Abstract
In this paper, we present the details of a optical fiber sensor, based on evanescent wave absorption in Long periodic grating (LPG) in multimode optical fibers, for measuring trace amounts of chromium concentration in water. The detection limit of the sensor is a few parts per billion (ppb) and the operating range is more than 3 orders of magnitude. The sensing length of the sensing element is only 10mm, but the dynamic range is as good as that of the conventional unclad evanescent wave sensors. A plastic clad silica core (PCS) fiber with LPGs on its middle region acts as the sensing element. Exposure of this sensing region to water samples containing spectroscopic reagents for the detection of chromium causes evanescent wave absorption, which increases with increasing concentration of chromium. The use of a super-bright green light emitting diode (LED) as light source reduces the size and cost of the system considerably. In order to reduce the effect of power supply fluctuations or ambient variations of the LED, a reference arm and ratio detector are employed.
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