*Article* **High Sensitivity Optical Fiber Mach–Zehnder Refractive Index Sensor Based on Waist-Enlarged Bitaper**

**Na Zhao 1,\* , Zelin Wang <sup>1</sup> , Zhongkai Zhang 1,\* , Qijing Lin 1,2,\*, Kun Yao <sup>1</sup> , Fuzheng Zhang <sup>1</sup> , Yunjing Jiao <sup>1</sup> , Libo Zhao <sup>1</sup> , Bian Tian <sup>1</sup> , Ping Yang <sup>1</sup> and Zhuangde Jiang <sup>1</sup>**


**Abstract:** A Mach–Zehnder fiber optic sensor with high refractive index response sensitivity was developed. By fabricating a waist-enlarged bitaper structure on the interference arm of a single mode–multimode–single mode (SMS) Mach–Zehnder interferometer (MZI), the spectral contrast and response sensitivity were improved. Subsequently, the response sensitivity was further improved by etching the interference arm. When a beam of light was introduced into the sensor, due to the structural mismatch between the multimode fiber and the normal transmission light, the difference between the low-order mode and the high-order mode was generated in the fiber core and the fiber cladding. In the process of transmission in the sensing arm, due to the different refractive indices of the core and cladding, the optical path difference of the high-order mode and the low-order mode was different, which eventually generated interference fringes. The experimentally measured response sensitivity of SMS MZI in the range of 1.351 RIU to 1.402 RIU is 57.623 nm/RIU; the response sensitivity of a single mode–multimode–bitaper–multimode–single mode (SMBMS) MZI is 61.607 nm/RIU; and the response sensitivity of the etched SMBMS (ESMBMS) MZI is 287.65 nm/RIU. The response sensitivity of the new ESMBMS MZI is three times higher than that of the original SMS MZI. The sensor has the characteristics of compact structure, high sensitivity, easy manufacture, and a wide range of refractive index measurements, and can be used in food processing, pharmaceutical manufacturing and other fields.

**Keywords:** fiber optic sensor; Mach–Zehnder interferometer; optical fiber waist-enlarged bitaper; the refractive index sensitivity; corrosion
