Optical microresonators are key enabling elements for many photonic integrated circuits (PICs) areas. Their applications include modulators, optical filters, optical delay lines, nonlinear optical devices, and optical sensors. In previous demonstrations, the coupling of the resonator and its input/output is generally fixed, or tuned using non- integrated alignment system. The ability to control and vary the optical coupling is highly desirable in the areas of emerging adaptive optical circuits as well as in ultra-compact tunable, switchable, and reconfigurable optical components and systems.
In this study, a tunable microresonator achieved by MEMS actuation is proposed on silicon platform. Compared with III-V or II-VI compound semiconductors, silicon has the advantages of low cost, mature fabrication technology, and potential monolithic integration with CMOS devices. Previously, microdisk-based resonators with tunable coupling have been demonstrated. One drawback of the microdisk-based devices is the lack of radial mode control, which could produce additional resonances due to high order modes. In this research, a novel tunable silicon microtoroidal resonator is proposed and demonstrated for the first time. Microtoroidal resonators offer tighter confinement of the optical mode and eliminate multiple radial modes observed in microdisks. By combining the hydrogen annealing and the wafer bonding processes, very compact and high-Q (quality factor) resonators are monolithically integrated with optical waveguides. The integrated micro-electro-mechanical-system (MEMS) actuators enable the coupling gap spacing to vary from 0 to 1 μm. Use hydrogen assisted surface tension induced annealing, smooth surface is created and high optical performance is attained.
We have achieved an unloaded Q of 110,000 for a 39-μm-diameter resonator with a toroidal radius of 200 nm. The device is able to operate in all three coupling regimes: under-, critical, and over-coupling. The loaded Q is continuously tunable from 110,000 to 5,400. Using this type of microtoroidal resonators we have successfully demonstrated several applications, including bandwidth-tunable filters and add-drop multiplexers. A 21.8 dB extinction ratio is attained for a dynamic add-drop multiplexer. Bandwidth is tuned from 2.8 to 78.4 GHz by voltage control, the highest bandwidth tuning range for this type of filter reported up to date.
The resonators can also be decoupled from the waveguide, enabling them to be cascaded without loading the waveguides. This device can be used as a building block for reconfigurable photonic integrated circuits.