The trend in increase in bandwidth consumption, as well as low latency applications, is pushing the network technology in the direction of a completely transparent optical transport layer. Optical switching elements and topologies are fundamental components to enable routing in a flexible, or even software-defined network. Due to their transparency they allow optical routing without requiring a more expensive and power consuming optical-to-electrical conversion of data packets. The advantages are numerous due to their small footprint, low energy-consumption and latency, as well as silicon photonic compatibility, making this class of components cardinal in the implementation of general purpose photonic transmission networks and data centers applications.