Tapered graded-index multimode-fiber coupled sapphire Fabry-Perot cavity for high-temperature dynamic strain measurement.

Real-time dynamic strain measurements under a high temperature environment is important for many applications, e.g. component fault diagnosis of aeroengines. The sapphire fiber is an ideal device for extreme sensing due to its high melting temperature up to 2050 °C. The multimode feature of the sapphire fiber, however, results in modal interference and hence the low visibility of reflection spectrum in Fabry-Perot (FP) cavities. Here we report a higher-order mode suppression technique using tapered graded-index multimode fiber (GIMMF) for the sapphire Fabry-Perot cavity. The tapered GIMMF is designed to couple the fundamental mode from the single-mode fiber to the sapphire FP cavity with low loss together with more than three times suppression on the excited energy percentage of the higher-order modes, significantly enhancing the contrast of the cavity interference signal. A three-wavelength laser-interference method is used to demodulate the cavity length of the multimode sapphire FP sensors, achieving a real-time strain measurement up to 1000 ℃ temperature with 160 kHz sampling frequency. Experimental results show that the sensor can achieve a dynamic range of ±1000 με with the maximum error less than ±10%.