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 $900\mu$m to $100\mu$m and thereby tune the microresonator FSR from $5$ pm to $50$ pm, with an estimated precision of better than $0.003$ pm. The microresonator Q-factor exceeds ${10}^5$ 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.