Suresh, Kumar P; Thomas Lee, S; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2002)
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Abstract:
The design and development of a cost-effective, simple, sensitive and portable LED based fiber optic evanescent wave sensor for simultaneously detecting trace amounts of chromium and nitrite in water are presented. In order to obtain the desired performance, the middle portions of two multimode plastic clad silica fibers are unclad and are used as the sensing elements in the two arms of the sensor. Each of the sensor arms is sourced by separate super bright green LEDs, which are modulated in a time-sharing manner and a single photo detector is employed for detecting these light signals. The performance and characteristics of this system clearly establish the usefulness of the technique for detecting very low concentrations of the dissolved contaminants.
Nampoori, V P N; Suresh, Kumar P; Unnikrishnan, K P; Thomas Lee, S; Girijavallabhan, C P; Sugunan, S; Radhakrishnan, P(International School of Photonics, May 15, 2003)
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Abstract:
In this communication, we discuss the details of fabricating an off-line fibre
optic sensor (FOS) based on evanescent wave absorption for detecting trace
amounts of Fe3+ in water. Two types of FOS are developed; one type uses
the unclad portion of a multimode silica fibre as the sensing region whereas
the other employs the microbent portion of a multimode plastic fibre as the
sensing region. Sensing is performed by measuring the absorption of the
evanescent wave in a reagent medium surrounding the sensing region. To
evaluate the relative merits of the two types of FOS in Fe3+ sensing, a
comparative study of the sensors is made, which reveals the superiority of
the latter in many respects, such as smaller sensing length, use of a double
detection scheme (for detecting both core and cladding modes) and higher
sensitivity of cladding mode detection at an intermediate range of
concentration along with the added advantage that plastic fibres are
inexpensive. A detection limit of 1 ppb is observed in both types of fibre and
the range of detection can be as large as 1 ppb–50 ppm. All the
measurements are carried out using a LabVIEW set-up.