Ultrasensitive fiber-optic refractive index sensor based on multimode interference with fiber-loop configuration

A novel ultrasensitive fiber-optic refractive index (RI) sensor is proposed and experimentally demonstrated for solute concentration measurement. To realize higher RI sensitivity, we insert a fiber-optic multimode interference sensor into a fiber-loop configuration to produce a steeper spectrum. The interaction with multimode interference is repeated and enhanced by the fiber loop, resulting in an output spectrum steeper than that obtained without a fiber loop. An ultrahigh RI resolution of 3.06 ' 10%⁷ refractive index unit (RIU) is obtained in the steepest RI range, and it is higher by approximately 53.6 times. We apply this sensor to the measurement of the concentration of a sodium chloride solution and obtain a resolution of 1.87 ' 10%⁴ wt %. This result agrees well with the RI measurement result. The results show that this ultrasensitive RI sensor has the potential for detection applications in the chemical and biomedical fields.