Distributed Fiber-Optic Chemical Sensor with Chemically Modified Plastic Cladding

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Abstract
A novel method for modification of the polymer cladding of an optical fiber has been developed for use in distributed chemical sensing. The usefulness of the new technique is illustrated by means of a distributed sensor for ammonium ions based on a 9 m long plastic-clad silica fiber modified with phenol red. The stability of the immobilized indicator made it possible, for the first time, to use a chemically modified fiber for the reversible detection of ammonium ions in highly alkaline solution (pH 11–14). The new sensor offers a broader dynamic range (four orders of magnitude) and shorter response time (30 s) than point sensors for ammonium ions reported in the past.