6 Wavelength Division Multiplexing (WDM)

WDM provides the capability to utilize a greater portion of the fiber capacity by extending the application through the wavelength domain. Two or more signals can transmitted on a unique wavelength through the fiber. The fiber can be separated into wavelength domain channels analogous to channels interleaved in time (time domain multiplexing). WDM has the capability of providing multiple, multi-gigabit channels on a single fiber. This capability could be used to allow different transmission protocols (ATM, FDDI and Fibre Channel, for example) to be multiplexed on the same fiber. Although the basic concept of WDM is relatively straightforward, a number of non-trivial challenges are involved, including:

• Four-Wave Mixing – two signals interfere through the refractive index of fiber and generate spurious signals much like intermodulation products.
• Raman Scattering of the optical signal, leading to loss of signal.
• Precise optical amplification such that signals are not amplified with an unacceptably large variance.

A number of commercial WDM components and systems have become available on the market from companies such as IBM, E-tek, and Pirelli Cable Corp. Additionally, WDM multiplexers, demultiplexers, splitters, and combiners are also available.

For NASA, WDM has the potential to permit multiple instrument designers each utilizing a different fiber optic bus heritage to share a common bus without redesigning heritage hardware. It is proposed that part of HSFOTB be dedicated to an evaluation of commercial WDM components and their suitability for spaceflight applications.