Highly sensitive temperature sensor based on a polymer-infiltrated Mach–Zehnder interferometer created in graded index fiber

A highly sensitive temperature sensor is proposed and demonstrated based on a UV-curable polymer-infiltrated Mach-Zehnder interferometer (MZI) created in a graded index fiber (GIF). The device was constructed by splicing a half-pitch GIF between two single-mode fibers and creating an inner air cavity in one lateral side of the GIF core by means of femtosecond laser micromachining. The air cavity and the residual GIF core functioned as two interference arms of the MZI. Moreover, the GIF was used as a miniature in-fiber collimator to reduce insertion loss of the air cavity. Experimental results show such an MZI device has a high refractive index (RI) sensitivity of 24611.54 nm/RIU (RI=1.545-1.565). Subsequently, thermo-sensitive polymer liquid was infiltrated into the air cavity, then cured with UV illumination, and annealed at 50°C for 12 h. The infiltrated MZI exhibits a high temperature sensitivity of -13.27 nm/°C. In addition, this MZI also has excellent thermal stability and repeatability, compact structure, low insertion loss, and high fringe visibility. As such, the proposed MZI could be developed for high-accuracy temperature measurements in many areas such as biomedical or oceanographic applications.