Frequency-domain displacement sensing with a fiber ring-resonator containing a variable gap

Ring resonators are in general not amenable to strain-free (noncontact) displacement measurements. We show that this limitation may be overcome if the ring resonator, here a fiber-loop, is designed to contain a gap, such that the light traverses a free-space part between two aligned waveguide ends. Displacements are determined with nanometer sensitivity by measuring the associated changes in the resonance frequencies. Miniaturization should increase the sensitivity of the ring resonator interferometer. Ring geometries that contain an optical circulator can be used to profile reflective samples. Manuscript