Fig. 1. Schematic diagram of LITES measurement system

Fig. 2. Frequency response curve of QTF

Fig. 3. Variation of modulation currents with 2f signal amplitudes

Fig. 4. （a） Nonlinear background and its polynomial fitting under purity N₂ condition and （b） fitting residuals

Fig. 5. （a）2f signal of 100×10^-6 CH₄ and polynomial fitting of its nonlinear background； （b） 2f signal with deduction of nonlinear background

Fig. 6. （a） 2f signals under different light intensities； （b） Amplitudes of nonlinear background under different light intensities； （c） Amplitudes of normalized 2f signals under different light intensities

Fig. 7. （a） 2f signals under different concentrations； （b） Concentration calibration curve； （c） Evaluation of detection limit