高性能与实用性极致融合的THz光谱学测量

太赫兹(频率0.1~10 THz,波长30~3000 μm)光谱学技术能够在这一传统光学和微波技术不能覆盖的频段提供独特和重要的信息。尽管经过了自上世纪八十年代以来的长期发展,低复杂度、高性能的太赫兹(THz)光谱学技术仍有待出现,THz光谱学的更多实际应用仍有待开发。

对混合有害气溶胶(例如烟雾、烟灰、灰尘、薄雾、浓雾、雾霾、烟气等)的气体分子进行监测是气体光谱学领域中一项非常有趣的应用。例如,为了从全球环境和节能减排的角度了解气体燃烧的效率,需要对气体燃烧过程进行动态分析。另一方面,实时检测烟雾中的有毒有害或易燃气体,对于避免火灾中的次生灾害具有非常重要的意义。

在这种背景下,有必要寻求一种能够在不受气溶胶影响的情况下,对气体分子进行动态分析的方法。以往采用气相色谱法具有很高的灵敏度,但难以进行实时测量,并存在样品预处理中使用的气溶胶捕获技术带来的诸多限制。另一方面,尽管红外吸收光谱法可以快速测量,但是气溶胶导致的光散射会造成分析性能的下降。因此,迫切需要一种无需进行样品预处理即可对气溶胶中目标气体进行快速、高精度测量的技术。

THz频带包含极性气体分子的旋转跃迁谱线,在其中会出现众多吸收谱线。在THz区域,采用测量分子旋转跃迁光谱代替在红外区域中观察分子间振动光谱,可以获得更高的选择性和灵敏度。此外,就THz辐射波长和微粒尺寸之间的关系而言,该方法对气溶胶造成的光散射影响较小。因此,在气溶胶与待测气体混合的条件下,THz光谱学技术是一种可以直接、快速分析目标气体分子种类的有效测量技术。

为了鉴别目标气体,所采用的光谱测量技术必须具有高光谱分辨率、高精度,且在THz区域内具有宽光谱覆盖范围。传统的THz光谱学测量技术不能满足以上要求,而THz双频梳光谱学技术(THz-DCS)可以实现高精度、高分辨率、宽光谱覆盖以及快速数据采集。然而,THz-DCS需要两套重频相互锁定的双光梳光源,高昂的成本与系统复杂度阻碍了其实际应用。近年来,双光梳光纤激光作为一种全新的模式,采用不需要借助稳频控制的单个激光器,极大地降低了双光梳光源的成本和复杂度。然而,当单腔双光梳光源应用于THz-DCS时,激光器残留的时间抖动会影响THz-DCS的高光谱分辨率性能。

日本德岛大学安井教授研究组与北京航空航天大学郑铮教授团队合作,将单腔双光梳光纤激光器与自适应采样THz-DCS方法有机结合,有效地补偿残余时间抖动所引起的光谱分辨率性能下降。研究成果发表在Advanced Photonics 2020年第3期上(Jie Chen, Kazuki Nitta, Xin Zhao, et al. Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser[J]. Advanced Photonics, 2020, 2(3): 036004

通过对具有逼近多普勒极限吸收特征、吸收线宽低至25 MHz的低压下氰化甲烷(CH3CN)分子的光谱学测量,系统具有的高光谱分辨性能得到了验证。这是首次使用单腔双光梳光纤激光器测量到如此窄的吸收线宽。这一兼具高光谱分辨率和低系统复杂度的THz-DCS的极致实现形式将大大降低实际使用的门槛,推进THz光谱学技术在气体分析等应用中的实际使用。

在(a)0.2~0.72 THz和(b)约0.3310 THz时,总压为360 Pa的CH3CN和空气的模式分辨吸收光谱。(c)(1-6)在430 Pa、330 Pa、280 Pa、256 Pa、149 Pa和115 Pa下CH3CN在0.331 THz附近的模式分辨吸收特性。

Ultimate form of THz spectroscopy featuring both high performance and wide versatility

Terahertz (THz) spectroscopy (frequency range 0.1–10 THz, wavelength range 30–3000 μm) can provide unique and crucial information in a spectral region less covered by the more mature optical and microwave technologies. However, despite of its developments since 1980's, a form of THz spectroscopy that can overcome the performance bottleneck with an affordable platform still remains elusive to unlock more real-world applications.

