Hybrid fiber interferometer sensor for simultaneous measurement of strain and temperature with refractive index insensitivity

A hybrid fiber interferometer sensor is proposed and demonstrated for simultaneous measurement of strain and temperature with insensitivity to external refractive index. This sensor consists of a 5 cm few-mode fiber (FMF) cascaded with a 37 mu m hollow-core fiber (HCF). The other end of the FMF is offset spliced to the lead-out single-mode fiber (SMF) to excite high-order modes inside the core. A Fabry-Perot interferometer (FPI) micro-cavity is formed by the HCF and two splicing interfaces, the interference pattern of which can be observed in the reflection spectrum. The FMF-based core-mismatched structure forms a Mach-Zehnder interferometer (MZI) and the interference pattern can be collected in the transmission spectrum. The simulation result shows that the interference in MZI is mainly generated from the LP11 and LP21 mode in the FMF. A cross coefficient matrix for dual-parameter sensing can be achieved by the different responses of FPI and MZI to strain and temperature. Experimental results show that proposed sensor has the maximum strain sensitivity of 5.89 pm/mu epsilon and the maximum temperature sensitivity of -38.8 pm/degrees C. We also tested the refractive index characteristics of this sensor and results show that it is insensitive to the variation of external refractive index. In addition, the cross coefficient matrix of this scheme has a good condition number of 6.72.