Hollow-Core-Fiber Delivery of Broadband Mid-Infrared Light for Remote Spectroscopy

High-resolution multi-species spectroscopy is achieved by delivering broadband 3–4-μm mid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from H³⁷Cl, H³⁵Cl, H₂O and CH₄ present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C₃H₆O and C₃H₈O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the intra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters.