Carbon Dioxide Real-time Tracking Exploiting Light-Induced Thermoelastic Spectroscopy

In this work, the detection of 330 parts-permillion (ppm) carbon dioxide (CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) concentration in indoor environments and its real-time monitoring were carried out exploiting the Light-Induced Thermo-Elastic Spectroscopy (LITES) technique by means of (i) an Interband Cascade Laser (ICL) having a central emission wavelength of 4,234 nm (2,361.83 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>); (ii) a custom T-shaped Quartz Tuning Fork (QTF) as photo-thermo-elastic detector; and (iii) a Gradient-INdex (GRIN) lens used for focusing the laser beam on the QTF surface. Considering a free-space optical path length of 30 cm (11.811 in.) for interaction between ICL radiation and CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> molecules, the developed sensing system achieved a Signal-to-Noise Ratio (SNR) over 1,000 and a Minimum Detection Limit (MDL) of almost 300 parts-per-billion (ppb) with a lock-in amplifier integration time constant of 0.1 s.