Femtosecond Laser Fabrication and Refractive Index Sensing Characteristics of Highly Sensitive Fiber Michelson Microcavity

In this article, a compact optical fiber refractive index (RI) sensing probe based on Michelson interference and Fabry–Pérot interference is proposed through fabricating a microcavity in the multimode optical fiber (MMF) by using the femtosecond laser-induced water breakdown technique. This probe not only exhibits ultrahigh RI sensitivity but also possesses high accuracy of RI measurement by compensating temperature crosstalk. The RI and temperature sensing characteristics of this probe are investigated theoretically and experimentally. The theoretical RI sensitivity of the fiber probe is up to
$-10\,\,000$
nm/RIU, and the theoretical temperature sensitivity is 33 pm/°C. In the experiment, the RI sensitivity of Michelson interference peak is −11523.8 nm/RIU in the range of 1.333–1.3387, and temperature sensitivity is 49.43 pm/°C in the range of 18.75 °C–55.38 °C in the air. Theoretical and experimental results agree well. At the same time, the RI and the temperature sensitivities of Fabry–Pérot interference peak are measured. The RI sensitivity of Fabry–Pérot interference peak is 422.75 nm/RIU, and temperature sensitivity is 8.06 pm/°C. The accuracy of RI measurement can be further improved by using the method of sensing matrix. The proposed RI sensing probe exhibited small volume, ultrahigh sensitivity, and high accuracy. It will be adapted to the RI detection in many fields and exhibits a widely practical application prospect.