Comparative Study on Software Demodulation for Continuous Wave and Quasi-Continuous Wave Wavelength Modulation Spectroscopy
SHAO Xin1, 2, LIU Fu-gui1, CHEN Wen-liang3
1. Province-Ministry Joint Key Lab of Electromagnetic Field and Electrical Apparatus Reliability，Hebei University of Technology，Tianjin 300401，China 2. Electrical and Electronics Technology Application Department, Tianjin Institute of Mechanical and Electronic Technology, Tianjin 300131，China 3. State Key Laboratory of Precision Measuring Technology and Instruments (Tianjin University), Tianjin 300072, China
Abstract：According to the modulation signal applied on laser diodes, wavelength modulation spectroscopy (WMS) can be divided into continuous wave (CW) WMS and quasi-continuous wave (QCW) WMS. In order to deeply compare CW-WMS and QCW-WMS, we used a specific software-realized lock-in amplifier for continuous and quasi-continuous modulation signal demodulation. The invalid signal in quasi-continuous modulation spectrum was filtered off, and then the effective detection signal was demodulated to obtain the second harmonic signal (WMS-2f). It was compared with the 2f signal demodulated continuous laser modulation spectrum with software. The results show that while the same system parameters are set, the signal-to-noise of the quasi-continuous modulation spectrum is 5% higher than the continuous modulation spectrum with software demodulation measurements, and the detection limit is 11.3% lower. And without the invalid signal in quasi-continuous modulation spectrum, the standard WMS-2f signal can be demodulated, which has potential to be used for the investigation of gas absorption profile. This work has provided accurate reference for selections of the laser modulation spectroscopy.
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