High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core single-mode fiber for biosensing

A fiber-optic refractive index (RI) sensor using multimode interference (MMI) composed of an unclad multimode fiber (MMF) sandwiched by smallcore single-mode fibers (SMFs) has been proposed and demonstrated. The small-core SMF at the input port enhances the diffraction of the light outputted to the MMF, and that at the output port provides a steep coupling characteristic of interfered light at the end of the MMF, resulting in the generation of a sharp signal. This leads to high sensitivity to small RI changes. A very high sensitivity of less than 6 ' 10%6 at an RI of more than 1.35 has been realized.