The possibility of utilizing certain polymers, known as variable-index materials, as cladding in tapered optical fibers for gas sensing is explored. We have shown that any small variation in the refractive index of the cladding of a tapered single-mode fiber has a direct influence on the transmission characteristic of the tapered region. A 4 deviation in the refractive index of a certain polymer is obtained upon the exposure of the polymer to ammonia. Sensitivity, repeatability, and selectivity of several fabricated sensors are also illustrated. # 1998 Society of Photo-Optical Instrumentation Engineers. S0091-3286(98)00504-2 Subject terms: tapered optical fibers; variable-index materials; fiber-optic sen-sors. Paper INT-05 received Jun. 1, 1997; revised manuscript received Aug. 28, 1997; accepted for publication Oct. 27, 1997.
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