Gas sensing using terahertz time-domain spectroscopy

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. Spectroscopic methods for the sensing and identification of gases have shown great promise, owing to their inherent non-invasive nature, relative simplicity, and high selectivity. The vast majority of the work in this area has relied on the “fingerprint” absorption in the mid-infrared (λ≈ 2–20 μm), where molecular vibrations often provide a unique signature. Both incoherent (for example, Fourier-transform infrared, FTIR) [1] and laser-based coherent sources [2] have been commonly employed. These have demonstrated sensitive detection of many gases including greenhouse gases such as CO, CO2, and CH4, chemical etchants such as HCl and HF, and common smokestack pollutants such as SO2 and N2O. Despite these promising results, significant challenges remain, particularly in expanding the range of gases amenable to spectroscopic detection. In contrast to mid-infrared gas sensing, the use of farinfrared or terahertz (1 THz= 1012 Hz) radiation for sensing purposes is a field in relative infancy. In this frequency range, from λ≈ 3 mm to 50 μm (corresponding to frequencies between 0.1 and 6 THz), many polar molecules exhibit unique spectral signatures arising from transitions between rotational quantum levels. The use of these absorption signatures for detection or identification of gases is very much complementary to the more well-established mid-IR techniques, and will greatly expand the number of gas species that can be detected via laser-based methods. Yet, the development of THz sensing tools has been hindered in large part due to the lack of suitable radiation sources and detectors for use in the farinfrared. Within the last decade, a number of new approaches to the generation and detection of THz radiation have been pursued with increasing interest. These techniques, based on frequency conversion using nonlinear optics [3–8], are often simpler, more reliable, and potentially much less expensive than the more traditional approaches such as molecular vapor lasers, free electron lasers, and synchrotrons. Although many of these nonlinear optical techniques were pioneered in the late 1960s or early 1970s [9], it is only relatively recently that it has become plausible to consider “real-world” applications based on these THz technologies. This excitement has been spurred in part by a number of important advances. First, a range of new fabrication techniques have been developed for nonlinear optical materials, such as the low-temperature growth of semiconductors and periodic poling. Second, there have been substantial improvements in the reliability, stability, size, and cost of the lasers required for these devices. The advances in the ultrafast pulsed laser systems are particularly notable, with the cumbersome argon-ion-pumped Kerr-lensmode-locked Ti:sapphire lasers giving way to more compact and less costly all-solid-state systems [10] or to mode-locked fiber lasers [11]. In this paper, we present a review of recent results in automated real-time gas sensing involving one of the more promising of these new THz techniques, known as terahertz time-domain spectroscopy, or THz-TDS. The THz-TDS system relies on the use of femtosecond laser pulses for the generation and detection of THz radiation, and has been a beneficiary of the rapid progress in this enabling technology. Using THz-TDS, one can obtain a rapid spectral measurement over a very large bandwidth in the terahertz range, where many gases exhibit “fingerprint” absorption spectra