Optical fiber Fabry-Perot interferometer coupled to a 3-D integrated waveguide for 3-D position sensing.

Optical fiber extrinsic Fabry-Perot interferometers (EFPIs) have been extensively demonstrated for the measurement of displacement and displacement-related physical quantities, e.g., acceleration, pressure, with high sensitivity and resolution. Despite its wide and successful applications, a conventional EFPI is limited to measuring only one-dimensional (out-of-plane) movement of its external reflector. In this Letter, a new strategy for optical fiber sensing, particularly for EFPI sensing, is proposed and demonstrated, allowing for three-dimensional (3-D) measurements based on a hybrid and compact EFPI device. A 3-D integrated optical waveguide array is aligned against a lead-in optical fiber with an air gap, where an EFPI is formed by the end facet of the optical fiber and the end facet of the waveguide array. As a proof of concept, we experimentally demonstrate that 3-D positioning can be achieved from the EFPI with sub-micron resolution by simultaneously measuring the reflection and transmission of the device. The proposed strategy of using an optical waveguide as an external reflector for an optical fiber EFPI, combined with machine learning-based analysis, opens new avenues in the development of compact yet multi-dimensional sensors.