Miniature optical fiber refractometer using cladded multimode tapered fiber tips

Cladded multimode tapered fiber tips with mirrored end are proposed as refractometers. The working mechanism of such devices is based on radiation of the modes guided by the cladding, which is induced by the refractive index of the sample medium. Using conventional graded-index multimode fiber, a low-power 850 nm LED, and a simple detection scheme some inexpensive, robust, and versatile refractometers are demonstrated. The length of the tips is typically 2 mm, while their diameter is less than 80 μm. The refractometers can cover a refractive index range between 1.36 and 1.46 with resolution up to 3 × 10−5.