Autler-Townes splitting and induced transparency windows in a multimode microfiber knot

In this Letter, Autler-Townes splitting and induced transparency windows are observed in a multimode microfiber knot. The microfiber knot is fabricated using tapered single-mode fiber, with the knot position located at the transition area of the tapered fiber. The spectrum, in analogy to Autler-Townes splitting, derives from the mode splitting of two high-order excited modes, which is theoretically explained by the multimode transfer matrix method. Moreover, without adding resonators, two induced transparency windows are realized with the tunable coupling coefficients and phase difference of excited knot modes. The tunable, easily fabricated, compact, and robust microfiber knot has potential applications in optical sensing, filters, slow light, and optical switching.