We report on a novel all-fiber refractometer sensor based on multimode interference (MMI) effects. The operating mechanism is based on the self-imaging phenomena that occur in the multimode fiber (MMF) section, which basically replicates the field at the input of the MMF on the output of the MMF for a specific wavelength. However, the longitudinal position of this image is highly dependent on the MMF diameter (D), since there is D2 dependence on the longitudinal position of this image. For the refractive index measurement a section of no-core multimode fiber, whose cladding is air, is surrounded by the liquid sample. The liquid sample now works as the cladding medium and as a result of the Goes-Hanchen shift the effective width (fundamental mode width) of the No-Core fiber is increased. As a result, the maximum coupling resulting from the imaging phenomena occurs at a different wavelength, and this can be used to measure the refractive index of the liquid. Using this scheme we can achieve a resolution on the order of 1x10-5 for a refractive index range from 1.333 to 1.434. The device was used here to measure refractive index in liquids, but can also be applied for measuring concentration of liquids. These sensors are promising and attractive in chemical and biotechnological applications because of their high sensitivity, immunity to electromagnetic interference, and compact size.
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