Liebermeister Lars, Nellen Simon, Kohlhaas Robert B, Lauck Sebastian, Deumer Milan, Breuer Steffen, Schell Martin, Globisch Björn
Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Berlin, Germany.
Institut für Festkörperphysik, Technische Universität Berlin, Berlin, Germany.
Nat Commun. 2021 Feb 16;12(1):1071. doi: 10.1038/s41467-021-21260-x.
Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.
宽带太赫兹光谱技术在科学和工业领域都有着许多前景广阔的应用。然而，现有太赫兹系统的复杂性至今仍阻碍着这项技术的突破。特别是，成熟的太赫兹时域光谱（TDS）方案依赖于复杂的飞秒激光器和光学延迟线。在此，我们提出一种用于光电调频连续波（FMCW）太赫兹传感的方法，这是一种用于宽带光谱分析和工业无损检测的强大工具。在我们的方法中，一个扫频光拍信号产生太赫兹场，然后利用同一拍信号的延时副本通过光混频对其进行相干检测。因此，接收器电流在无需额外调制器的情况下固有地进行相位调制。由于这项技术，我们的宽带太赫兹光谱仪（测量速率为200 Hz，或带宽为4 THz且平均时峰值动态范围为117 dB）与最先进的太赫兹TDS系统性能相当，但复杂度显著降低。对层厚低至23 µm的多层介电样品进行厚度测量，显示出其在实际应用中的潜力。在仅0.2 s的测量时间内，实现了小于2%的不确定度，这是连续波太赫兹光谱分析所报道的最高精度。因此，光电FMCW方法为结合光纤和光子集成技术的宽带紧凑型太赫兹光谱仪铺平了道路。
