Multiple harmonics extraction of wavelength modulated laser spectrum by single step empirical mode decomposition

Wavelength modulation spectroscopy (WMS) is a noise immune method for gas detection in Tunable diode laser absorption spectroscopy (TDLAS). Harmonic extraction is the key to obtain the normalized harmonics in WMS. A harmonic extraction method is proposed in this work by using a single step empirical mode decomposition (SS-EMD). And simultaneous extractions of multiple complete harmonics are realized by using only the raw WMS signals once. An analytic expression for the WMS signal is derived from only a small number of raw laser intensities. Both simulation and experiment results are used to validate the noise immunity of the proposed method, and performance comparisons with the lock-in amplification (LIA) method are also made. Complete and accurate multi-harmonics extraction of SS-EMD requires only one fourth of sampled laser intensity points compared to LIA, and the difference of signal to noise ratio between these two methods is less than 2.4%.