Coupling of angle polished waveguides to high-Q whispering gallery mode resonators

High-Q whispering gallery mode resonators (WGMs) made both in glasses or crystals attract growing interest not only for the investigation of fundamental processes but also for the implementation of advanced photonics devices and biological sensors [1]. For practical applications of WGM resonators the crucial issue is the implementation of an efficient and robust coupling scheme. Fiber taper based couplers are particularly efficient and flexible but strongly lack of the necessary robustness. Prism coupling technique, which involves free space beams, is typically used to efficiently couple light in crystalline resonators. Planar waveguide coupling to glass microspheres and more recently to high-Q lithium niobate resonators [2] has been also demonstrated. Finally an alternative known coupler is based on angle polished fibers [3]. The tip of an optical fiber is polished according to a proper angle θ so that the evanescent field after the total internal reflection has the same propagation constant of the resonator mode we want to excite. The application of this method is limited by the availability of fibers with the proper refractive index. In this work we propose the alternative use of robust angle polished waveguides which can be made on a number of different substrates, which, in turn, can be selected with the proper index in order to fulfil the phase matching condition with the resonator. The simple equation θ=acos(n
WGM
/n
WG
) holds, where n
WGM
and n
WG
are the WGM and the waveguide effective indices, respectively.