Application of TDM and FDM methods in TDLAS based multi-gas detection

Multi-gas tunable diode laser absorption spectroscopy (TDLAS) system based on time division multiplexing (TDM) demodulation technology which combine with frequency division multiplexing (FDM) technology is proposed, called F-TDM (frequency- time division multiplexing) system. In addition, traditional TDM technology and FDM technology are introduced to analyze and compare their performance. As for the performance of F-TDM system, multi-gas detection of CH 4 and C 2 H 2 has been accomplished with minimum detection limits of 10.24 ppmv for CH 4 at 1653.72 nm and 0.763 ppmv for C 2 H 2 at 1532.83 nm respectively. The relationships (CH 4 : R-square = 0.9989; C 2 H 2 : R-square = 0.9995) between gas concentration and second harmonic signal amplitude are proved as a good linear response. It is consistent with the data obtained by traditional TDM and FDM demodulation methods, indicating that the F-TDM system is feasible. By comparison, it is known that different methods have their advantages in different applications. The TDM system uses less equipment and is the most cost-effective. The system based on FDM technology is the most time-saving. The F-TDM system is a compromise method that combines the advantages of both and has no obvious disadvantage at the detection limit.