Interference characteristics in a Fabry–Perot cavity with graphene membrane for optical fiber pressure sensors

Due to higher mechanical strength and ultra-thin thickness, graphene is used as a sensitive diaphragm in a Fabry–Perot cavity to improve the sensitivity of pressure sensors. In accordance with the working principle of Fabry–Perot interferometer, a load–deflection mathematical model of fiber-tip pressure sensor with graphene membrane is established based on the large deflection elastic theory of circular membrane. The effects of membrane parameters, including prestressing force and membrane layer number, on deflection mechanical behaviors are studied by using finite element method. Also, the effects of graphene membrane layer and incident light angle on the film reflectivity are obtained according to the refractive index characteristics of the membrane. The simulation results show that an approximate linear relation between loads and deflections exists in the simulated pressure range from 0 to 3.5 kPa, and a theoretical pressure sensitivity of 1,096 nm/kPa for a single-layered graphene membrane can be achieved. To estimate the performance of multi-layered graphene membrane as the diaphragm, an extremely thin 13-layered 125-μm diameter graphene diaphragm is fabricated on the tip of the fiber end, which forms a low finesse Fabry–Perot interferometer. The Fabry–Perot cavity with a length of 40 μm can exhibit a fringe visibility of approximate 0.56 with a measured membrane reflectivity of 1.49 %. The experimental results demonstrate that the use of graphene as diaphragm material would allow highly sensitive and miniature fiber-tip sensors.