Polymer Waveguide Couplers for Fluorite Microresonators

We report on the development of a planar evanescent-field coupler for calcium fluoride (CaF
₂
) whispering gallery mode resonators using polymer rib waveguides and demonstrate 80% coupling efficiency with resonators characterized with the loaded quality factors exceeding 10
⁸
. The coupling efficiency is limited due to emission of light from the resonator mode to the unconfined modes of the waveguide.