Ultra-precise, sub-picometer tunable free spectral range in a parabolic microresonator induced by optical fiber bending.

Surface nanoscale axial photonic (SNAP) microresonators are fabricated on silica optical fibers, leveraging silica's outstanding material and mechanical properties. These properties allow for precise control over the microresonators' dimension, shape, and mode structure, a key feature for reconfigurable photonic circuits. Such circuits find applications in high-speed communications, optical computing, and optical frequency combs (OFCs). However, consistently producing SNAP microresonators with equally spaced eigenmodes has remained challenging. In this study, we introduce a method to induce a SNAP microresonator with a parabolic profile. We accomplish this by bending a silica optical fiber in a controlled manner using two linear stages. This approach achieves a uniform free spectral range (FSR) as narrow as 1 pm across more than 45 modes. We further demonstrate that the FSR of the SNAP microresonator can be continuously adjusted over a range nearly as wide as one FSR itself, specifically from 1.09 to 1.72 pm, with a precision of ±0.01 pm and high repeatability. Given its compact size and tuning capability, this SNAP microresonator is highly promising for various applications, including the generation of tunable low-repetition-rate OFC and delay lines.