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A fiber optic sensors technique evanescent wave fiber optic sensor (EWFS) for detecting trace amounts of nitrite compounds in water was designed and constructed and evaluated for nitrite determination which based on evanescent field absorption in a test solution formed by the reaction of nitrite compounds in water with suitable chemical reagents. A short unclad portion of a plastic clad silica fiber around (12cm) length acts as the sensing region from (20 cm) total fiber length. The absorption of the evanescent wave changes when the waist cladding refractive index n 2 is slightly modified. Consequently, the coupling visibility also changes. By putting the device in the water polluted, the trace amounts of nitrite in water can be detected. The experimental results clearly establish the usefulness of the present technique for detecting very low concentrations of the order of 1 ppb (parts per billion) of nitrite compounds with a large dynamic range of 1-1000 ppb. Such a high sensitivity enables the present device to be used for measuring the nitrite content in drinking water. The results were consistent with the data obtained by standard spectrophotometric method, showing potential of the proposed sensor for practical application.

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