A Chinsese research group at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science developed a novel method of laser frequency locking and intensity normalization for wavelength modulation spectroscopy (WMS), which significantly improves the stability and sensitivity of the WMS based gas sensor. The proposed method has a potential application in long-term trace gas measurement.
This work was done by GAO Xiaoming's research group and was published in Optics Express.
WMS, combined with long optical path length designs, have been widely used for a variety of applications in environmental sensing, medical diagnostic and industrial process control. However, two key factors, laser frequency shift and intensity fluctuations, may affect the stability and sensitivity of the WMS based system.
In this study, the center spacing between two second harmonic peaks demodulated from the rising and falling edges of a scanning triangular wave (for wavelength scan) was 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 was used as an intensity normalization reference.
Compared to conventional WMS system without frequency locking, the optimal integration time and the Minimum measurement precisions were both significantly improved by a factor of ~4.
In addition, the DS-sine based 2f normalization method showed a more accurate concentration determination than commonly used 1f normalization method under frequency locking conditions.
This work was supported by National Key Research and Development Program of China and National Natural Science Foundation of China.
Link to the paper: Laser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing