Sub-ppm CO detection in a sub-meter-long hollow-core negative curvature fiber using absorption spectroscopy at 2.3 μm

•Hollow-core negative curvature fiber (HC-NCF) provides sub-ppm CO detection using absorption spectroscopy.•Coupling the 2.3-μm laser into HC-NCF with free-space optics provides a single-mode light delivery.•The fiber sensor features a fast gas exchange rate of < 5 s. We demonstrated the sensitive CO detection in a novel hollow-core negative curvature fiber (HC-NCF) with infrared laser absorption at 2.3 μm. The HC-NCF consists of a single ring of eight nontouching silica capillaries around the air core, providing a single-mode light delivery of the 2.3-μm distributed feedback laser. A high coupling efficiency of 90% was achieved with the aid of optimal free-space coupling optics. The hollow-core fiber was used as a gas cell for gas absorption measurement of a total path length of 85 cm. Both direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) were adopted to demonstrate the sensor performance by detecting the CO line R(10) at 4297.7 cm−1. In scanned-wavelength DAS, we obtained a minimum detection limit (MDL) of 13 ppm CO, which was limited mainly by the existing mode noise in the HC-NCF. By applying a pressure difference of 0.8 bar between the two ends of the fiber, we demonstrated a very short gas loading time of only 5 s. Finally, we achieved a MDL of 0.4 ppm CO using the WMS technique, corresponding to a noise equivalent absorption of 1.6 × 10-7 cm−1.