Fig. 1. (a) Cross-sectional micrograph of the FSDF. (b) Profiles of four supermodes excited in the FSDF. (c) Optical coupling in SMF into FSDF with and without the core expanded region. (d) Schematic diagram of the FSDF-MZI.

Fig. 2. (a) Transmission spectra of the FSDF-MZIs with L of 2.40, 2.45, and 2.50 cm. (b) The corresponding FFT spectra of the FSDF-MZIs: short wavelength range and long wavelength range.

Fig. 3. Schematic diagram of a high-temperature strain measurement system for the FSDF-MZI sensor.

Fig. 4. Transmission spectra under different strains at room temperature.

Fig. 5. Strain responses of the FSDF-MZI at room temperature: (a) dip1; (b) dip2. Forth and back represent increasing and decreasing strains, respectively.

Fig. 6. (a) Transmission spectra and (b) temperature responses of dip1 and dip2 from room temperature to 1000°C. (c) Dependence of the strain sensitivities of dip1 and dip2 at different temperatures. (d) Wavelength shift versus time for each applied strain at 1000°C.