A new publication from Opto-Electronic Advances, 10.29026/oea.2024.230230 discusses a high-sensitivity LITES-based gas sensor empowered by a novel quartz tuning fork and a multi-pass cell with dense spots pattern.
Trace gases are gases with a volume fraction much less than 1%, which are not very abundant but have a huge impact on many fields. For example, with the rapid development of industrialization and urbanization, atmospheric pollution has become one of the serious problems faced by the world. Harmful gases such as nitrogen oxides (NOx) and sulfur dioxide (SO2), as well as greenhouse gases such as ozone (O3), are typically in the range of 10-12-10-6 in volume fraction. An increase in their concentration will directly lead to ecological changes. In addition, trace gas detection has very important research and application value in industrial production, medical diagnosis and fire early warning. The development of new gas sensing technology will create a safer, healthier and more sustainable living environment for human beings.
Laser-induced thermoelastic spectroscopy (LITES) is an emerging gas sensing technology in recent years, which was first proposed by the team of Yufei Ma of Harbin Institute of Technology in 2018. A quartz tuning fork (QTF) is used as a detector to acquire absorption signals. When a modulated laser is absorbed by the target gas in a gas chamber, the transmitted laser will carry the concentration information of the gas. After absorbing the energy of this part of the laser, the QTF will perform a signal conversion process from light to heat and then to electricity. By demodulating the electrical signal, the concentration information of the gas can be obtained. LITES has many advantages such as high selectivity, high sensitivity, fast response and providing non-invasive measurements. In addition, the detector based on thermoelastic effect of QTFs has a great response bandwidth, which overcomes the shortcomings of traditional photodetectors that have a narrow response bandwidth. Therefore, LITES may be used for full-band spectral detection of gases.
In recent years, LITES-based gas sensing technology has developed very rapidly. With in-depth research, LITES has been proved to be a stable and sensitive gas sensing technology. Optimizing the structure of the innovative system and improving the detection sensitivity is the way to promote the further development of LITES technology.
The authors of this article have optimized and innovated the structure of the LITES-based gas sensing system. The system was upgraded by increasing laser power, enhancing gas absorption range, and using a novel low-frequency QTF detector, which realized the high sensitivity measurement of the target gas.
An erbium-doped fiber amplifier (EDFA) was employed to amplify the optical power so that the signal level was further enhanced. A multi-pass cell (MPC) with dense spots pattern was used as a gas chamber, which was able to provide an effective optical path length (OPL) of 37.7 m and an optical path length to volume (RLV) of 13.8 cm-2. The spots pattern of the MPC is shown in Fig. 1. A commercial QTF with a frequency of ~32 kHz and a self-designed QTF with trapezoidal tips were used as detectors, respectively. The photograph and frequency response curve of the QTFs are shown in Fig. 2. The unique structure of the trapezoidal-tipped QTF results in a resonance frequency of only 9641.83 Hz, which gives a longer energy accumulation time and produces a stronger signal in the LITES system.
Fig. 3 shows the concentration test results of the C2H2-LITES sensor, and the system has an excellent concentration linear response. The detection sensitivity of the system is only 24.6 ppb when using trapezoidal-tipped QTF as the detector, and the detection performance is ~2 times higher than that of using commercial QTF. Meanwhile, according to the results of Allan deviation, the detection sensitivity can be further improved to 1.29 ppb when the averaging time was 140 s.
This work optimizes the LITES-based gas sensing system in three dimensions: excitation, absorption, and detection, resulting in a significant improvement in detection performance. The detection performance of this system can be further enhanced by designing MPCs with a larger RLV and adopting laser with better output beam quality. In addition, research on the design of new QTFs with low resonance frequency and high Q-factor can further promote the development of LITES technology.
Keywords: light-induced thermoelectric spectroscopy / quartz tuning fork / multi-pass cell / gas sensing
