High-temperature sensor based on fiber with inner cladding

We proposed and demonstrated a Mach-Zender interferometer based on an optical fiber with an inner waveguiding cladding spliced between two standard fibers. The interferometer is suitable for sensing temperatures up to 1000 degrees C. The interference spectrum was formed because of the interference of the core mode and one of the inner cladding modes. The wavelength shift of the interference dip with temperature was well-fitted by the second-order polynomial function lambda dip = 1514.0 + 0.041.T + 6.65.e-5.T2 with R2Adj = 0.999. The temperature sensitivities achieved by the linear fitting of this curve in the ranges of 20-200 degrees C, 200-500 degrees C, and 500-1000 degrees C were 56 pm/ degrees C, 89 pm/ degrees C, and 141 pm/ degrees C, respectively. Owing to its double-cladding waveguide structure, the sensor is immune to changes in the external refractive index. The measured sensitivity of the proposed sensor to bending and tension is also low.