Tunable Micro-Optical Resonator Based on MEMS Actuators
Ming-Chun Tien and Ming C. Wu
Microring and microdisk resonators are key components to realize silicon monolithic optoelectronic devices because they have the potential of integration with current CMOS electronic devices. The applications include most wavelength-division-multiplexing photonic circuits, such as optoelectronic transceivers, wavelength filters, add-drop multiplexers, optical delay lines, and group velocity dispersion compensators. Furthermore, in order to achieve dynamic switching functions, tunable capability is desired. Here we propose a tunable microring resonator with integrated MEMS actuators, which control the power coupling between optical waveguides and microring resonators. By tuning the gap between the waveguide and microring, it can be operated in four different coupling regimes: uncoupling, under-coupling, critical coupling, and over-coupling.
The fabrication of such a tunable microring resonator includes a two-mask process using silicon-on-insulator (SOI) wafers. Due to the high refractive index contrast of silicon waveguides, sub-micron dimension is necessary to maintain a single mode waveguide. Instead of using the e-beam writing technique, sub-micron optical waveguides were achieved utilizing deep UV photolithography together with thermal oxidation, which reduces the surface roughness caused by the etching process. The fabricated tunable microring resonator consists of a microring resonator, optical waveguides, and electrostatic MEMS actuators as shown in Figure 1. By applying different bias voltages, tunable coupling between the resonator and waveguides can be achieved. Figure 2 shows two different transmission spectra which correspond to under- and over-coupled regimes, respectively. The intrinsic quality factor of the resonator is as high as 88,400, which is extracted from the transmission spectra using coupled mode theory.
Figure 1: SEM picture of fabricated tunable resonators, which consists of a microring resonator, optical waveguides, and electrostatic actuators
Figure 2: Transmission Spectra with different coeffieients between the resonator and waveguides at different bias voltages. The resonator is in the under-coupled regime at 65 V while it's in the over-coupled regime at 70.2 V