The fabrication and characterization of a fibre optic pH sensor based on evanescent wave absorption is presented. The unclad portion of a multi-mode optical fibre is coated with a pH sensitive dye, which is immobilized by the sol-gel route. The sensitivity of the device has been found to increase when multiple sol-gel coatings are used as the sensing region. The dynamic range and the temporal response of the sensor are investigated for two different dyes, namely bromocresol purple and bromocresol green. The performance of the device is evaluated in terms of the results obtained during actual measurements.

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 Fe 3+ 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 Fe 3+ 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.

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.

We propose and demonstrate a new technique for evanescent wave chemical sensing by writing long period ~ in a bare multimode plastic clad silica tiber. The sensing length of the present sensor is only IQ mm, but is as sensi1iIt; a conventional unclad evanescent wave sensor having about 100 mm sensing length. The minimum measurabkl1! centratioll of the sensor reported here is 10 nmoUl and the operating range is more than 4 orders of magnitl Moreover, the detection is carried out in two independent detection configurations viz., bright field detection that detects the core-mode power and dark field detection scheme that detects the cladding mode power. The used a double detection scheme definitely enhances the reliability and accuracy of the results. Furthennore, the~. the present tiber need not be removed as done in conventional evanescent wave fiber sensors.