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

Abstract

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 μm. To test our sensor we used a quantum cascade laser as the 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⁻¹. 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 × 10⁻¹⁰ W cm⁻¹ Hz^−1/2, comparable with the best result of mid-IR QEPAS systems.