Frequency Division Multiplexing and Normalized Harmonic Method for WMS Detection System in Multi-gas Detection

A frequency division multiplexing (FDM) WMS system for multi-gas detection is reported in this paper. The distributed feedback lasers with different wavelengths adopt different modulation frequencies to achieve multi-components gas detection by the FDM technology. Combining optical path extension, normalized harmonic detection and wavelength modulation spectroscopy technology, the highly self-made system realized high sensitivity, high precision and short response time. The performance of this system was evaluated through the mixed gas of methane and hydrogen chloride. Experimental results show great linear response between gas concentration and normalized second harmonic (2f/1f) signal, the minimum detection limit (MDL) of the system is calculated to be 30.16 ppb for CH4 and 31.6 ppb for HCl with the integration time of 15 s. The MDL could be further enhanced to 11 ppb for CH4 and 13.1 ppb for HCl with an extended integration time. The maximum relative error is 2.16% and 3.19% for CH4 and HCl respectively.