Fiber optic chemical sensor with modified cladding refractive index

We report a miniature optical fiber chemical sensor that consists of a fused-biconical single-mode optic fiber coupler. 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 chromium and nitrite in water can be detected. The coupler fabrication and design of the chemical sensor are described. The performance and characteristic of this system clearly establish the usefulness of the technique for detecting very low concentrations of the dissolved contaminants. And the sensor has shown preferable sensitivity and time response.