  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 microresonator
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 (OFC). However, consistently producing
SNAP microresonators with equally spaced eigenmodes has remained challenging.
In this study, we introduce a method to create 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
pm 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.
