Near-ultraviolet evanescent-wave absorption sensor based on a multimode optical fiber.

Fiber-optic near-ultraviolet evanescent-wave sensors have been constructed, and their feasibility for practical applications has been demonstrated. The sensors, used for the detection of ozone near the 254-nm peak of the Hartley absorption band, were fabricated from coiled segments of low-cost multimode plastic-clad silica optical fibers. The sensing sections were produced alternatively by stripping only the protective jacket from the fiber to expose the gas-permeable silicone cladding or by stripping the jacket and the cladding to expose the bare-silica fiber core. Response characteristics are given, including sensitivity to ozone, reversibility, and aging effects. The useful lifetime was unacceptably short for the sensor that employed the bare-silica core, whereas the exposed-cladding sensor demonstrated good stability over the entire two-month period of investigation. The latter, more useful sensor demonstrated a linear response to ozone over the range 0.02-0.35 vol% and a reversible response with a time constant on the order of 1 min. Differences in ozone absorption spectra obtained in the transmission and evanescent-wave modes are discussed. Projected applications of the new exposed-cladding sensor include ozone determination in water-treatment processes and ozone production plants.