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Tunable diode laser absorption spectroscopy (TDLAS) combined with wavelength modulation spectroscopy (WMS) is extremely important for trace-gas detection in a wide range of fields.In circuit design, lock-in amplifiers (LIAs) are employed to extract extremely weak signals.Derivative of the absorption function can be obtained by Fourier decomposition.Gas concentration has a relationship with the amplitude of harmonic signals,among which the amplitude of first-harmonic signal is the highest.Therefore first-harmonic detection is employed to optimize signal-to-noise ratio (SNR) in our research.The experiment results agree well with theoretical analysis.In addition, good reliability and good linearity with gas concentration have been shown in the experimental results.