Off-axis integrated cavity output spectroscopy for real-time methane measurements with an integrated wavelength-tunable light source

Abstract

Off-axis integrated cavity output spectroscopy (OA-ICOS) is a sensitive technique for trace gas sensing. A wavelength modulated off-axis integrated cavity output spectroscopy (WM-OA-ICOS) based CH4 detection system was presented in this paper. An integrated near-infrared (NIR) light source consisting of a digital temperature controller, a current modulator and a 1.65 μm distributed feedback (DFB) laser was developed. A 50.8 mm long assembly Fabry–Perot (F-P) cavity was designed and used as the gas chamber. Wavelength modulation spectroscopy (WMS) was employed to improve the signal to noise ratio (SNR) of the system and a LabVIEW-based orthogonal lock-in amplifier was developed to extract the second harmonic (2f) signal. The experimental values of the optimum modulation parameters were determined and CH4 measurements were carried out to demonstrate the effectiveness and sensitivity of the sensor system. A quantitative characterization of long-term stability and detection sensitivity is presented. Based on Allan variance analysis, the limit of detection is 10.5 ppm when the averaging time is 30 s which can be improved easily by changing the cavity lens with higher reflectivity or increase the distance between the cavity mirrors. The sensor system was insensitive to optical alignment and can be easily serviced on site. Furthermore, this system can be further integrated by using self-developed lock-in amplifier based on our previous work, which enables the system to have good prospects in industrial field.