|Silica-based optical fibers with refractive-index profiles tailored in a region of 1.45 - 1.62 for fiber-optic chemical detection|
|Ivan Kašík, Ph.D.; Vlastimil Matějec, Ph.D.; Miroslav Chomát, Ph.D.; Daniela Berková, Ph.D.; Jana Proboštová, MSc.|
Chemical detection by means of optical fibers has many advantages over
other ways of sensing and shows great potential for environmental
monitoring and medical sensing. For this purpose, polymer-clad silica
(PCS) fibers have been predominantly used. However, PCS fibers reach
high sensitivity to refractive-index or absorption-coefficient changes
only for refractive indexes of the medium surrounding the fiber core
which are close to that of silica (i.e. 1.457 at a wavelength of 633
nm and temperature of 25 °C). In order to extend the operation range
of optical fibers for chemical sensing to refractive indexes below and
above that of silica, novel types of sensing fibers have been designed
and prepared in the Institute -. These fibers are of two types –
multimode large-core Inverted Graded-Index (IGI) fibers, and
Polymer-Clad Glass (PCG) fibers.
IGI and PCG fibers have been drawn from preforms prepared by the Modified Chemical Vapor Deposition (MCVD method) and liquid-phase etching or from commercially available rods of optical glasses (e.g. F2 glass, Shott) . Typical refractive-index profiles of the MCVD preforms are shown in Fig. 1. Calibration curves in Fig. 2 show that IGI and PCG fibers enable us to tailor the detection sensitivity for refractive indexes of the fiber surroundings ranging from 1.45 to 1.62 . In this way, a group of detectable chemical substances can be enlarged and the number of materials suitable for the preparation of detection membranes on the fibers can be increased. IGI fibers coated with polysiloxane detection membranes have been tested for the detection of toluene dissolved in water in concentrations of about 3-5 mg/l . PCG fibers coated with special Ormosil membranes have also been tested for the detection of glucose and oxygen in water.