Artificial olfaction with hollow core bragg fiber arrays
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An optoelectronic nose for the analysis of alcohols (ethanol and methanol) in chemically complex environments is reported. The cross-responsive sensing unit of the optoelectronic nose is an array of three distinct hollow core infrared (IR) transmitting photonic band gap fibers (Bragg fibers), which transmit a specific band of IR light depending on their Bragg mirror structures. Presence of alcohol molecules in the optofluidic core quench the fiber transmissions if there is an absorption band of the analyte overlapping with the transmission band of the fiber. The cumulative response data of the fiber array enables rapid, reversible and accurate discrimination of alcohols in chemically complex backgrounds such as beer and fruit juice. In addition, we observed that humidity of the environment has no effect on the response of the optoelectronic nose, which is rarely achieved in gas sensing applications. Consequently, it can be reliably used in virtually any environment without precalibration for humidity or drying the analytes. To further improve the performance, we engineered the photonic bandgap of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes such as the optoelectronic nose. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications.