Microresonators induced at the optical fiber intersections

Abstract: A widely tunable free spectral range (FSR) is essential for many optical microresonator applications, but achieving it remains a significant challenge. Recently, it has been experimentally demonstrated that side-coupling between two optical fibers can induce a high-Q whispering-gallery-mode (WGM) microresonator. In contrast to broadly explored monolithic optical microresonators, this configuration enables extensive tuning of the microresonator FSR through fiber bending, tilting, and twisting. Beyond fundamental interest, this class of microresonators is particularly important for a range of critical applications, including tunable delay lines, frequency comb generators, and reconfigurable optical sensors. Here, we develop the theory of such microresonators, which has remained largely unexplored. We consider weakly twisted fibers, whose geometry can be decomposed into tilting and bending. We show that an extremely small curvature of fibers critically affects the shape and spectrum of the induced microresonators. We discuss the physical origin of this curvature and show that taking it into account leads to excellent agreement between the developed theory and the experimental results.