Special Report: Next Generation Internet Applications

Wavelength Division Multiplexing (WDM) Wavelength division multiplexing (WDM) is the optical equivalent of frequency division multiplexing. Frequency and wavelength are inversely proportional, and the multiplexing schemes associated with both are relatively similar. Wavelength division multiplexing is the multiplexing of different separate wavelengths of light onto a common light channel such as a fiber optic medium. The light is multiplexed by means of a diffraction grating or optical prism. Figure 2 depicts the optical wavelength multiplexing process. Grating Grating Figure 2: Wavelength Multiplexing and Diffraction Gratings [9] WDM technology initially multiplexed two wavelengths for multiplexing, 1310 nm and 1550 nm. These wavelengths are common in devices that interface to single mode fiber, since the fiber has lowest optical loss at these wavelengths. As the multiplexing technology has progressed, WDM devices have grown to accommodate greater than two wavelengths. As the number of supported wavelengths per fiber continues to grow, a new name for the technology has emerged: "dense wavelength division multiplexing" (DWDM). DWDM involves the multiplexing of additional wavelengths around the 1310 nm and 1550 nm wavelengths, to yield more optical channels per fiber. DWDM technology's bandwidth expansion is limited only by the resolution of the wavelength division of the optical network elements.[3]

2.1.3 Optical Cross Connects (OXC) Reliable and affordable optical cross connects (OXC) are essential to the proliferation of all-optical switches. The evolution of OXCs stemmed from advances in microelectromechanical systems (MEMS). MEMS is a technology that enables the control of miniature mechanical devices such as very small mirrors. In OXCs, MEMS mirrors are used to reflect the optical signal from one input to the chosen output. Early design utilized a binary-controlled two-dimensional mirror array that would either implement a mirror or remove it from the light path. Recently, analog controlled three-dimensional mirror matrices have been designed that allow for finer control of optical signal direction. Three- dimensional optical cross connects utilize three-dimensional mirror matrices to connect any input to any output and reduce cost by utilizing fewer mirrors. Twodimensional optical cross connects require N2 mirrors, while three-dimensional optical cross connects require 2N mirrors, where N is the number of inputs or outputs.[5] Figure 3 depicts the two-dimensional OXC structure and Figure 4 depicts the three-dimensional OXC structure.

2.1.4 Optical Add-Drop Multiplexers (OADM) Optical Add-Drop Multiplexers (OADM) are responsible for adding or removing individual signals (wavelengths) at individual points along the optical transport channel. OADMs function fully in the optical domain without performing an OEO conversion. OADMs operate as peripherals to the OXCs, providing the OXCs with the appropriate signals to direct. There are two types of OADMs. The first operates with fixed optical filters and is deemed a fixed wavelength OADM. The second operates with tunable optical filters and is referred to as a dynamic OADM.[10] While fixed wavelength OADMs are the more mature variant, researchers are actively pursuing dynamic OADMs with features such as acousto-optic tunable filters (AOTF). The AOTF removes the need for an optical switch typically found in a dynamic OADM and thus reduces cost. Figure 5 illustrates a fixed OADM and Figure 6 illustrates a dynamic OADM.

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