An in-fiber modal interferometer based on coaxial dual-waveguide optical fiber was demonstrated for temperature sensing. The modal interferometer is composed of a coaxial fiber, which is sandwiched in between two single mode fibers with small lateral core-offsets. Such a modal interferometer is insensitive to external refractive index (RI) and can work when the surrounding RI is larger than the RI of the fiber cladding. It has high temperature sensitivity of 78 pm/°C and low crosssensitivity to strain of 0.004°C/με.

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