Serial-tilted-tapered fiber with high sensitivity for low refractive index range.

We propose an optical fiber sensor for low refractive index (RI) based on a serial-tilted-tapered fiber (STTF), which can be considered as two tightly concatenated micro Mach-Zehnder interferometers (MZIs). The STTF has a compact length of 959.8 μm, and can realize point detection and sensing in limited space. Numerical simulations reveal that a significantly strong evanescent field occurs around the STTF, making it to have the high sensitivity for surrounding RI. In the experiments, the interference dips show the nonlinear wavelength and intensity responses with increasing RI from 1.3395 to 1.3538. In the RI range of 1.3532~1.3538, the RI sensitivities reach the highest value of 2300 nm/RIU and -16183.33 dB/RIU. Moreover, the transmission spectrum of the STTF is low sensitive to temperature. These results indicate that our proposed sensor can be an appropriate candidate in most chemical and biological applications.