Highly Sensitive Refractive Index Sensor Based on an In-Fiber Droplet-Shape Air-Cavity

We present a highly sensitive refractive index (RI) sensor based on an ultra-compact in-fiber Mach-Zehnder interferometer (MZI) with a droplet-shape air-cavity. The dropletshape air-cavity is fabricated by fusion splicing and single-side fused tapering. The fabricated MZI is with a very short interference arm of ~330μm and the taper waist diameter of ~40μm. The interference happens between the silica-cladding mode and the air-core mode. The trough wavelength of the interference with specific order in the transmission spectrum shifts with the change of surrounding refractive index. The experimental results show that this sensor offers high sensitivity (up to 2803.6 nm/RIU at RI~1.42) for RI measurement in the range of 1.3164-1.4270, which match well with the theoretical analysis. High RI sensitivity, high-spatial resolution, temperature insensitivity, easy fabrication, and low-cost make it a competitive RI sensor in the biochemical and physical sensing fields.