The growing need for highly sensitive and robust refractive index (RI) sensors engaged in a wide range of fields, especially in biochemical analysis, food safety, environmental monitoring, and industry process monitoring, has driven the study of various sensing schemes. For instance, the monitoring of contaminants and impurities such as preservatives in natural and daily foods is critical in food safety. Also, the measurements of salinity in the aspects of oceanics, marine environment monitoring, seasonal climate prediction, mariculture, and solar engineering are a complex subject. Moreover, there is a need to measure small RI changes in small volumes of liquid in the bio-sensing field. Traditional bulk refractometers are then not appropriate due to their inconvenient size and weight. Among the demonstrated sensing schemes, fiber-optic RI sensors (FORISs) offer distinctive light–matter interactions, along with their unique advantages of compact footprint, short response time, immunity to electromagnetic interferences, and the possibility of distributed sensing[2–11].