In light of the increasing demand for sensing devices with larger sensitivities and precision, this work presents an optical fiber sensor based on multimode interference, fabricated by manipulating the fusion parameters for a silica capillary tube spliced between two sections of single-mode fiber. The sensor, with a length of ~2.5 mm, is studied for a variation of refractive index ranging from 1.3342 RIU to 1.3452 RIU. To circumvent the complex spectral response of the sensor, a frequency filtering of the response strategy is implemented, resulting in sensitivities of 1391.0 nm/RIU and 2807.5 nm/RIU, and a correspondent resolution of 7.6 × 10−5 RIU and 9.4 × 10−4 RIU, when considering the first two main frequencies of the spectral response.