In this paper, Baiad and co-authors take this method a step forward, and not only use the internal modes of the core, but harness also guided cladding modes. This is done by incorporating a double-clad fiber. Using the cladding modes is a novel and simple solution for enhancing sensitivity, mainly for two reasons. First, it does not rely on the same channel for exciting and reading the SPR. The gold, used to excite the SPR, is coated only on the cladding which does not hinder the fiber core. Second, it is possible to measure the cladding modes with reflection, which greatly enhances the signal-to-noise ratio, in comparison to measuring the transmission of the core.
What can be measured with such a small prop as a SPR? In this paper, the authors show that by measuring the cladding modes’ reflection it is possible to achieve a sensitivity of 508 nm/RIU, a number that stands for the response of the SPR wavelength to a change of the surrounding refractive index. Though others have reported higher sensitivity, this method can do this for a much larger range of refractive indices, from 1.335 to 1.432, due to its high signal-to-noise ratio. This method is a leap forward since it paves the path for multiple channel sensing, where some channels are used for launching and others for sensing, enabling to measure different properties at once. The possibility of creating a multiclad tilted fiber grating coated with gold shows that technology is thoroughly keeping pace with the speed of material science.
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