Hollow-Core Fiber-Based Broadband Absorption Spectroscopy With Dual-Wavelength Measurements to Remove the Effect of Multigas Interference

A hollow-core fiber (HCF) combined with a broadband supercontinuum laser-based direct absorption spectroscopy technique is pro- posed and demonstrated for concentration measurements of gas mixtu- res. A dual-wavelength technique is used to eliminate gas interference in multigas sensing. The concentrations of methane (CH
₄
) in two bands, 1.6 and 2.3 µm, ammonia (NH
₃
) in two bands, 2 and 2.3 µm, and carbon dioxide (CO
₂
) in the 2-µm band are measured. The measurements were performed using 0.5, 1, 5, and 10 m long HCF covering multiple absorption lines in the given band. The minimum detection limit (MDL) is improved as the signal-to-noise ratio was enhanced up to 3.9 dB by considering cumulative absorbance obtained from multiple absorption lines in the broadband measurement. The MDL were 13 and 15 parts-per-million (ppm) for CH
₄
at 1.6 and 2.3 µm, respectively; 13 and 17 ppm for NH
₃
at 2 and 2.3 µm, respectively; and 154 ppm for CO
₂
at 2 µm band. The proposed dual-wavelength measurement technique was further extended to measure concentrations of gases in biogas samples. CH
₄
, CO
₂
, and hydrogen sulfide were measured to be 54.7%, 46.1%, and 2.1%, respectively, and atmospheric CO
₂
of 421 ppm was also measured using a 10-m long HCF. The system offers excellent long-term stability for over continuous measurement of 4 h.