Integrated near-infrared QEPAS sensor based on a 28 kHz quartz tuning fork for online monitoring of CO2 in the greenhouse

Abstract

In this paper, a highly sensitive and integrated near-infrared CO₂ 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 CO₂ 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 CO₂ absorption line of 4991.25 cm^-1 was achieved with an integration time of 1 s. The long-term performance and stability of the CO₂ 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 CO₂ sensor system based on QEPAS was developed for 24 h continuous monitoring of CO₂ in the greenhouse located in Guangzhou city. The CO₂ concentration variations were clearly observed during day and night. Photosynthesis and respiration plants can be further researched by the portable CO₂ sensor system.

Keywords: Photoacoustic spectroscopy; Quartz enhanced photoacoustic spectroscopy; Quartz tuning fork; Trace gas sensing.

Graphical abstract

Fig. 1

(a) Diagram of traditional QEPAS ADM. (b) Diagram of Integrated QEPAS ADM in this work, only optical part. (c) Front view of integrated ADM. (d) Back view of integrated ADM.

Fig. 2

(a) Schematic diagram of CO₂ gas sensor system based on QEPAS (b) Top view of the ADM (c) Side view of the ADM.

Fig. 3

Frequency response curve of custom 28 kHz QTF.

Fig. 4

The 2 f QEPAS signals under dry and wet conditions.

Fig. 5

The QEPAS 2 f signals for 32 kHz QTF and 28 kHz QTF.

Fig. 6

(a) QEPAS signal amplitude of different CO₂concentrations. (b) Linear fitting for different CO₂ concentrations.

Fig. 7

Allan variance analysis for the CO₂ sensor system based on QEPAS.

Fig. 8

(a) Location of the sensor. (b) The photograph of the greenhouse. (c) The picture of the CO₂ sensor system (d) The real-time concentration of CO₂ in the greenhouse in 24 h.