For example, one interesting application of gas analysis is in monitoring of gas molecules mixed with unwanted aerosols, such as smoke, soot, dust, mist, fog, haze, and fume. For example, dynamic analysis of combustion gas is required to realize the efficiency of combustion process from viewpoints of global environment and saving energy. On the other hand, real-time sensing of hazardous, toxic, or flammable gas in the smoke is important to avoid the secondary disaster in the fire accident. Against this backdrop, there is a strong demand for methods that allow dynamic analysis of molecular gases without the influence of unwanted aerosols. Although gas chromatography has high sensitivity, the measurement is not real time and it has many limitations due to the capture technology of unwanted aerosols used in sample preprocessing. On the other hand, infrared absorption spectroscopy is rapid, but the influence of light scattering by aerosols reduces the analysis performance. Therefore, a technology that allows the target gas in aerosols to be analyzed quickly and with high precision without the need for sample preprocessing is highly desirable.

Terahertz region is a characteristic frequency band in which many absorption lines due to rotational transitions of polar gas molecules appear. Instead of the intermolecular vibration spectrum observed in the infrared region, if the molecular rotational transition spectrum observed in the THz region could be used, high selectivity and high sensitivity would be expected. In addition, from the relationship between the wavelength of THz radiation and the size of minute particles, there is less susceptibility to optical scattering by aerosols. Therefore, under conditions in which aerosols are mixed with the gas to be analyzed, THz spectroscopy is considered to be a useful technique for simultaneously analyzing the target gas molecule species in a straightforward and rapid manner.

In order to discriminate the target gas, a spectroscopic technique employed must have high spectral resolution, high spectral accuracy, and a broad spectral coverage in the THz region. While conventional THz spectroscopic techniques could not meet all requirements, THz dual-comb spectroscopy (THz-DCS) can achieve all of high accuracy, high resolution, broad spectral coverage, and rapid data acquisition. However, the practical use of THz-DCS is still hampered by the need for a pair of repetition-rate-stabilized optical frequency comb sources with expensiveness and complexity. Recently, a dual-comb fiber laser has appeared as a new mode to reduce the expensiveness and complexity due to a single laser without the need for stabilization control. However, when the dual-comb fiber laser is used for THz-DCS, the residual timing jitter of the laser hampers high spectroscopic performance in THz-DCS.

Prof. Yasui's group at Tokushima University, Japan collaborating with Prof. Zheng's group at Beihang University combined the dual-comb fiber laser with the adaptive sampling method in THz-DCS because the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter. The related research results are published in Advanced Photonics, Vol. 2, Issue 3, 2020 (Jie Chen, Kazuki Nitta, Xin Zhao, et al. Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser[J]. Advanced Photonics, 2020, 2(3): 036004).

The achieved performance is highlighted by low-pressure spectroscopy of acetonitrile gas with Doppler-limit-approaching absorption features with linewidth down to 25 MHz. It is the first time to observe such narrow absorption linewidth with a dual-comb fiber laser. This is the ultimate form of THz-DCS for high spectroscopic performance and largely-reduced system complexity, lowering the barriers for practical use and accelerating real-world application such as gas analysis.

Mode-resolved absorption spectra of CH3CN and air with a total pressure of 360 Pa within the frequency range of (a) 0.2 to 0.72 THz and (b) around 0.3310 THz. (c) (1-6) Mode-resolved absorption characterization of CH3CN around 0.331 THz at 430 Pa, 330 Pa, 280 Pa, 256 Pa, 149 Pa, and 115 Pa.