Porous optical fibers for high-sensitivity ammonia-vapor sensors

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Abstract
A new porous glass optical fiber has been developed for use as a sensor for the detection of ammonia vapors at low concentrations. The porous structure that remains after selective heat treatment, phase separation, and chemical leaching of a borosilicate glass imparts a high surface area to the fiber core. Ammonia vapors permeating into the porous zone, which is pretreated with a reversible pH dye indicator, produce a spectral change in transmission. The resulting pH change is measured by in-line optical absorbance and is proportional to the ambient-ammonia concentration. Ammonia-vapor concentrations as low as 0.7 part in 106 have been detected.