Gas sensing using terahertz time-domain spectroscopy

Abstract

A method for detection and identification of polar gases and gas mixtures based on the technique of terahertz time-domain spectroscopy is presented. This relatively new technology promises to be the first portable far-infrared spectrometer, providing a means for real-time spectroscopic measurements over a broad bandwidth up to several THz. The measured time-domain waveforms can be efficiently parameterized using standard tools from signal processing, including procedures developed for speech recognition applications. These are generally more efficient than conventional methods based on Fourier analysis, and are easier to implement in a real-time sensing system. Preliminary results of real-time gas mixture analysis using a linear predictive coding algorithm are presented. A number of possible avenues for improved signal processing schemes are discussed. In particular, the utility of a wavelet-based signal analysis for tasks such as denoising is demonstrated.

Compound Explosives Detection and Component Analysis via Terahertz Time-Domain Spectroscopy

“…Therefore, THz pulses, localized in both time and frequency, seem to be naturally suited to wavelet-based signal processing methods [19]. The defining equation of wavelet transform is presented as…”

“…Especially, signal processing of THz data based on wavelet transform is known to enhance signal quality without losing the essential information as compared to conventional filtering or smoothing algorithms [18]. Although research on utilizing wavelets in THz data post processing for gas sensing [19], and imaging [20,21] have formerly been conducted, practical application on compound plastic explosives have rarely been reported, therefore they are still at an introductory level. Moreover, analyzing the 'recipe' of plastic explosives is known to help determine where the explosives were originally produced [22].…”

Uncertainty and Ambiguity in Terahertz Parameter Extraction and Data Analysis

“…Developed in the 1980s and 1990s [1,2], THz time-domain spectroscopy (THz TDS) has been established as a general spectroscopic technique to study liquids [3], solids [4], gases [5], biomolecules [6,7], aqueous salt solutions [8], nanoparticles [9], semiconductors [10] and plasmas [11]. The covered frequency bandwidth ranges from 3 cm −1 to 140 cm −1 (0.1 THz to 4 THz).…”

Terahertz Frequency Spreading Filter via One-dimensional Dielectric Multilayer Structures

“…Among the recent discoveries in THz science and technology, one can easily find many future applications in communications, material characterizations, biological and medical imaging, and precision spectroscopy of molecules [2]. Also, along with the commercial development of femtosecond lasers, THz time-domain spectroscopy (THz-TDS) has been widely adopted, in particular, for the measurement of absorption and dispersion of materials in the THz frequency range [3][4][5][6][7]. However, a THz pulse is extremely broadband in frequency and its usage beyond simple spectroscopy and imaging can be also easily found in its applications, especially with THz waves in designed waveforms.…”