Ultrasensitive Refractive Index Sensor Based on Microfiber-Assisted U-Shape Cavity

A novel in-fiber Mach-Zehnder interferometer formed by a microfiber-assisted U-shape cavity (MFA-USC) with ultrahigh refractive index (RI) sensitivity and an ultra-wide RI measurement range is demonstrated. The MFA-USC is fabricated by splicing a section of micro-fiber (diameter is ~ 63 μm, length is 688 μm) tapered from single-mode fiber (SMF) between two sections of SMF with designed fiber geometry relationship. An ultrahigh sensitivity (higher than 126 000 nm/RIU at a RI of 1.445) within a RI range 1.320-1.446 and 1.462-1.738 has been experimentally achieved. This sensitivity is the highest ever reported to date to the best of our knowledge.

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