Noise Immune TDLAS Temperature Measurement through Spectrum Shifting by Using a Mach-Zehnder Interferometer

A noise immune temperature measurement method is proposed by using tunable diode laser absorption spectroscopy (TDLAS). Noise immunity is achieved through spectrum shifting by a Mach-Zehnder interferometer. The driving current of a distributed feedback (DFB) laser is tuned for linear wavenumber scanning and followed by a saw-tooth climbing laser signal. The laser signal travels through the interferometer, and a high-frequency carrier signal of megahertz level modulates the saw-tooth signal away from background noises. When the output laser of the interferometer passes through the target gas, the absorption spectrum is imposed on the intensity profile. In this way, the absorption spectrum is shifted to a high-frequency spectral band from a low-frequency band where both the absorption spectrum and the background noises appear. A part of the output laser of the interferometer is split to provide a reference for baseline extraction and hence absorption spectrum extraction. For the dual-color temperature extraction method, laser absorption spectra of the target gas centered at two different wavenumbers are achieved. Numerical simulations and actual experiments at different temperatures, e.g., using a heating device and Bunsen burner verified the noise immunity of the proposed method. Performance comparisons with the direct absorption spectroscopy (DAS) showed that the proposed method generated more precise temperature values in a noisy background.