Trace gas detection based on dual-QTFs laser-induced thermoelastic spectroscopy

Abstract

A laser-induced thermoelastic spectroscopy (LITES) gas detection structure based on dual-quartz tuning forks (dual-QTFs) is developed for trace gas detection. In order to obtain the best response of the sensor, two QTFs with a similar resonance frequency were selected as photodetectors. Due to the reflection characteristic of the metal layer on the surface of the QTF, the reuse of laser energy was achieved by using two QTFs. By measuring the signal response and reflection effect of the front surface of QTF, the optimal incident position of the laser beam is determined. The laser beam injected on the surface of QTF1 was used to excite the LITES signal, and part of the laser beam was reflected back to the surface of QTF2 through the metal layer for exciting the LITES signal again. The signals generated by QTF1 and QTF2 were superimposed by an adder to realize the enhancement of the LITES signal. Choosing acetylene (C₂H₂) as the target analyte verified the detection performance of the sensor. At the C₂H₂ concentration of 1000 ppmv, the LITES signal amplitude of the dual-QTFs was 1641 µV, which had a 2.21 times improvement compared to QTF1. The long-term stability was evaluated by using the Allan variance analysis method; a minimum detection limit of 53 ppbv was achieved with an optimal integration time of 91 s. The corresponding to a normalized noise equivalent absorption coefficient of 7.59 × 10⁻⁹ cm⁻¹∙W∙Hz^−1/2.