TDLAS second harmonic demodulation based on Hilbert transform

Abstract

A demodulation method for tunable diode laser absorption spectroscopy (TDLAS) second harmonic based on the Hilbert transform is proposed in this paper. The second harmonic of the TDLAS signal can be easily obtained without a reference signal. The TDLAS signal is firstly processed by band-pass filtering, then the envelope of the processed signal is obtained with Hilbert transform. And finally, the second harmonic is extracted from the 1f component of the envelope. The validity of the proposed method is confirmed by simulation and verified by experiment, and the result shows that the error is acceptable under the cases of weak absorbance, which indicates that the proposed method is applicable to practical trace gas detection.

Data processing method for improving the in-situ detection performance of flue gas concentration in laser modulation spectroscopy

“…Second harmonic normalization requires two harmonic signals demodulated simultaneously from the absorption spectrum, which increases the number of channels of the lock-in amplifier [9,10] to two, and the unequal delay time will make the harmonics demodulated by the two channels inconsistent, which makes the second harmonic normalization produce systematic errors [11] and reduces the detection accuracy of the system. To avoid the use of reference signals, many alternative methods for demodulating the harmonic signals have been developed, for example, methods based on the Hilbert transform [12] can only extract the second harmonic, methods based on the Fast Fourier Transform (FFT) [13,14] , where the separation of the harmonic channels requires the sampling of a complete scan period, and wavelet transform (WT)-based [15] methods may suffer from the fact that the harmonic frequencies cannot be precisely matched to the generated sub-wavelet sequences. WT is developed on the basis of the idea of STFT localization, and the STFT-based method is considered to extract the harmonic signals due to the small number of harmonic components needed for in-situ measurements and the low requirement of frequency resolution.…”

Flame Imaging Technology Based on 64-Pixel Area Array Sensor

“…Characteristic changes in physical parameters use the physical parameters as carriers to represent temperature information outwards. Commonly used non-contact temperature measurement methods mainly include the radiation temperature measurement method [ 10 , 11 , 12 ], the absorption spectrum temperature measurement method [ 13 , 14 , 15 , 16 ], and the acoustic wave temperature measurement method [ 17 , 18 ].…”