Wavelength selective switching for optical bandwidth management

Optical transport capacities have grown significantly in the last decade to meet the increased demands on communications networks. This growth has been achieved both by increases in individual channel rates, which are based on time division multiplexing (TDM), and by increased channel counts, through the use of dense wavelength division multiplexing (DWDM). Yet increasing optical transport capacity alone is insufficient to scale the network; the underlying data needs to be delivered from numerous geographically diverse originating locations to similarly diverse terminating locations, requiring increasing switching capacity to facilitate this networking need. Since the growth in the individual TDM channel rates is driven by the capabilities of electronics, it is reasonable to expect that the switching capacity of electronics will tend to track this trend, although because of the challenges in high data rate interconnects it is unlikely to exceed it. This leaves the challenge of managing the increased bandwidth attained through the use of DWDM. Management of this bandwidth in the optical layer is an attractive proposition if eliminating unnecessary high-speed electronics in the path of an optical signal can reduce the complexity of the network and the associated equipment costs. These optical bandwidth management elements are classified according to the degree of switching, as either reconfigurable optical add/drop multiplexers (ROADM) or wavelength selective cross-connects (WSXC), analogous to the add/drop multiplexers and digital cross-connects of the TDM domain. We generalize these elements and describe whether the switching provides functions that are multicolored, colorless, or colored, and whether the channels are fixed data rate or rateless. We review the wavelength selective switch (WSS) components that perform the necessary switching function and present two successful technology platforms that can be used to construct them: planar lightwave circuits (PLC) and micro-electromechanical systems (MEMS). We also discuss future directions for WSS technologies and device functionality to more flexibly manage bandwidth in the optical layer.