Towards practical gas sensing with micro-structured fibres

In this work we realize a compact, low volume gas sensor utilizing commercially available hollow core photonic band-gap fibre (PBGF) guiding at around 1550 nm. The device is demonstrated for the detection of methane and acetylene gas. Hollow core PBGF offers near free-space beam guidance. This gives an advantage for gas sensing applications providing a longer interaction length than conventional gas cells and the potential to detect very low analyte concentrations whilst also being inherently safe for operation in remote or hazardous environments. However, one difficulty with such a device is filling a long fibre core with the test gas. Allowing the test gas to enter by diffusion alone can be a lengthy process and forced gas flow may be undesirable. A compact sensor has been designed and built to provide gas detection in an enclosed environment. Standardized connectors incorporating single mode fibre are used to couple light to and from the device. This is spliced to PBGF which has been arranged in sections connected by specially designed splices allowing gas into the fibre and therefore reducing the diffusion length. The fill time by diffusion is minimized while maintaining a long (~1 m) interaction length. Results from demonstration gas concentration measurements are presented and the performance of the device is evaluated. Results are limited by the presence of interferometric noise which is believed to arise from the interaction between core and surface modes within the PBGF; despite this, acetylene concentrations down to 0.05% are measured.