Numerical removal of water vapour effects from terahertz time-domain spectroscopy measurements

Abstract

The use of T-rays, or terahertz radiation, to identify substances by their spectroscopic fingerprints is a rapidly moving field. The dominant approach is presently terahertz timedomain spectroscopy. However, a key problem is that ambient water vapour is ubiquitous and the consequent water absorption distorts the T-ray pulses. Water molecules in the gas phase selectively absorb incident T-rays at discrete frequencies corresponding to their molecular rotational transitions. When T-rays propagate through an atmosphere, this results in prominent resonances spread over the T-ray spectrum; furthermore, in the time domain, fluctuations after the main pulse are observed in the T-ray signal. These effects are generally undesired, since they may mask critical spectroscopic data. So, ambient water vapour is commonly removed from the T-ray path by using a closed chamber during the measurement. Yet, in some applications, a closed chamber is not always feasible. This situation, therefore, motivates the need for an optional alternative method for reducing these unwanted artefacts. This paper represents a study on a computational means that is a step towards addressing the problem arising from water vapour absorption over a moderate propagation distance. Initially, the complex frequency response of water vapour is modelled from a spectroscopic catalogue. Using a deconvolution technique, together with fine tuning of the strength of each resonance, parts of the water vapour response are removed from a measured T-ray signal, with minimal signal distortion, thus providing experimental validation of the technique.

High effective THz-TDS method for the detection and identification of substances in real conditions

“…), the packaging material can significantly attenuate and distort the spectrum of the measured THz signal [7], [8]. The inhomogeneity of the substance surface and high humidity during the measurements also decrease the efficiency of this method [6], [9], [17].…”

“…THz spectroscopy and imaging were actively used for this purpose during the past twenty years in the wide area of security applications [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. At the same time, this technique starts to be widely used for the non-destructive testing, for quality inspection of pharmaceuticals and food products as well as for medical applications [20][21][22][23][24][25][26][27].…”

Differentiation of the Coordination Chemistry of Metal Chlorides in Catalytic Conversion of Glucose in Ionic Liquids

“…FIR offers great advantages in the characterization of those inorganic compounds [51][52][53][54], as well as coordination complexes [55], lattice vibrations [56,57], hydrogen bonds [58,59], and the skeleton vibration of ring molecule [58], due to the lower energy in the FIR than the MIR spectral region. Just as with MIR, FIR analysis also suffers from the interference of water vapor in the air especially in low frequency [60,61]. In order to reduce the interference of water, several methods may be adopted [60], such as background subtraction, reduction in moisture level by purging with dry inert gases, resolution reduction, and isolation of the workplace from the air with a physical barrier.…”

“…Just as with MIR, FIR analysis also suffers from the interference of water vapor in the air especially in low frequency [60,61]. In order to reduce the interference of water, several methods may be adopted [60], such as background subtraction, reduction in moisture level by purging with dry inert gases, resolution reduction, and isolation of the workplace from the air with a physical barrier. In addition, the spectral information of far infrared below 200 cm −1 may be neglected as there is little meaningful absorption by most compounds of interest, but the absorption of water is very intense in this region [55].…”

Low Dimensional Embedding of Climate Data for Radio Astronomical Site Testing in the Colombian Andes

“…Another recent surge in interest in technology at this frequency range has appeared recently due to the saturation of the "classical radio window" in telecommunications (Hosako et al 2007). For this reason, atmospheric models for absorption of THz photons (Rosenkranz 1998, Slocum et al 2013 and artifact removal models (Withayachumnankul et al 2008) have been developed. Countries near the Equator face a challenge in using this frequency band for astronomical observations and telecommunications using terabit satellite links (Suen 2016) due to the presence of a tropical belt of dense water vapor which efficiently absorbs THz radiation (Seidel et al 2008).…”