Wavelength-Selective Switching for Optical Bandwidth Management

Optical transport capacities have grown significantly in the last decade to meet the increased network demand. This growth has been achieved by increases in individual channel rates, which are based on time-division multiplexing (TDM) and increased channel counts, from the use of dense wavelength division multiplexing (DWDM). Yet large optical transport capacity alone is insufficient to scale the network; the underlying data carried by optical means needs to be delivered from numerous geographically diverse originating locations to similarly diverse termination locations, requiring the optical layer facilitate this networking need. An optical network is composed of a series of switching nodes connected together by transport links, which spans a geographical region of interest. 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, 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 bandwidth in the optical layer is an attractive proposition if the complexity of the network, and the associated equipment costs, can be reduced by eliminating unnecessary high-speed electronics in the path of an optical signal. 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 multipliers and digital crossconnects of the TDM domain. We will generalize these elements, describing whether the switching provides functions, which are multi-colored, colorless, or colored, and whether it is fixed data rate or rateless. We review the wavelength selective switches (WSS) components that perform the necessary switching function and present two successful technology platforms that can be used to construct them: planar light wave circuits (PLC) and Micro- Electro-Mechanical Systems (MEMS).