Etalon fringe removal of tunable diode laser multi-pass spectroscopy by wavelet transforms

Tunable diode laser absorption spectroscopy (TDLAS) is an important technique for trace gas detection. The sensitivity of TDLAS system, especially for multi-pass absorption, is usually limited by optical fringes. It is an effective approach to suppressing optical fringes of spectral lines using digital filtering. The parameters derived from filtering simulations are key factors and they are related to the noise characteristic. In the paper, the precise Airy functions were used to simulate optical interference and the interference signal was effectively minimized by the wavelet transforms. The root-mean-square error and the residual standard deviation are introduced to quantify the suppression ratio of etalon fringes. It is demonstrated that the optimum wavelet function in denoising the observed CO2 lines is fk18, in comparison with haar, db02, coif1 etc. In addition, the absorption signals recorded by direct absorption spectroscopy and wavelength modulation spectroscopy at second-harmonic (WMS-2f) with a 10 m White-type cell were subjected to low-pass, averaging and fk18 wavelet filtering, respectively. The results show that only wavelet transform is capable of suppressing optical interference noise. The detection limit of WMS-2f can reach 7 ppm and the perfect calibration curves between signal intensity and gas concentration are obtained after wavelet filtering. To further demonstrate the feasibility of the approach, spectral signals at different pressures and temperatures were acquired and the etalon interference is substantially eliminated by fk18 wavelet at 13th level of decomposition.