Frequency-Division Multiplexing and Main Peak Scanning WMS Method for TDLAS Tomography in Flame Monitoring

The wavelength modulation spectroscopy (WMS) method is typically used in the tunable diode laser absorption spectroscopy (TDLAS) for spectrum extraction in a noisy background. Usually, two entire absorption spectra for target species are scanned in a cascade way in the WMS method, and it limits the temporal response of the TDLAS sensor. In this article, the frequency-division multiplexing and main peak scanning method were introduced to the WMS method and enabled a high frame rate six times that in the classical TDLAS sensor at the same scanning speed. For the first time, two wavelengths modulated at different frequencies were used to extract the absorptions in parallel along various laser paths and implemented successfully on a TDLAS tomography hardware, to reconstruct the distributions of both temperature and species concentrations, at a frame rate up to 10 kHz. A five-view fan-beam tomographic sensor was utilized in the tomographic imaging. In each view, lasers at two wavelengths were selected and modulated at different sinusoidal frequencies on the same sawtooth signal. Numerical simulations varied the high-quality tomographic imaging of both temperature and H2O molar concentration distributions for different phantoms. Experiments were also carried out on dynamic flames, which were generated by exciting a Bunsen burner acoustically. The acoustic excitation frequency varied from 40 to 750 Hz, and the results of the tomographic images agreed well with the flame evolutions.