locked laser beams with different wavelengths and different repetition frequencies were generated from this laser and were used to generate dual PC-THz combs having different frequency spacings in photoconductive antennae. Based on the dual PC-THz combs, the absolute frequency of CW-THz radiation was determined with a relative precision of 1.2 × 10−9 and a relative accuracy of 1.4 × 10−9 at a sampling rate of 100 Hz. Real-time determination of the absolute frequency of CW-THz radiation varying over a few tens of GHz was also demonstrated. Use of a single dual-wavelength mode-locked fibre laser, in place of dual mode-locked lasers, greatly reduced the size, complexity, and cost of the measurement system while maintaining the real-time capability and high measurement precision.

Abstract-Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy

Yi-Da Hsieh

, Shota Nakamura

, Dahi Ghareab Abdelsalam

, Takeo Minamikawa

, Yasuhiro Mizutani

, Hirotsugu Yamamoto

, Tetsuo Iwata

, Francis Hindle

 & Takeshi Yasui

http://www.nature.com/articles/srep28114

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH3CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz. The spectral overlapping of closely packed absorption lines significantly boosted the detection limit to 200 ppm when considering all the spectral contributions of the numerous absorption lines from 0.2 THz to 1 THz. Temporal changes of the CH3CN gas concentration were monitored under the smoky condition at the atmospheric pressure during volatilization of CH3CN droplets and the following diffusion of the volatilized CH3CN gas without the influence of scattering or absorption by the smoke. This system will be a powerful tool for real-time monitoring of target gases in practical applications of gas analysis in the atmospheric pressure, such as combustion processes or fire accident.

Abstract-Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

Takeo Minamikawa, Kenta Hayashi, Tatsuya Mizuguchi, Yi-Da Hsieh, Dahi Ghareab Abdelsalam, Yasuhiro Mizutani, Hirotsugu Yamamoto, Tetsuo Iwata, Takeshi Yasui,

http://link.springer.com/article/10.1007/s10762-015-0237-6

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.