Networking and routing in space-division multiplexed systems

Optical networks serve as the cornerstone of our connected society, enabling the information superhighway that delivers the internet all across the globe. As the number of users and data services increase, the network technology and architecture must adapt for it to continue to efficiently and economically support the larger traffic loads. Currently these optical networks consist of optical transceivers of different wavelengths whose signals are wavelength-division multiplexed (WDM) together and transported using fiber-optic cables with optical amplifiers. Each fiber can potentially support more than 100 wavelengths, each carrying hundreds of gigabits per second for a per fiber capacity in the tens of terabits per second. The paths these optical signals traverse through the network can be selected by using reconfigurable optical add-drop multiplexers at network nodes. In this chapter we address current architecture of WDM networks and how it may evolve in the future to support even greater capacities through the use of additional spatial paths, an approach referred to as space-division multiplexing as an analogy to WDM. The use of SDM in terrestrial networks is an inevitable consequence of the demand for increased bandwidth. While the addition of SDM to optical transport simply adds linearly to the number of channels in an unused degree of freedom, the challenges in switching are more significant. The spatial degrees of freedom are already being used in the optical switches to enable wavelength demultiplexing and switching to multiple ports. This means that adding SDM compromises some aspect of the switching performance. To manage increased numbers of spatial channels, it may be necessary to compromise on the spectral resolution and therefore the channel count. It seems that the most likely approach to SDM, at least initially, will be to use increasing numbers of uncoupled spatial channels, again most likely in the form of multiple SMFs. It also seems likely that as the number of these spatial channels increases, there will be a corresponding reduction in the number of wavelength channels to be switched, ultimately leading us to a purely spatial switching network scenario. A purely spatial switched network will lead to significant changes not only in switch architecture and network control but also due to the elimination of combining diverse wavelength routed signals, in optical line system design.