Michelson interferometer based on a fiber with a germanium-doped core and inner cladding for high-temperature sensing

We proposed and demonstrated a temperature sensor based on a fiber Michelson interferometer. The Michelson Interferometer contains a special fiber with a germanium-doped core and pure silica inner cladding, so the interference spectrum is formed as a result of interference of the mode of the core and one of the modes of the inner cladding. The existence of inner cladding reduces the number of interacting cladding modes. When annealing the sensor for 300 min at a temperature of 600 °C, no thermal hysteresis of the interference spectrum was observed. The temperature sensitivities of the proposed sensor are 48 pm/°C in the range of 20–250 °C and 78 pm/°C in the range of 250–600 °C. But when the sensor was exposed to a constant temperature of 700 °C and above for a few hours, a shift in the wavelength of the dip of the interference spectrum is observed. This wavelength shift, which occurs even at a fixed temperature, depends on the temperature of annealing. It can be explained by a number of processes occurring in glass at high temperatures: the changing fictive temperature of the glass, diffusion of germanium oxide from the core, and formation of mechanical stresses resulting from the difference in thermal expansion coefficients of the core and cladding, and relaxation of these mechanical stresses at an elevated temperature.