M. Scheucher, K. Kassem, A. Rauschenbeutel, P. Schneeweiss, and J. Volz
Optical fibers play a key role in many different fields of science and technology. For many of these applications it is of outmost importance to precisely know and control their radius. In this manuscript, we demonstrate a novel technique to determine the local radius variation of a 30 micrometer diameter silica fiber with sub-Angström precision over more than half a millimeter in a single shot, by imaging the mode structure of the fiber’s whispering gallery modes (WGMs). We show that in these WGMs the speed of light propagating along the fiber axis is strongly reduced, which enables us to determine the fiber radius with significantly enhanced precision, far beyond the diffraction limit. By exciting several different axial modes at different probing fiber positions, we verify the precision and reproducibility of our method and demonstrate that we can achieve a precision better than 0.3 Angström. The demonstrated method can be generalized to other experimental situations where slow light occurs and, thus, has a large range of potential applications in the realms of precision metrology and optical sensing.