Sensitive handheld refractometer by using combination of a tapered fiber tip and a multimode fiber

This work presents a sensitive handheld refractometer using a tapered fiber tip, a multimode fiber, and a tiny mirror. The taper fiber tip was made using a modified wet etching method. The angular intensity distribution of the transmission light from the tapered fiber is sensitive to the refractive index of the surrounding solution. The low angular wave was reflected and collected by a multimode fiber. By setting a small collection angle and 10-mm distance between the fibers and the mirror, the maximum intensity sensitivity reaches up to 7.8 × 105%/RIU (refractive index unit). The sensor resolution of the sensor probe can achieve 8.7 × 10−5 RIU under 0.25% laser intensity stability. The simple handheld refractometer can be used to determine the properties of various liquids. For the glucose solution measurement under handheld condition, 0 to 50 °Bx can be distinguished by monitoring the intensity change. Considering the hand vibration and environmental disturbance, the sensor resolution was 1.22 °Bx.