Simultaneous detection of atmospheric CO and CH₄ based on TDLAS using a single 2.3 μm DFB laser

A laser-based spectrometer with a physical size of 60× 30 ×25 cm³ has been developed to continuously monitor CO and CH₄ in atmosphere based on tunable diode laser absorption spectroscopy (TDLAS). Two neighboring lines of CO and CH₄ around 2.3 μm were selected as candidates for simultaneous measurement by a single diode distributed feedback (DFB) laser. A special Herriott-type multipass absorption cell, with a 72 m optical path length, was designed and used to enhance the absorption signals of sample gases. Normalized wavelength modulation spectroscopy was applied to improve the sensitivity and robustness of the spectrometer and it was implemented on a home-made electronic system based on field programmable gate array (FPGA). Meanwhile, the electronic system controlled the temperature and current of DFB laser with the precision of 0.01 °C and 40 ppm. The 2nd-harmonic signals normalized by the corresponding 1st-harmonic signals for both CO and CH₄ are of high linear response to their concentrations in the range of 0.046-4.6 ppm and 0.487-48.7 ppm, respectively. According to the Allan variance, respective minimum detection limits for CO and CH₄ are 0.73 ppb and 36 ppb at 122 s and 137 s. As an application example, the spectrometer has been validated through real-time and in-situ measurement of atmospheric CO and CH₄ for 48 h.