Open AccessLaser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing
Author(s) -
Guishi Wang,
Jianwei Mei,
Xing Tian,
Kun Liu,
Tu Tan,
Weidong Chen,
Xiaoming Gao
Publication year - 2019
Publication title -
optics express
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.394
H-Index - 271
ISSN - 1094-4087
DOI - 10.1364/oe.27.004878
Subject(s) - optics , normalization (sociology) , wavelength , materials science , laser , intensity modulation , spectroscopy , frequency modulation , physics , phase modulation , phase noise , bandwidth (computing) , telecommunications , quantum mechanics , sociology , anthropology , computer science
A novel method for laser frequency locking and intensity normalization in wavelength modulation spectroscopy (WMS)-based gas sensor system is reported. The center spacing between two second harmonic peaks demodulated from the rising and falling edges of a scanning triangular wave (for wavelength scan) is employed as a frequency locking reference. Amplitude of the directly acquired sine signal (for wavelength modulation) in the spectral region far away from the absorption feature is employed as an intensity normalization reference. A 50 ppm CH 4 :N 2 sample sealed in a multi-pass cell at 1 atm was employed as the target analyte for demonstration. The frequency locking significantly improves measurement accuracy, and the introduced intensity normalization method realized a ~3 times SNR improvement as compared to the commonly used 1f normalization method under frequency locking conditions. A minimum measurement precision of ~2.5 ppbv was achieved with a normalized noise equivalent absorption coefficient of 1.8 × 10 -9 cm -1 Hz -1/2 .