Whispering-gallery resonators (WGRs) are most promising for nonlinear-optical frequency-conversion due to their intensity enhancement by small mode volumes and high Q-factors. This has been shown frequently by millimeter-sized diamond-blade cut and polished bulk WGRs. For reproducible batch fabrication, however, the integration of WGRs into lithium-niobate-on-insulator (LNOI) substrates became of great interest. Here we report on integrated WGRs made by batch processes like lithography and reactive-ion etching. Since the Q-factor of integrated WGRs is limited by scattering losses, we focused on developing a polishing process for the waveguide sidewalls that allowed us to enhance the unloaded Q-factors already to more than 106 with room for further improvements. Furthermore we employ a coupling scheme with two waveguide chips, one comprising a linear coupling waveguide and one with the integrated WGR. By adjusting the distance between the coupling waveguide and the WGR, we can reproducibly and stably tune the coupling-efficiency between 0 and 95 %.
Richard Wolf, Ingo Breunig, Hans Zappe, and Karsten Buse, "Q-factor enhancement of integrated lithium-niobate-on-insulator ridge waveguide whispering-gallery-mode resonators by surface polishing," Proc. SPIE 10090, Laser Resonators, Microresonators, and Beam Control XIX, 1009002 (Presented at SPIE LASE: January 30, 2017; Published: 20 February 2017); https://doi.org/10.1117/12.2251516.
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