Long period grating in multimode fiber for ammonia gas detection

This article describes the design, development and characterization of a fiber optic sensor based on evanescent wave absorption in Long Period Grating (LPG) on multimode optical fibers, for the detection of toxic ammonia gas. The sensing length of the sensing element is only 10mm, but the dynamic range is as good as that of the conventional evanescent wave sensors. Sol-gel technology is employed for immobilizing a reversible ammonia sensitive dye on the grating region of a multimode fiber. Ammonia gas permeating into the immobilized dye, the color of the dye changes reversibly from yellow to blue with increasing concentration of ammonia gas. The concentration of ammonia gas can be determined by measuring the absorption at a given wavelength. The dynamic range of the sensor is from 0.027 to 2.04 mM/Ltr.