Ultra-compact microdisk resonator filters on SOI substrate.

The evanescent coupling of a 1.5 mum radius silicon microdisk with one or two Silicon-On-Insulator waveguides is studied. Thanks to the high refractive index contrast between Silica and Silicon materials, this very-small-diameter microdisk exhibits the highest quality factor measured in wavelength range from 1500 nm to 1600 nm. Coupled to a single monomode waveguide, the optical resonator behaves as a stop-band filter. Even if the microdisk is a largely multimode resonator, only its fundamental modes are efficiently excited. The filter's transmission is measured for different gap between the waveguide and the resonator. The critical coupling is clearly observed and gives access to 1.63 nm linewidth. A 20 dB decrease of the transmission signal is also observed. Coupled to two waveguides, the resonator becomes a compact symmetric wavelength-demultiplexer. In this case, the optimal response comes from a compromise between the gap and the desired linewidth dropped in the second waveguide. Finally, our measurements are also compared to analytic models showing a good agreement especially for the critical gap prediction.