A Temperature Sensor based on All-Fiber Mach-Zender Interferometer with Indium Fluoride Glass

A high sensitivity temperature sensor exploiting indium fluoride optical fibers is designed and characterized. It is based on a non-adiabatic tapered optical fiber, acting as a Mach-Zender interferometer. The sensitivity of the sensor is predicted via mode analysis, performed with Finite Element Method, and then computing the phase delay between the LP01 mode and the LP02 mode. By considering the effect of the thermal expansion and of the thermo-optical properties of the glass, respectively on the waist length and on the core and the cladding refractive indices, the sensing mechanism is explained. To the best of the authors knowledge, the value of the thermo-optic coefficient of the Indium Fluoride Glass (IFG) has not yet been measured. In this paper, the authors assess it starting from the Bragg wavelength shift with temperature of an indium fluoride fiber Bragg grating, already reported in literature. The recovered value is used in the developed thermal model. The non-adiabatic tapered optical fiber (Le Verre Fluoré IFG SM [2.95] 7.5/125) sensor is fabricated with Vytran GPX-2400 glass processing system, addressing the difficulties of indium fluoride glass, including its inclination to crystallize, its limited temperature range for fabrication, and its low glass transition temperature. The sensor is characterized in the mid-infrared spectral range with an interband cascade laser, emitting at the wavelength λ = 3.34 µm. This kind of Mach-Zender interferometer could be employed for refractive index measurement in the mid-infrared, too. The possibility to sense in this spectral range is strongly valuable due to the presence of the absorption lines of many molecules.