Long period gratings in multimode optical fibers: application in chemical sensing

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. © 2003 Published by Elsevier B.v. PACS: 42.8I.Pa; 42.79.Dj