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

Abstract

We demonstrate a low-loss, optical four port resonant coupler (add-drop geometry), using ultrahigh Q (>10 8 ) 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. DOI: 10.1103/PhysRevLett.92.253905 PACS numbers: 42.79.Gn, 42.60.Da, 42.82.Et, 42.79.Sz Minimizing optical loss is of crucial importance in various studies, as it is often the main obstacle in realizing distinct physical functionalities. This is true in quantum optical applications of microcavities where parasitic loss can both inhibit the generation of quantum states and interfere with intended coupling to a transport medium such as optical fiber [1][2][3][4][5][6][7][8]. Also, in photonic applications of these devices [9-12] the ability to attain high efficiency power transfer between two distinct waveguides is of great interest. The ability to attain coupling coefficients between the resonator and waveguides that are greater than the intrinsic round-trip loss of the cavity (usually called the overcoupled regime) is fundamental to achieve high waveguide-to-waveguide resonant power transfer efficiency. High waveguide coupling efficiency and high intrinsic quality factor are hence essential in all applications of waveguide-coupled resonator systems. Such characteristics ensure that the overall quality factor of the system can be dominated by the intentional control of waveguide loading (coupling into and out from the resonator) as opposed to parasitic mechanisms which include intrinsic losses of the cavity and scattering losses at the waveguide-resonator junctions. Ultrahigh-qualityfactor (UHQ) microresonators (Q > 10 8 ) have been extensively studied [7,8,13], and the ability to provide high efficiency coupling to UHQ devices by use of low-loss tapered fiber waveguides has also been verified [14]. In this Letter, we explore a new realm of performance enabled by the combination of these results. Toroidal microcavities with intrinsic Q factors in excess of 10 8 were fabricated and studied in the so-called add-drop geometry where a whispering-gallery mode enables resonant power transfer between two distinct waveguides. Greater than 93% power transfer efficiency is measured in devices having loaded Q factors of 3:3 10 6 (overall quality factor of the resonator including coupling to waveguides), while the nonresonant insertion loss remains less than 0.02%. The transfer efficiency is predicted to be even higher for lower loaded Q values. This represents a substantial improvement compared to all prior work on similar microcavity-type structures, elevating their performance to a level at whic...

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