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A refractive index (RI) and temperature or a temperature and axial strain sensor based on an inline Mach–Zehnder interferometer with thin core fiber (TCF)–thin fiber (TF)–TCF structure is proposed and experimentally demonstrated, requiring only the cleaving and fusion splicing methods. The operation principle depends on the effect that the TF cladding modes interfere with the core mode as an optical coupler. The RI, temperature, or axial strain variations can lead to resonance dip variations in the interferometer spectra, and the RI, temperature, or axial strain sensitivity can be measured by monitoring the wavelength shifts of resonance dips. Then we can measure both RI and temperature, or temperature and axial strain through the demodulation matrix. Four sensors with different TF lengths are fabricated based on numerical simulation. A 15 mm long TF sensor displays an RI sensitivity as high as − 174.357 n m / R I U , temperature sensitivities in the glycerin solution and the air of 12.47 and 26.19 pm/°C, and axial strain sensitivity of − 3.43 × 10 − 4 n m / µ ε . Moreover, due to its simple manufacture, high cost-effectiveness and compactness, the proposed sensor has a broad application prospect in physical, chemical, and biological sensing.

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