부산대학교 도서관

A noise immune tunable diode laser absorption spectroscopy (TDLAS) measurement method for temperature detection was proposed by coherent demodulation of modulated intensities. For TDLAS intensities distorted by background thermal emissions, a Mach-Zehnder interferometer was used to modulate these intensities, and they were kept away from background noises in the frequency domain. In the intensity modulation, the laser wave number was linearly varied to extract the absorption spectral profile. The demodulation was realized by using an analog multiplier and spectral profile was demodulated at a high signal to noise ratio (SNR). The modulated signal was multiplied by itself in a multiplier to achieve coherent demodulation. Also, the same multiplication was carried out on the reference intensities. In this way, the multiplier replaced the lock-in amplifier to realize the signal demodulation and simplified the demodulation. Experiments showed that the noise immunity of the proposed method was equivalent to that of the digital lock-in amplifier with a high sampling rate for accurate absorption spectrum and calculated temperatures of the proposed method were consistent with those of direct absorption spectroscopy (DAS).