Diabetes has recently become the leading cause of death worldwide. So far, there is no effective treatment to cure or prevent diabetes, but good blood control through glucose monitoring can...

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An optical fiber is a flexible, transparent, and cylindrical waveguide made of plastic or silica, with diameters slightly thicker than that of a human hair (Figure 1a). [1] Optical fibers deliver/guide light for long distances with low losses. Single-index optical fibers consist of a transparent core covered with a transparent cladding material of a lower refractive index. The total internal reflection allows for guiding light in the fiber core; however, based on the waveguide analysis the energy of the transferred light is not fully trapped in the fiber core but a portion of the light travels in the cladding as evanescent waves. When the incident light hits the core-clad interface at angles larger than its critical angle, the light is completely reflected and guided in the fiber. In contrast, the incident light which meets the interface at smaller angles is refracted into the cladding to be lost. The critical angle is the minimum angle required to attain the total internal reflection, which is determined by the refractive index difference between the core and cladding materials. [2] Fibers can be classified into two categories based on the number of guided modes: single-mode and multimode fibers.

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