Multimode interference (MMI) devices have attracted a great deal of interest due to their simplicity of fabrication. The MMI device is ready for testing after splicing a section of multimode fiber (MMF) between two single-mode fiber (SMF). In this chapter we provide an overview of the fundamentals behind the formation of self-images in MMI fiber devices, as well as the basic mechanisms for tuning their operational wavelength which is related with their application for sensing applications. The sensitivity enhancement of these MMI fiber sensors is also investigated by reducing the diameter of the MMF via wet chemical etching, as well as coating the MMF with a high refractive index overlay. The MMI fiber sensors are applied to the quality control of gasolines and in particular the real time monitoring of gasohol, mixtures of gasoline and ethanol, which is critical for the proper operation of flexible-fuel vehicles (FFV). The results demonstrate that MMI fiber sensors are well suited for such applications, as well as other applications were the binary mixture of liquids has to be controlled or monitored.
Rodriguez-Rodriguez, A. J., May-Arrioja, D. A., Hernandez-Romano, I., & Matías, I. R. (2017). Multimode interference fiber sensors for the monitoring of gasoline/ethanol blends. In Smart Sensors, Measurement and Instrumentation (Vol. 21, pp. 329–346). Springer International Publishing. https://doi.org/10.1007/978-3-319-42625-9_15