Highly sensitive refractive index sensing based on a novel Mach-Zehnder interferometer with TCF-PCF composite structure

•A novel RI sensor based on MZI with TCF-PCF structure is proposed.•A sensitivity of 252.88 nm/RIU in the range of 1.3735 ∼ 1.3994 is measured.•RI sensing properties of the MZI with different length are studied.•The sensor has merits of simple fabrication, high resolution and robust. We have proposed and demonstrated a highly sensitive refractive index (RI) sensor based on photonic crystal fiber Mach-Zehnder interferometer (PCF-MZI), which was obtained by splicing a piece of thin core fiber (TCF) and PCF with the same length between two single mode fibers (SMFs). The proposed MZI exhibited good fringe visibility as high as 21 dB in air and a highly RI sensitivity of 252.88 nm/RIU in the range of 1.3735 ∼ 1.3994. Additionally, the RI sensing properties of different length of PCF were also investigated experimentally. It was found that the RI sensitivities were weekly dependent on the length of MZI. The measured sensitivities of the MZI with 35 mm PCF, 40 mm PCF, 45 mm PCF were 112.99 nm/RIU, 119.25 nm/RIU, and 117.52 nm/RIU in the range of 1.3333 ∼ 1.3735 and 228.77 nm/RIU, 252.88 nm/RIU and 183.66 nm/RIU in the range of 1.3735 ∼ 1.3994, respectively. Moreover, the RI sensitivities were increased by about 62% and 2.4 time than the MZI with SMF-PCF-SMF structure in the above RI intervals, respectively. In addition, the sensor shown advantages of simple fabrication, high fringe visibility, high sensitivity and low temperature sensitivity, which was attractive for the development of RI sensing application.