Widely tunable SNAP microresonators via translation of side-coupled optical fibers

We demonstrate free spectral range (FSR) tunable Surface Nanoscale Axial Photonics (SNAP) microresonators induced by side-coupled, parallel optical fiber segments. By translating one segment relative to the other, we tune the coupling length from 900 µm to 100 µm and thereby tune the microresonator FSR from ∼5 pm to ∼50 pm, with an estimated precision of <0.003 pm. The microresonator Q-factor exceeds 10⁵ and can potentially be significantly increased in a clean lab environment. Possible applications of the demonstrated device include miniature and low-loss tunable delay lines and optical frequency comb generators, as well as ultraprecise tunable optical sensors.