Sensitivity-Enhanced Photonic Crystal Fiber Refractive Index Sensor With Two Waist-Broadened Tapers

A new sensitivity-enhanced refractive index (RI) sensor based on photonic crystal fiber Mach–Zehnder interferometer (PCF-MZI) was proposed and experimentally demonstrated. It was realized by introducing a normal PCF-taper between two waist-broadened single mode fibers (SMF)-PCF-tapers, which were fabricated at the joints of a PCF and two SMFs using “overlap” splicing technique. Via tapering the PCF, the maximum RI sensitivity is increased by a factor of 1.73 owing to the evanescent field. Consequently, a maximum RI sensitivity of 281.6 nm/RIU is successfully achieved in the RI range of 1.3333–1.3737, which is around ten times higher than the RI sensitivity of SMF-MZI with two or three-cascaded-taper. Moreover, the proposed three-taper PCF-MZI is independent on temperature from 20 to 55 °C. In addition to temperature immunity, it has advantages such as high RI sensitivity, ease of fabrication and measurement, compactness, and robustness.