We demonstrate a miniaturized fiber in-line Mach–Zehnder interferometer based on an inner air cavity adjacent to the fiber core for high-temperature sensing. The inner air cavity is fabricated by femtosecond laser micromachining and the fusion splicing technique. Such a device is robust and insensitive to ambient refractive index change, and has high temperature sensitivity of ∼43.2 pm/°C , up to 1000°C, and low cross sensitivity to strain.
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