Silica-based optical fibres with refractive index profiles tailored in a region of 1.46–1.62 for fibre-optic chemical detection

The paper deals with special optical fibres suitable for evanescent-wave sensing. Novel kinds of polymer-clad glass (PCG) fibres are proposed to overcome the limitation imposed by the refractive index of pure silica core of the conventional polymer-clad silica (PCS) fibres that makes impossible their use for the detection of a number of analytes with refractive indices above 1.457 (at 630 nm). The first structure proposed is the step-index (SI) one drawn from rods of soft silicate glasses with refractive index higher than that of silica. As an alternative to this kind of fibres, novel structures including inverted graded-index (IGI) and SI fibres based on doped silica with refractive indices above 1.457 and tailored by the MCVD method were devised. It was found that the sensitivity of the SI structures with the refractive index higher than 1.457 to refractive index changes was comparable to that of conventional PCS fibres. The prepared special optical fibres could be employed for evanescent-wave chemical sensing of analytes with refractive indices in a wide range, up to 1.62.