An In-Fiber Dual Air-Cavity Fabry–Perot Interferometer for Simultaneous Measurement of Strain and Directional Bend

A simultaneous strain and directional bend sensor based on an in-fiber dual air-cavity Fabry-Perot interferometer (FPI) is presented and demonstrated. The hybrid structure is composed of a fraction of dual side-hole fiber fusion spliced in between a seven-core fiber and a multimode fiber. The bending sensitivities of the two air-cavity FPIs at two opposite most sensitive directions are 120.5 pm/m^-1 and -122 pm/m^-1, 110.4 pm/m^-1, and -121.1 pm/m-1, respectively. The strain sensitivities of the two FPIs are 1.59 pm/με and 1.52 pm/με, respectively. By using the method of difference between the two mutually independent air-cavity FPIs, the highest bending sensitivities can be improved to be 242.5 pm/m^-1 and -231.5 pm/m^-1, respectively, and the axial strain effect can be eliminated simultaneously.