Detection of refractive-index changes by using a sensing fiber with an inverted parabolic index profile

TLDR: In this paper, a novel sensing fiber with an inverted parabolic-index profile was used for the detection of refractive-index changes of a medium surrounding the fiber core, and the results showed that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers.

Abstract: The application of a novel sensing fiber with an inverted parabolic-index profile to the detection of refractive-index changes of a medium surrounding the fiber core is investigated. The detection experiments were made with a set- up consisting of a fiber connector-based coupler allowing the excitation of the sensing fiber by a light source of a relatively small size placed on the fiber axis in a preset distance from the fiber end face, and an optical cell containing a declad segment of the sensing fiber in an immersion whose refractive index could be varied in a known way by heating. Various light sources, such as outputs of multimode and single-mode fibers were tested as more or less accurate approximations to the ideal point light source. For overcoming problems with the profile imperfections in the central core region appearing in some of the produced fibers and originating from the fiber manufacturing technology, the excitation of the fiber by an inclined beam from a bevelled single-mode fiber was studied as well. For comparison, PCS fibers were used in the experiments, too. The results have shown that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers. The possibility of shifting the measuring range down to a more practical lower index values by using inclined-beam fiber excitation and confinement of the index profile to the outer region at the core will be shown.