High sensitivity gas sensing using hollow core photonic bandgap fibres designed for operation at mid-IR wavelengths

High sensitivity gas detection is of great interest in the fields of security, medical diagnostics, and environmental and industrial monitoring. Optical absorption methods are well established and the use of fibre based devices has advantages in terms of small footprint, relatively low cost, immunity to electromagnetic interference and potential for distributed sensing; however, they generally suffer from poor sensitivity due to the difficulty of achieving an efficient spatial overlap with gas analytes. The use of Hollow Core Photonic Bandgap Fibres (HC-PBGFs), i.e. a special type of optical fibres that guide light in a hollow core through bandgap effects, enables an extremely efficient sensing platform due to close to 100% overlap between the light and gas in the hollow core and the possibility for long interaction lengths. Furthermore, HC-PBGFs can be designed to operate at mid-IR wavelengths, where conventional fibres cannot operate. We recently demonstrated HC-PBGFs with loss as low as 0.05dB/m at 3.3µm and more than 100nm 3dB transmission bandwidth [1]. The capability of accessing the mid-IR wavelengths is critical for gas sensing, as the fundamental vibrational transitions of many gas molecules (for instance those containing C-H, O-H, N-H groups) fall in this spectral region. By detecting the fundamental absorption bands as opposed to their much weaker overtones in the near IR often targeted by fibre based sensor systems, substantially higher sensitivity can be achieved [2]. Additionally, mid-IR HC-PBGFs have relative large core diameters (~50 µm), which is beneficial in allowing a comparatively faster gas filling and evacuation