Comparison of preprocessing technique of second harmonic detection with tunable diode laser

Tunable diode laser absorption spectroscopy combined with harmonic detection has been extensively used to measure trace-gas. However, some problems are inevitable, such as background absorption and residual amplitude modulation. Subtraction and division are two common preprocessing techniques used before harmonic extraction in gas detection. The performances of two preprocessing techniques are discussed from a problem solving perspective. Study results demonstrated that both of the two preprocessing techniques have the abilities of eliminating the background absorption caused by air which is sealed in the gas gap of the optical components. However, the division method has a greater advantage of eliminating residual amplitude modulation. The experimental results give good agreement with the theoretical analysis.