Calibration-free measurement of absolute gas concentration and temperature via light-induced thermoelastic spectroscopy

The calibration process has always been an inevitable key step of light-induced thermoelastic spectroscopy (LITES) technology, which is also the current primary bottleneck hindering the widespread application. The complexity, high cost, and environmental sensitivity of such a process severely limit the sensing system’s ability to perform long-term, stable, and accurate measurements of the target. We propose a breakthrough calibration-free LITES sensing technique. The core innovation lies in the first-time utilization of quartz thermoelastic effect as the sensing mechanism, combined with full-range light intensity modulation to acquire the complete target gas absorption spectrum, from which absolute gas concentration and temperature values are directly extracted, eliminating the need for calibration. The concentration and temperature in this work were measured separately, among which, the temperature was retrieved by targeting two gas absorption lines. Theoretical research with experimental studies on gas concentration and temperature sensing was conducted. In terms of gas sensing, this technology can achieve the inversion of the different absolute gas concentrations without a calibration process using a quartz tuning fork (QTF). In terms of temperature measurement, this technology successfully achieves simultaneous inversion of two gas absorption lines with a single QTF and obtains the temperature field information. The experimental results also verified that this technology can accurately detect the target even when the laser power, QTF characteristics, and the optical path of the system change, providing an effective technical proposal for the long-term, stable, and accurate measurement of the analyte. Furthermore, it also offers the advantages of small size and wavelength independence, which make it well suited for miniaturization and extending sensing to mid-infrared or far-infrared wavebands.