Detection of methane at 1670-nm band with a hollow-core photonic bandgap fiber

In recent years, hollow-core photonic bandgap fibers (HC-PBFs) have been demonstrated to be a promising technology for gas sensing. In particular, the long interaction path lengths available with these fibers are especially advantageous for the detection of weakly absorbing gases such as methane. In the near-infrared region, methane has the strongest absorption band, 2ν3, at 1670 nm. However, HC-PBFs were not available until recently in this wavelength range and gas sensing devices based on HC-PBFs were previously made in the weaker band of 1300 nm. In this paper, we report the demonstration of a methane sensor based on a 1670-nm-band HC-PBF. A strong spectral feature, the R(6) manifold (near 1645 nm), was selected for sensing purposes as it shows a good signal-to-noise ratio. This absorption line is comprised of six energy transitions, strongly overlapped at our experimental conditions. For that reason, we applied a multiline algorithm that used information from the six transitions to fit the manifold. The goodness of the fitting was assessed measuring the concentration of different methane samples. With this method, a minimum detectivity of 10 ppmv for the system configuration was estimated.