Refractive Index Sensor Using a SMS Fiber in an Erbium-Doped Fiber Ring Laser

In this letter, a refractive index (RI) sensor was developed and experimentally validated using an interferometric design that integrates single-mode fiber (SMF), no-core multimode fiber, and another segment of SMF. The single-mode–multimode–single-mode structure was employed as a filter and inserted into an erbium-doped fiber ring laser to enhance the sensor's detection accuracy. Experimental findings show that the sensor provides an outstanding linear response, with an RI sensitivity of 96.639 nm/RIU over a measurement range of 1.3468–1.4061, along with strong stability.