  Surface nanoscale axial photonics (SNAP) microresonators with nanoscale
effective radius variation (ERV) along optical fiber axis can be fabricated by
inscribing axially oriented lines inside the fiber with a femtosecond laser.
The optimization of variable parameters in the femtosecond laser inscription
technique is of great significance for flexible and ultra-high precision
control of ERV, which is vital to the performance of SNAP devices. Here, we
present the first systematical investigation on the relationships between the
various controllable fabrication parameters and the introduced ERV of the SNAP
microresonators. Specifically, both the qualitative and quantitative processing
principles were revealed. As a proof-of-principle, by comprehensively
optimizing the fabrication parameters, we realized a SNAP microresonator with
the characteristics of both small axial size and maximal ERV, which is almost
impossible to realize with other SNAP fabrication techniques. Our work promotes
the fs laser inscription technology to be a flexible and versatile approach for
fabricating the SNAP devices with ultra-high precision, ultra-low loss and high
robustness.
