Fibre optical sensor for C₂H₂ gas using gas-filled photonic bandgap fibre reference cell

We present results from the first optical fibre correlation spectroscopy gas sensor which uses no lenses other than in the measurement gas cell. The sensor measures acetylene (C2H2) gas concentration. A 1m length of photonic bandgap fibre (PBGF) was filled with acetylene at atmospheric pressure, then fusion spliced to standard, solid-core, single-mode SMF-28 fibre to form a compact, coilable, reference cell without the need for any lenses or bulk optics. As the cell contained a gas volume <5 µL, it presents a greatly reduced hazard even when filled with explosive or harmful gases. Acetylene concentration measurements were made by comparing the absorption spectrum of this reference cell with that of a bulk optics measurement cell containing test gases, using a simple sensing topology requiring no spectrally selective components. Our sensor had a noise-limited sensitivity of 100 ppm acetylene using 4 s averaging time. Pure carbon dioxide gave a cross-sensitivity equivalent to 150 ppm acetylene