Phase-shifting multimode FLRD technique with MSM structure for RI measurement

A high-sensitivity fiber-optic refractive index (RI) sensor based on phase-shifting multimode fiber-optic loop ring down (PS-MFLRD) technology and multimode-single-mode-multimode (MSM) structure is proposed. The multimode coupler and combiner form a loop with intense light, eliminating the need for an additional Erbium Doped Fiber Amplifier (EDFA) in this system. In addition, The MSM structure's loss varies linearly with the RI of the surrounding environment, enabling RI measurement through phase monitoring. Experimental results demonstrate a sensitivity of 10 deg/RIU within the range of 1.353 to 1.388, and the corresponding linearity R-2 of 0.9927 is obtained, it also has excellent temperature stability at temperatures from 20 degrees C to 80 degrees with a fluctuation range of only 0.006 deg. More importantly, under the same configuration, the single mode structure has a sensitivity of only 0.6 deg/RIU, a difference of 17 times. In addition, the resolution limit of this system is 1.0 x 10(-6) RIU. The proposal of MFLRD provides an effective solution for improving the sensitivity of subsequent RI measurements and simplifying devices.