Ultralow Loss, High Q, Four Port Resonant Couplers for Quantum Optics and Photonics

We demonstrate a low-loss, optical four port resonant coupler (add-drop geometry), using ultrahigh Q (>10⁸) toroidal microcavities. Different regimes of operation are investigated by variation of coupling between resonator and fiber taper waveguides. As a result, waveguide-to-waveguide power transfer efficiency of 93% (0.3 dB loss) and nonresonant insertion loss of 0.02% (<0.001  dB) for narrow bandwidth (57 MHz) four port couplers are achieved in this work. The combination of low-loss, fiber compatibility, and wafer-scale design would be suitable for a variety of applications ranging from quantum optics to photonic networks.