Terahertz quartz enhanced photo-acoustic gas sensor employing a quantum cascade laser source

摘要

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) will be described. The QTF dimensions are 3.3 cm × 0.4 cm × 0.8 cm, with the two prongs spaced by ～ 800 μm. To test our sensor we used a quantum cascade laser as light source and selected a methanol rotational absorption line at 131.054 cm~(-1) (～3.93 THz), with line-strength S = 4.28·10~(-21) cm/mol. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q factor was 74760. 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 2·10~(-10) W·cm~(-1)·Hz~(-1/2), comparable with the best result of mid-IR QEPAS systems.