Dear Colleagues,

The expanding function of refractive index measurement in numerous disciplines, such as environment monitoring, food control, process control, and biomedical research, has led to an increase in research and development in this area.  Optical fibers provide a great deal of promise as a platform for refractive index sensing because of their desirable characteristics. These include high sensitivity, a tiny fingerprint, compatibility with difficult settings, high degrees of integration, remote analysis, and the possibility to enable real-time measuring and monitoring. Numerous fiber-optic refractive index sensors have been suggested and created as of late using various designs and processes. Combining nanostructured thin films as a transformer-sensitive layer and optical fiber as a signal carrier to create a miniaturized refractive index sensor is recognized as a promising strategy for addressing environmental characteristics and comprehending novel sensor concepts. However, the most challenging area of optical fiber sensor technology is still the construction of an efficient one. This special issue aims to provide significant opportunities for the next phase of optical fiber refractive index sensor development.