High-Temperature Properties of a Thin-Core Fiber MZI With an Induced Refractive Index Modification

A fiber-optic Mach-Zehnder interferometer is proposed and demonstrated experimentally. The device consists of a short piece of thin-core fiber (TCF) sandwiched between two standard single mode fibers. A microscope direct-writing technique with femtosecond laser exposure is utilized to induce a refractive index change near the interface between the core and cladding of the TCF. This added refractive index profile is helpful for the coupling of the core-to-cladding modes, resulting in a well-defined interference spectrum. The ultra-high temperature property of the proposed interferometer is demonstrated up to 900 °C with a sensitivity of 94.3 pm/°C over repeated measurements. And the repeated operations of thermal annealing are employed to improve the thermal stability and measurement performance of the interferometer.