Abstract-Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

 Lars Liebermeister, Simon Nellen, Robert B. Kohlhaas, Sebastian Lauck, Milan Deumer, Steffen Breuer, Martin Schell, Björn Globisch 

https://www.nature.com/articles/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.

Abstract-Continuous-wave terahertz spectrometer without active phase modulation (Conference Presentation)

Björn Globisch, Lars Liebermeister, Simon Nellen;,  Robert B. Kohlhaas,  Martin Schell

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11279/112790U/Continuous-wave-terahertz-spectrometer-without-active-phase-modulation-Conference-Presentation/10.1117/12.2545856.short?SSO=1

We present a novel system architecture for coherent cw THz spectrometers. The system features 2.5 THz bandwidth with an acquisition rate of 58 Hz and provides full phase information without active phase modulation. We achieve passive amplitude and phase modulation at a fixed intermediate frequency by using a fast sweeping laser in combination with a static optical fiber delay: By heterodyning the incoming THz signal with the frequency-shifted optical beatnote on a photomixing receiver, we can extract amplitude and phase of the signal with a lock-in detector. To the best of our knowledge, this is the fastest coherent cw THz system demonstrated so far.

Abstract-Ultra-fast, High-Bandwidth Coherent cw THz Spectrometer for Non-destructive Testing

Lars Liebermeister, Simon Nellen, Robert Kohlhaas Steffen Breuer,  Martin Schell, Björn Globisch

https://link.springer.com/article/10.1007/s10762-018-0563-6

Continuous wave THz (cw THz) systems define the state-of-the-art in terms of spectral resolution in THz spectroscopy. Hitherto, acquisition of broadband spectra in a cw THz system was always connected with slow operation. Therefore, high update rate applications like inline process monitoring and non-destructive testing are served by time domain spectroscopy (TDS) systems. However, no fundamental restriction prevents cw THz technology from achieving faster update rates and be competitive in this field. In this paper, we present a fully fiber-coupled cw THz spectrometer. Its sweep speed is two orders of magnitude higher compared to commercial state-of-the-art systems and reaches a record performance of 24 spectra per second with a bandwidth of more than 2 THz. In the single-shot mode, the same system reaches a peak dynamic range of 67 dB and exceeds a value of 100 dB with averaging of 7 min, which is among the highest values ever reported. The frequency steps can be as low as 40 MHz. Due to the fully homodyne detection, each spectrum contains full amplitude and phase information. This demonstration of THz-spectroscopy at video-rate is an essential step towards applying cw THz systems in non-destructive, in line testing.