Preliminary investigation into feasibility of dissolved methane measurement using cavity ringdown spectroscopy technique

• Published in: Frontiers of physics Vol. 11; no. 6; pp. 159 - 166
• Main Authors: Wang, Zhen-Nan, Ye, Wang-Quan, Luan, Xiao-Ning, Qi, Fu-Jun, Cheng, Kai, Zheng, Ronger
• Format: Journal Article
• Language: English
• Published: Beijing Higher Education Press 01.12.2016; Springer Nature B.V
• Subjects: Analog to digital converters; Astronomy; Astrophysics and Cosmology; Atomic; cavity; Cavity ringdown; Chemical analysis; Condensed Matter Physics; Continuous radiation; continuous-wave; continuous-wave cavity ringdown spectroscopy; current-modulation method; current-modulationmethod; Data acquisition; Degassing; gas; gas hydrate; Gas hydrates; hydrate; Methane; Modulation; Molecular; Optical and Plasma Physics; Optical switching; Particle and Nuclear Physics; Physics; Physics and Astronomy; Piezoelectric transducers; Research Article; ringdown; Seawater; Spectroscopy; Spectrum analysis; Stability analysis; Water analysis; gas hydrate; methane; current-modulation method; continuous-wave cavity ringdown spectroscopy
• ISSN: 2095-0462; 2095-0470
• DOI: 10.1007/s11467-016-0592-3

For the exploration of gas hydrate resources by measuring the dissolved methane concentration inseawater, a continuous-wave cavity ringdown spectroscopy （CW-CRDS） experimental setup was con-structed for trace methane detection. A current-modulation method, rather than a cavity-modulationmethod using an optical switch and a piezoelectric transducer, was employed to realize the cavityexcitation and shutoff. Such a current-modulation method enabled the improvement of the experi-mental setup construction and stability, and the system size and stability are critical for a sensor tobe deployed underwater. Ringdown data acquisition and processing were performed, followed by anevaluation of the experimental setup stability and sensitivity. The obtained results demonstrate thatgreat errors are introduced when a large fitting window is selected if the analog-to-digital converterhas an insufficient resolution. The ringdown spectrum of methane corresponding to the 2v3 bandR（4） branch was captured, and the methane concentration in lab air was determined to be 2.06 ppm.Further experiments for evaluating the quantitative ability of this CW-CRDS experimental setup areunderway from which a high-sensitivity methane sensor that can be combined with a degassing systemis expected.