Abstract
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.
Highlights
Broadband terahertz spectroscopy enables many promising applications in science and industry alike
Compact femtosecond lasers and the use of devices and components, which were originally developed for fiber-based telecommunications, enabled the development of table-top THz time-domain spectroscopy (TDS) systems, the best of which attain a bandwidth of 6.5 THz and peak dynamic range (DR) of more than 100 dB15–17
We demonstrated that the optoelectronic frequency-modulated continuous-wave (FMCW) THz system can be used for broadband terahertz spectroscopy with high DR
Summary
Broadband terahertz spectroscopy enables many promising applications in science and industry alike. The receiver current is inherently phasemodulated 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. Owing to the SDSH technique, an active phase or amplitude modulation—the standard technique for coherent cw THz measurements so far—is no longer required This technique can be regarded as an optoelectronic analog of frequency-modulated continuous-wave (FMCW) radar[27,28]. Our THz spectrometer achieves a bandwidth of 4 THz, a peak DR of 117 dB, and a measurement speed of 200 Hz, i.e., a performance comparable to that of state-of-the-art THz TDS systems, yet without their complexity With this spectrometer, we demonstrate broadband spectroscopy and highly accurate NDT, such as contract-free multilayer thickness measurements
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