Fiber-Optic Refractive Index Sensor Based on Multi-Tapered SMS Fiber Structure

A new refractive index (RI) sensor based on multi-tapered single mode–multimode–single mode (SMS) fiber structure is proposed. The sensor is fabricated by tapering several tapers in multimode fiber part of the SMS fiber structure. In the taper region, the evanescent wave is generated and permeates to the surrounding liquid. Thus, the sensor is sensitive to the RI variation and the sensing characteristic is investigated theoretically and experimentally. Through fabricating three tapers, five tapers, and eight tapers, it can be concluded that the more the tapers, the higher the measuring sensitivity. According to the experimental results, when the number of tapers is up to eight, the RI sensitivity of 261.9 nm/RIU in the RI range of 1.3333–1.3737 can be achieved. Since the fabricated process of this all-fiber RI sensor can be completed just using a fusion splicer, the sensor has a series of advantages, such as high sensitivity, low cost, and simple operation.