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Experimental analysis for refractive index sensing by using a compact, simple and robust Mach-Zehnder interferometer based on an air gap inside of a fiber
•Fabrication of optical fiber devices with air gap inside optical fiber with a simple procedure.•High sensitivity values for refractive index sensing.•Compact (a few μm) and simple optical fiber device for refractive index sensing.
Compact and straightforward construction of optical fiber Mach-Zehnder interferometer and its application as a refractive index sensor is presented. The interferometer is composed of an air gap inside an ultra-high Numerical Aperture (NA) fiber. As the light propagates through the air gap, the fundamental mode travels in the air gap. Simultaneously, high-order modes are excited in the section constructed by the air gap and the outside air-cladding interface. The propagating high-order modes are sensitive to the surrounding media because their intensity distribution can extend to the surrounding media. Cargille calibrated oils of different refractive indexes are used to test the refractive index change response of various interferometers with several air gap lengths over refractive index changes in the surrounding media. A maximum sensitivity response of 226.8 nm/RIU within the range from 1.43 to 1.45 for a sample device with 12 min of etching time and inconsistent behavior of the intensity (dBm/RIU) for the different devices are reported.