Simultaneous measurement of temperature and strain using multi-core fiber (MCF) with an in-line cascaded symmetrical ellipsoidal fiber balls structure of Mach-Zehnder interferometer (MZI) is presented. The sensor is fabricated by using an ordinary fusion apparatus. The thermo-coupling effect is realized through Germanium (Ge)-doped central and hexagonal distributed outer cores of MCF. A high-quality transmission spectrum is obtained with a fringe visibility of 12-15 dB and higher extinction ratio. The sensor exhibits superior mechanical strength compared with the fragile structures, such as tapered, etched, misaligned and offset fibers. The temperature sensitivity of 137.6 pm/°C and 68.1 pm/°C in the range of 20-90°C, and the strain sensitivity of -0.42 pm/με and -1.19 pm/με in the range of 0-801 με are obtained, when probe ``L'' is 40 mm and 20 mm, respectively. Simultaneous measurement of temperature and strain can be achieved by solving the coefficient matrix and tracing the wavelength shifts in the interference spectrum. Besides, the sensor has many advantages, such as high sensitivity, easy fabrication, simple structure, being stable and inexpensive, which may find potential applications in the field of optical sensing.
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