Temperature-compensated high-sensitivity refractive index, liquid level and PH sensing realization enabled by a small-period microfiber long period grating cascaded with microfiber Bragg grating

Conventional reported optical fiber sensors couldn't have both good temperature-compensated characteristic and high PH sensing sensitivity. In this study, we proposed and fabricated a small-period microfiber long period grating (SP-mFLPG) cascaded with a line-by-line microfiber Bragg grating (mFBG) by using femtosecond (fs) laser directly writing technique for high-sensitivity temperature-compensated sensing refractive index(RI), liquid level(LL), PH value. To be the different from traditional LPG devices, the temperature sensitivities of SP-mFLPG was measured as -8.61 pm/℃ owing to the strong coupling occurred between the fundamental mode of TE11 mode and the higher-order mode of TE111, which could be compensated with the mFBG device with a temperature sensitivity of 7.85 pm/℃. On the basis of this temperature-insensitive sensing strategy, our proposed SP-mFLPG sensor can sense RI, LL and pH with high sensitivities of 866.343 nm/RIU, 7.224 nm/mm, -5.70 nm/PH, respectively. Our obtained PH sensitivity is much higher than numerous reported state-of-the-art optical fiber PH sensors. Thus, our proposed optical microfiber device can be found potential applications in real-time measuring some important biochemical reaction procedures.