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

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

doi:10.1007/s10762-018-0563-6

Lars Liebermeister, Simon Nellen + Show 4 more

Open Access

https://doi.org/10.1007/s10762-018-0563-6

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Abstract

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.

Highlights

Terahertz technology is pushing forward strongly into industrial applications
Previous demonstrations of optoelectronic continuous-wave THz-spectroscopy (cw THz) systems aiming at high update rates and broadband THz-spectra (> 1 THz bandwidth), commercial and scientific, can be categorized by the technology and tuning mechanism used by the laser source
An ultrafast cw THz spectrometer is presented showing homodyne acquisition of broadband cw THz spectra with high update rate

Summary

Introduction

Terahertz technology is pushing forward strongly into industrial applications. This is due to its extraordinary features: Contact-free determination of multi-layer thicknesses of coatings [1,2,3] as well as sensing for thickness variations or defects in polymers, foams, and other non-conductive materials [4]. Previous demonstrations of optoelectronic cw THz systems aiming at high update rates and broadband THz-spectra (> 1 THz bandwidth), commercial and scientific, can be categorized by the technology and tuning mechanism used by the laser source. The relative phase of the optical beat signal at emitter and receiver is modulated, enabling instant phase sensitive, coherent detection [19] This gives a very robust and alignment-free system, which fits into a standard desktop case (see Fig. 1 b)

Acquisition Scheme

System Performance

Conclusion