Integrated near-infrared QEPAS sensor based on a 28 kHz quartz tuning fork for online monitoring of CO(2) in the greenhouse

In this paper, a highly sensitive and integrated near-infrared CO(2) sensor was developed based on quartz-enhanced photoacoustic spectroscopy (QEPAS). Unlike traditional QEPAS, a novel pilot line manufactured quartz tuning fork (QTF) with a resonance frequency f(0) of 28 kHz was employed as an acoustic wave transducer. A near-infrared DFB laser diode emitting at 2004 nm was employed as the excitation light source for CO(2) detection. An integrated near-infrared QEPAS module was designed and manufactured. The QTF, acoustic micro resonator (AmR), gas cell, and laser fiber are integrated, resulting in a super compact acoustic detection module (ADM). Compared to a traditional 32 kHz QTF, the QEPAS signal amplitude increased by > 2 times by the integrated QEPAS module based on a 28 kHz QTF. At atmospheric pressure, a 5.4 ppm detection limit at a CO(2) absorption line of 4991.25 cm(−1) was achieved with an integration time of 1 s. The long-term performance and stability of the CO(2) sensor system were investigated using Allan variance analysis. Finally, the minimum detection limit (MDL) was improved to 0.7 ppm when the integration time was 125 s. A portable CO(2) sensor system based on QEPAS was developed for 24 h continuous monitoring of CO(2) in the greenhouse located in Guangzhou city. The CO(2) concentration variations were clearly observed during day and night. Photosynthesis and respiration plants can be further researched by the portable CO(2) sensor system.