All Single-Mode Fiber Mach–Zehnder Interferometer Based on Two Peanut-Shape Structures

A peanut-shape fiber structure that can realize the coupling and recoupling between the fiber core mode and the cladding modes is proposed in this paper. Based on the structure, a simple and low-cost Mach-Zehnder interferometer (MZI) formed by cascading two peanut-shape structures in the single-mode fiber is demonstrated. The theory and the experimental results show that the first peanut-shape structure can couple the light energy of the core mode into the cladding modes and the second peanut-shape structure can recouple the light in the cladding modes into the core mode. A high-quality interference spectrum with a fringe visibility of about 13 dB is observed. Moreover, it has very good mechanical strength compared with the MZIs based on the tapers or the offset structures. When the interferometer length L = 22 mm, the temperature sensitivity of the device is ~ 46.8 pm/°C and the strain sensitivity is ~ 14 pm/με. Such kind of interferometer would find potential applications in communication and sensing fields.