High sensitivity refractive index sensing of Mach-Zehnder interferometer based on multimode fiber core sandwiched between two waist-enlarged fiber tapers

A taper based Mach-Zehnder interferometer (MZI) for high sensitivity refractive index (RI) measurement is investigated. The sensor head consists of two waist-enlarged tapers, in which a section of multimode fiber core (MMFC) is sandwiched to improve the sensitivity of the MZI. Theoretical analysis shows that the RI sensitivity could be enhanced by reducing the MMFC diameter, which is verified in the experiments. A high RI sensitivity of 430.94 nm/RIU and resolution of 4.64×10−5 are achieved in the RI range of 1.33-1.39 for the MZI with MMFC core diameter of 68.5 um. The temperature response of the MZI is also demonstrated.