Microring resonators for very large scale integrated photonics

Microring resonators have three key attributes which make them desirable for Very Large Scale Integrated Photonic Circuits (VLSI-PCs). They provide numerous wavelength selective functions including channel dropping filters, true ON/OFF routing switches, and notch filters, to name a few. The responses of the foregoing devices can always be improved by using multiple rings in coupled cavity arrangements. With reported devices having radii of from 5 μm to 25 μm, device integration densities of 10⁴ to 10⁵ devices/cm² can be envisioned. In ring resonator based devices, the rings are the wavelength selective elements, while the bus waveguides are simply optical conduits that interconnect rings among themselves or to the outside world. Since rings and bus waveguides serve two different roles, it is desirable to control the parameters of each independently. Vertical coupling allows such flexibility in design. In vertical coupling, air-clad ring resonators are integrated vertically above lower index contrast buried bus waveguides. Vertical integration allows one to optimize the buried waveguides for high input/output fiber coupling efficiency, and for low junction scattering induced cross-talk when bus waveguides cross, while independently optimizing the ring resonator for compactness and low bending loss. It also allows for precision control of the ring-to-waveguide interaction strength