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In-Fiber Quasi-Michelson Interferometer Based on Waist-Enlarged Fiber Taper for Refractive Index Sensing

A novel refractive-index (RI) Michelson interferometer based on a waist-enlarged taper is achieved. Such a device is fabricated by splicing a section of multimode fiber (MMF) at one end of single-mode fiber (SMF). Due to the fiber bitaper at the coupling point of SMF and MMF, the light is coupled into the MMF from lead in fiber core, and the intermodal interference will occur for the optical path difference between core mode and cladding mode. Then, the light will be reflected at the end of the fiber and recoupled back into the lead out fiber core by the fiber bitaper. When the lights return back to the lead out fiber, the intermodal interference will occur for the optical path difference between core mode and high order mode. The sensor has a linear response to RI with a sensitivity of -178.424 dB/RIU in the range of 1.351-1.4027 RIU. The temperature cross sensitivity is also analyzed, and the influence can be eliminated by simultaneous measurement of RI and temperature through the matrix equation. The proposed sensor features the advantages of easy fabrication, low cost, high mechanical strength, which make it a good candidate for bio-chemical measurement.