Developing CH4 detection limit at λ = 1.654 μm by suppressing optical interference fringes in wavelength modulation spectroscopy

Optical fringes tend to cause difficulty in gas detection based on wavelength modulation spectroscopy (WMS). In this article, we introduce a convenient fringes suppression method. Through modulation depth optimization and two-frequency modulation technology, we substantially improve the CH4 detection limit. Our experiment adopts a distributed feedback laser operated at λ = 1.654 μm in an open-path environment. The fluctuation of the sensor measurement is dramatically reduced by approximately 10-20 times. The minimum detection limit of the open-path environment improves to 130 ppb m for a 50-min period. Compared with other previously reported near-infrared CH4 detection systems, our proposed method improves the detection limit further. In contrast to traditional fringe suppressing methods, our method has more advantages in CH4 detection in open-path environments.