Silicon active microring resonators for optical switching

摘要

Microring resonators are basic building blocks for a variety of functions such as modulators, switches, buffers, and filters. Active tuning of resonators and external coupling are widely used to change the resonance properties. In this lecture, we will present the basic elementary switch devices and the NxN switching fabric made of various types of silicon active microring resonators. An elementary switch can be formed by integrating a thermo-optic (TO) or an electro-optic (TO) tuner into a ring resonator coupled with two bus waveguides. During the switching operation, the resonance wavelengths are shifted to provide optical power exchange between the two output ports. Due to the slow roll-off of the resonance passband for the single ring resonator, the switching extinction ratio is relatively low. To improve the switching extinction ratio, two approaches can be adopted. The first one is to use three directly coupled ring resonators in which the central ring is actuated. The second one is to let the rings side-couple with a Mach-Zehnder interferometer (MZI), and due to the interference, the passband has a much shaper edge, greatly improving the switching extinction ratio. Besides the direct cavity tuning, the inter-coupling between ring resonators or the external coupling to bus waveguides can also be modified to perform switching. The inter-ring coupling could be enabled by using a directional coupler or a MZI coupler. A three-waveguides based directional coupler can result in irregular resonance selective switching, which can find applications in duplex communication systems. The drop power of an add-drop ring is significantly affected by the ring-waveguide coupling strength. Hence, optical switching can also be achieved by tuning the external coupling. High switching extinction ratio is resulted as long as the ring is tuned to meet the critical coupling condition. With the basic 1 × 2 or 2 × 2 switch elements, a large scale NxN switch fabric can be built using various topological architectures, such as crossbar, Benes, switch-and-select, etc.. The pros and cons of these different switch elements and architectures will be discussed for applications in datacenter and supercomputer interconnect networks.