A quartz enhanced photo-acoustic gas sensor based on a custom tuning fork and a terahertz quantum cascade laser

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) is described. The QTF dimensions are 3.3 cm 0.4 cm 0.8 cm, with the two prongs spaced by 800 mm. To test our sensor we used a quantum cascade laser as the light source and selected a methanol rotational absorption line at 131.054 cm1 (3.93 THz), with line-strength S ¼ 4.28 1021 cm mol1. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q-factor was 74 760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2 1010 W cm1 Hz1/2, comparable with the best result of mid-IR QEPAS systems.