Hybrid optical fiber Fabry-Perot interferometer for nano-displacement sensing

•A novel method for displacement sensing with 100 nm resolution is demonstrated.•Extrinsic Fabry-Perot consist of a fiber tip and a hybrid structure.•The lead-in fiber tip was made with a 7-cores fiber spliced to a single-mode fiber.•The supermode interferometer enhances the Fabry-Perot interferometer’s reflectance.•An envelope is exhibited when optical lengths of the two cavities are almost equal. Nano-displacement sensing based on an extrinsic hybrid fiber Fabry-Perot interferometer is proposed and demonstrated. The lead-in fiber tip of such an interferometer consists of a strongly-coupled multicore fiber section fusion spliced to a single-mode fiber. The referred lead-in fiber tip is placed in front of a microscope slide, whose rear surface is coated with a high reflecting layer. The gap between the end-face of the fiber tip and the layer is composed of an air cavity in series with a glass one. Light exiting from the lead-in fiber tip is partially reflected at the fiber-air and air-glass interfaces and the reflecting layer generating three beams that are recoupled into the multicore fiber and combined with supermode interference. By making the optical path length of the air cavity slightly different from the glass one, it is possible to generate an envelope in the interference spectra with a larger period. Thus, by tracking the shift of such an envelope, displacements of 0.47 nm can be resolved. The nano-displacement sensing approach reported here is easy to implement; moreover, the sensitivity, resolution, and dynamic range can be reconfigured by an appropriate selection of the glass cavity.