Figure 3. Simulation and experimental results of BF-QEPAS.

(a–c) First harmonic QTF output signal for different modulation frequencies and wavelength-scanning rates were simulated by MATLAB software with actual parameters of the QTF system. The different wavelength-scanning rates can be simulated by changing the value of t_a as this parameter represents the action time of the acoustic force to the QTF. The value of t_a was estimated by using the ratio of the absorption line width to the wavelength scanning rate. (d–f) The corresponding tests were carried out with 2.5% water vapour at room temperature and atmosphere pressure. The wavelength was scanned at a rate of 0.12 cm⁻¹ s⁻¹, 3 cm⁻¹ s⁻¹ and 72 cm⁻¹ s⁻¹, respectively by scanning the laser current. (a,b,d,e) The modulation frequency of the laser current was 32,760 Hz, while for c,f it was 32,960 Hz.